DESCRIPTION
[Title of Invention]
COMMUNICATION SYSTEM, COMMUNICATION DEVICE,
INTEGRATED CIRCUIT, AND COMMUNICATION METHOD
[Technical Field]
[0001]
The present invention relates to technology for transferring data between,
for example, a plurality of communication devices connected in a ring via a serial
link.
[Background Art]
[0002]
With recent development in technology for miniaturizing semiconductors
and accelerating the speed of the semiconductors, the amount of data communicated
between devices or LSIs (Large Scale Integrations) provided in the devices is
increasing more than ever. On the other hand, a strict restriction is posed on the
number of terminals (pads) in an LSI, which affects the packaging cost. Accordingly,
for the purpose of achieving high-speed data communication with a fewer number of
terminals in an LSI, the interface standards employing serial communication have
widely prevailed.
[0003]
In general, a bus connection is difficult in a serial line. One example of
network topologies used to connect a plurality of communication devices is a ring
topology. With the ring topology, a communication device that serves as a relay
station connected between a source communication device, from which data is
transmitted, and a destination communication device, to which the data is
transmitted, needs to be active at any time to perform relay processing. For example,

Patent Literature 1 discloses a conventional technique that, in order to reduce the
amount of power consumed by such a communication device serving as the relay
station, causes the relay station to switch to bypass mode. In this bypass mode, a
protocol processing unit that operates in a logical layer, which is unnecessary for the
relay processing, is made inactive.
[Citation List]
[Patent Literature]
[0004]
[Patent Literature 1]
JP Patent Application Publication No. 2005-065216
[Summary of Invention]
[Technical Problem]
[0005]
However, the communication device pertaining to the above-described
conventional technique is structured such that, while a reset state of the logical layer
is maintained, received data is bypassed at the very last stage of a
transmission/reception processing unit that operates in a physical layer. This makes
it impossible to reduce the amount of power consumed by the transmission/reception
processing unit while performing the bypass processing, an unused line, and a
communication device connected to the unused line.
[0006]
In view of the above, the present invention aims to provide a
communication system, a communication device, an integrated circuit and a
communication method that enable reduction in the amount of power consumed by
an unused line (serial link) and a communication device connected to the unused
line.

[Solution to Problem]
[0007]
In order to achieve the above aim, a communication system of the present
invention includes a first communication device and a plurality of second
communication devices, the first communication device and the second
communication devices being connected with one another in a ring via a serial link.
In the system, the first communication device comprises: a first command
processing unit operable to (i) exchange a command packet and a response packet
with a partner communication device with which the first communication device
performs communication, and (ii) issue a standby packet to the partner
communication device when a downlink is not used for data packet transfer, the
downlink being a part of the serial link via which the command packet is transferred;
and a first data transfer unit operable to (i) transmit a write data packet to be written
via the downlink, and (ii) receive a read data packet to be read via an uplink, which
is a part of the serial link via which the response packet is transferred. Also in the
system, each of the second communication devices comprises: a second command
processing unit operable to (i) exchange the command packet and the response
packet with a partner communication device with which the own device performs
communication, and (ii) issue the standby packet to the partner communication
device when the uplink is not used for the data packet transfer; a second data transfer
unit operable to receive the write data packet via the downlink and transmit the read
data packet via the uplink; a packet relay unit operable to, in accordance with a
destination of a packet input to the own device, relay the input packet to one of the
first and second communication devices that immediately succeeds the own device
in the ring; and a standby control unit operable to, when the input packet relayed by
the packet relay unit of the own device is the standby packet, cause the own device
to switch to standby mode.

[0008]
A communication device of the present invention is connected to a serial
link and comprises: a command processing unit operable to (i) exchange a command
packet and a response packet with a partner communication device with which the
own device performs communication, and (ii) issue a standby packet for causing a
communication device connected to a part of the serial link that is not used for data
packet transfer to switch to standby mode; a data transfer unit operable to transfer (i)
a write data packet to be written via a downlink, which is a part of the serial link via
which the command packet is transferred, and (ii) a read data packet to be read via
an uplink, which is a part of the serial link via which the response packet is
transferred; a packet relay unit operable to, in accordance with a destination of a
packet input to the own device, relay the input packet to a communication device
that immediately succeeds the own device in the ring; and a standby control unit
operable to, when the input packet relayed by the packet relay unit is the standby
packet, cause the own device to switch to the standby mode.
[0009]
An integrated circuit of the present invention is used by a communication
device included in a plurality of communication devices that are connected with one
another in a ring via a serial link and that constitute a communication system. The
integrated circuit comprises: a command processing circuit operable to (i) exchange
a command packet and a response packet with a partner communication device with
which the own device performs communication, and (ii) issue a standby packet for
causing a communication device connected to a part of the serial link that is not used
for data packet transfer to switch to standby mode; a data transfer circuit operable to
transfer (i) a write data packet to be written via a downlink, which is a part of the
serial link via which the command packet is transferred, and (ii) a read data packet
to be read via an uplink, which is a part of the serial link via which the response
packet is transferred; a packet relay circuit operable to, in accordance with a

destination of a packet input to the own device, relay the input packet to a
communication device that immediately succeeds the own device in the ring; and a
standby control unit operable to, when the input packet relayed by the packet relay
circuit is the standby packet, cause the own device to switch to the standby mode.
[0010]
A communication method of the present invention is used in a
communication system including a first communication device and a plurality of
second communication devices, the first communication device and the second
communication devices being connected with one another in a ring via a serial link.
The communication method causes the first communication device to perform the
steps of: exchanging a command packet and a response packet with a partner
communication device with which the first communication device performs
communication, and issuing a standby packet to the partner communication device
when a downlink is not used for data packet transfer, the downlink being a part of
the serial link via which the command packet is transferred; and transmitting a write
data packet to be written via the downlink and receiving a read data packet to be
read via an uplink, which is a part of the serial link via which the response packet is
transferred. The communication method causes each of the second communication
devices to perform the steps of: exchanging the command packet and the response
packet with a partner communication device with which the own device performs
communication, and issuing the standby packet to the partner communication device
when the uplink is not used for the data packet transfer; receiving the write data
packet via the downlink and transmitting the read data packet via the uplink; in
accordance with a destination of a packet input to the own device, relaying the input
packet to one of the first and second communication devices that immediately
succeeds the own device in the ring; and causing the own device to switch to
standby mode when the input packet that has been relayed in the own device is the
standby packet.

[Advantageous Effects of Invention]
[0011]
According to the above communication system, communication device,
integrated circuit and communication method, a communication device connected to
a part of the serial link that is not used for data transfer switches to standby mode by
relaying a standby packet to an immediately succeeding communication device in
the ring. This makes it possible to reduce the amount of power consumed by this
unused part of the serial link and by a communication device connected to this
unused part of the serial link.
[0012]
The above communication system may be configured as follows: when the
downlink is used for the data packet transfer, the first command processing unit
issues a loopback packet to the partner communication device with which the first
communication device performs communication; when the uplink is used for the
data packet transfer, each of the second command processing units issues the
loopback packet to the partner communication device with which the own device
performs communication, and when the input packet relayed by the packet relay unit
of the own device is the loopback packet, each of the standby control units causes
the own device to switch to loopback mode.
[0013]
According to the above communication system, a communication device
connected to a part of the serial link that is used for data transfer switches to
loopback mode by relaying a loopback packet to an immediately succeeding
communication device in the ring. This makes it possible to reduce the amount of
power consumed by a communication device connected to this part of the serial link
used for data transfer. As a result of the communication device switching to the
loopback mode, the transfer path via which a signal input from the serial link is

transferred within the communication device is shortened. This reduces
communication latency experienced in the communication device.
[0014]
The above communication system may be configured as follows: each of
the first command processing unit and the second command processing units
performs transmission and reception of a flow control packet via the downlink and
the uplink; the flow control packet is considered as the standby packet when
transmitted via one of the downlink and the uplink that is not used for the data
packet transfer; and the flow control packet is considered as the loopback packet
when transmitted via one of the downlink and the uplink that is used for the data
packet transfer.
[0015]
According to the above communication system, a flow control packet,
which is exchanged to perform flow control, is used to cause a communication
device to switch to standby mode or loopback mode. For this reason, there is no
need to transmit/receive a special packet for causing a communication device to
switch to standby mode or loopback mode. Consequently, an increase in the
communication traffic within the serial link can be effectively suppressed.
[0016]
The above communication system may be configured as follows: the first
command processing unit issues a flow control request packet to the partner
communication device with which the first communication device performs
communication in order to write the write data packet, and in response to the flow
control request packet, each of the second command processing units issues a flow
control ready packet to the partner communication device with which the own
device performs communication; each of the second command processing units
issues the flow control request packet to the partner communication device with
which the own device performs communication in order for the read data packet to

be read, and in response to the flow control request packet, the first command
processing unit issues the flow control ready packet to the partner communication
device with which the first communication device performs communication; and the
flow control request packet is considered as the loopback packet, and the flow
control ready packet is considered as the standby packet.
[0017]
According to the above communication system, a flow control request
packet and a flow control ready packet, which are exchanged to perform flow
control, are used to cause a communication device to switch to standby mode or
loopback mode. For this reason, there is no need to transmit/receive a special packet
for causing a communication device to switch to standby mode or loopback mode.
Consequently, an increase in the communication traffic within the serial link can be
effectively suppressed.
[0018]
The above communication system may be configured as follows: when the
own device is a transmitter of the write data packet or the read data packet, each of
the first data transfer unit and the second data transfer units transmits a loopback
cancellation signal at an end of transfer of data having a predetermined size, the
loopback cancellation signal cancelling the loopback mode of one or more of the
first and second communication devices that have switched to the loopback mode;
and each of the standby control units cancels the loopback mode of the own device
in accordance with detection of the loopback cancellation signal.
[0019]
According to the above communication system, a serial link for data
communication can be used as-is for transmission of a loopback cancellation signal
that cancels loopback mode of a communication device. Consequently, there is no
need to provide a dedicated line for the loopback cancellation signal, and the cost
increase can be effectively suppressed.

[0020]
The above communication system may be configured as follows: when the
own device is a receiver of the write data packet or the read data packet, each of the
first command processing unit and the second command processing units transmits a
wakeup signal after the transfer of the data having the predetermined size, the
wakeup signal cancelling the standby mode of one or more of the first and second
communication devices that have switched to the standby mode; each of the first
command processing unit and the second command processing units issues a polling
packet to which each of the first and second communication devices writes a status
of the own device; each of the second communication devices further comprises a
wakeup detection unit operable to detect the wakeup signal; and each of the standby
control units cancels the standby mode of the own device in accordance with the
detection of the wakeup signal, and when the input packet relayed by the packet
relay unit of the own device is the polling packet, causes the own device to switch to
the standby mode.
[0021]
According to the above communication system, a serial link for data
communication can be used as-is for transmission of a wakeup signal that cancels
standby mode of a communication device. Consequently, there is no need to provide
a dedicated line for the wakeup signal, and the cost increase can be effectively
suppressed. In addition, as the above communication system uses the same polling
packet to both (i) notify the status of a communication device and (ii) cause the
communication device to switch to standby mode, the amount of power consumed
by the communication device can be reduced while preventing an increase in the
communication traffic.
[0022]
The above communication system may be configured as follows: when the
own device is a transmitter of the write data packet or the read data packet, each of

the first data transfer unit and the second data transfer units transmits a data burst
end signal at an end of transfer of data having a predetermined size; and each of the
standby control units cancels the loopback mode of the own device in accordance
with detection of the data burst end signal.
[0023]
According to the above communication system, a data bust end signal
transmitted at the end of transmission of data having a predetermined size is used to
cancel loopback mode of a communication device. Consequently, the loopback
mode of the communication device can be cancelled while suppressing an increase
in the communication traffic of the serial link.
[0024]
The above communication system may be configured as follows: when the
own device is a receiver of the write data packet or the read data packet, each of the
first command processing unit and the second command processing units (i)
transmits a wakeup signal after the transfer of the data having the predetermined size,
the wakeup signal cancelling the standby mode of one or more of the first and
second communication devices that have switched to the standby mode, and (ii)
after transmitting the wakeup signal, issues a status packet for notifying a result of
the reception of the write data packet or the read data packet; each of the second
communication devices further comprises a wakeup detection unit operable to detect
the wakeup signal; and each of the standby control units cancels the standby mode
of the own device in accordance with the detection of the wakeup signal, and when
the input packet relayed by the packet relay unit of the own device is the status
packet, causes the own device to switch to the standby mode.
[0025]
According to the above communication system, a serial link for data
communication can be used as-is for transmission of a wakeup signal that cancels
standby mode of a communication device. Consequently, there is no need to provide

a dedicated line for the wakeup signal, and the cost increase can be effectively
suppressed. In addition, as the above communication system uses the same polling
packet to both (i) notify the status of a communication device and (ii) cause the
communication device to switch to standby mode, the amount of power consumed
by the communication device can be reduced while preventing an increase in the
communication traffic.
[0026]
The above communication system may be configured as follows: each of
the standby control units causes the own device to switch to the standby mode when
the input packet relayed by the packet relay unit of the own device is the command
packet or the response packet transferred to a communication device other than the
own device.
[0027]
The above communication system can reduce the amount of power
consumed therein.
[Brief Description of Drawings]
[0028]
FIG. 1 shows a system structure of a communication system pertaining to an
embodiment of the present invention.
FIG. 2A shows one example of a packet format of a packet. FIG. 2B shows
one example of a payload of a command packet. FIG. 2C shows one example of a
payload of a response packet. FIG. 2D shows one example of a payload of a data
packet. These packets are exchanged between the communication devices shown in
FIG. 1.
FIG. 3A shows one example of a payload of a device list packet. FIG. 3B
shows one example of a payload of a flow control packet. FIG. 3C shows one
example of a payload of a polling packet. These packets are exchanged between the

communication devices shown in FIG. 1.
FIG. 4 shows one example of how the special symbols of the 8b/10b
scheme, which is used by the communication devices shown in FIG. 1, are allocated
to functions.
FIG. 5 is a structural diagram of the communication devices shown in FIG.
1.
FIG. 6 shows one example of connection between a master and a slave in
the communication system shown in FIG. 1.
FIG. 7 is a flowchart of the operations performed by the master
communication device included in the communication system shown in FIG. 1.
FIG. 8 is a flowchart of the operations performed by the non-master
communication devices included in the communication system shown in FIG. 1.
FIG. 9 is a flowchart of the operations for the packet relay processing
shown in FIG. 8.
FIG. 10 shows an initialization sequence and an operational sequence
relating to command processing for an entirety of the communication system shown
in FIG. 1.
FIG. 11 shows an operational sequence for an entirety of the
communication system shown in FIG. 1, the operational sequence relating to flow
control, data transfer and polling and being performed upon data writing.
FIG. 12 shows an operational sequence for an entirety of the
communication system shown in FIG. 1, the operational sequence relating to flow
control, data transfer and polling and being performed upon data writing.
FIG. 13 shows an operational sequence for an entirety of the
communication system shown in FIG. 1, the operational sequence relating to flow
control, data transfer and polling and being performed upon data reading.
FIG. 14 shows an operational sequence for an entirety of the
communication system shown in FIG. 1, the operational sequence relating to flow

control, data transfer and polling and being performed upon data reading.
FIG. 15 shows one example of how the special symbols of the 8b/10b
scheme, which is used by communication devices in a communication system
pertaining to the first modification example, are allocated to functions.
FIG. 16 shows an operational sequence for an entirety of the
communication system pertaining to the first modification example, the operational
sequence relating to flow control, data transfer and reception status notification and
being performed upon data writing.
FIG. 17 shows an operational sequence for an entirety of the
communication system pertaining to the first modification example, the operational
sequence relating to flow control, data transfer and reception status notification and
being performed upon data writing.
FIG. 18 shows an operational sequence for an entirety of the
communication system pertaining to the first modification example, the operational
sequence relating to flow control, data transfer and reception status notification and
being performed upon data reading.
FIG. 19 shows an operational sequence for an entirety of the
communication system pertaining to the first modification example, the operational
sequence relating to flow control, data transfer and reception status notification and
being performed upon data reading.
FIG. 20 shows a system structure of a communication system pertaining to
the second modification example.
[Description of Embodiment]
[0029]
[Embodiment]
The following describes an embodiment of the present invention with
reference to the drawings.

[0030]

FIG. 1 shows a system structure of a communication system pertaining to an
embodiment of the present invention.
[0031]
The communication system shown in FIG. 1 includes four communication
devices 100a to lOOd. It should be noted here that "0" to "4" shown in FIG. 1 and
the like are device IDs assigned to the communication devices 100a to lOOd,
respectively.
[0032]
The communication devices 100a to lOOd include protocol processing units
101a to lOld and transmission/reception processing units 102a to 102d, respectively.
In the present embodiment, the communication device 100a is the master
communication device (the first communication device). Accordingly, the
communication devices 100b to lOOd are non-master communication devices (the
second communication devices). The protocol processing units 101a to lOld of the
communication devices 100a to lOOd are blocks that operate in a logical layer. The
transmission/reception processing units 102a to 102d of the communication devices
100a to lOOd are blocks that operate in a physical layer, and respectively include
serial transmission subunits (Tx) 103 a to 103 d, serial reception subunits (Rx) 104a
to 104d, and the like. With the serial transmission subunits 103 a to 103 d and the
serial reception subunits 104a to 104d in the communication devices 100a to lOOd
connected sequentially in a ring via a serial link 105, a communication system based
on a ring topology is established. Note that in the serial link 105, a serial signal is
transmitted in a direction indicated as "Forward direction" in FIG. 1.
[0033]
Specifics of the protocol processing units 101a to 101d and the
transmission/reception processing units 102a to 102d are described later. An

overview of these units is described below.
[0034]
Each of the protocol processing units 101a to lOld interprets a reception
packet received from another communication device based on a predetermined
protocol, and generates a transmission packet to be output to another communication
device. Each of the transmission/reception processing units 102a to 102d receives a
serial signal output from a communication device that immediately precedes the
own device in the ring via the serial link 105, converts the serial signal into a
reception packet, and outputs the reception packet to a corresponding one of the
protocol processing units 101a to 101 d. Also, each of the transmission/reception
processing units 102a to 102d converts a transmission packet input from a
corresponding one of the protocol processing units 101a to lOld into a serial signal,
and transmits the serial signal to a communication device that immediately succeeds
the own device in the ring via the serial link 105.
[0035]
Each packet exchanged between the communication devices 100a to lOOd
includes destination information. In a case where the destination information
included in a reception packet indicates another communication device different
from the own device, each of the protocol processing units 101a to 10Id performs
relay processing, i.e., outputs the reception packet as a transmission packet to a
communication device that immediately succeeds the own device in the ring. As
described above, with a communication device between a source communication
device and a destination communication device serving as a relay station, a packet
exchange between a master communication device and a given slave communication
device can be achieved.
[0036]

The following describes a packet format of a packet exchanged between the

communication devices 100a to lOOd shown in FIG. 1, with reference to FIGs. 2A to
2D.
[0037]
FIG. 2A shows one example of a packet format of a packet exchanged
between the communication devices 100a to lOOd shown in FIG. 1. As shown in
FIG. 2A, the packet format includes a header (HEADER) 201 and a payload
(PAYLOAD) 202.
[0038]
The header 201 includes a packet type (TYPE) 211, a destination ID (DID)
212 as the aforementioned destination information, a source ID (SID) 213, and a
transaction ID (TID) 214.
[0039]
The packet type 211 shows a type of the packet. Examples of a packet
include: a command packet (CMD) issued by a master as a request for starting data
transfer; a response packet (RES) issued by a slave, to which the command packet is
addressed, as a response to the command packet; a data packet (DAT) including
actual data transferred between the master and the slave; and a message packet that
is used to notify a status to another communication device via the serial link.
[0040]
Each of the destination ID 212 and the source ID 213 specifies, for example,
one of the device IDs assigned to the communication devices. The total number of
communication devices that can be connected in a ring is restricted by the field
lengths of the destination ID 212 and the source ID 213. Provided the destination ID
212 and the source ID 213 shown in FIG. 2A each have a field length of four bits,
any value in a range of "0" to "15" can be used as a device ID. In the present
embodiment, the device ID of the master communication device 100a is "0", and a
value "15" is used as a special device ID for performing broadcasting that places all
of the communication devices included in the communication system as destination

communication devices. Also, in the present embodiment, unique values are
assigned as device IDs of the non-master communication devices 100b to lOOd at
the time of initialization.
[0041]
In a case where a plurality of commands are issued between a pair of a
master and a slave (command queue) and multiple data transfers are performed
simultaneously (transactions), the transactions cannot be identified from the
destination ID 212 and the source ID 213. The transaction ID 214 is required for this
reason.
[0042]
The payload 202 has different fields depending on the packet type 211. The
following describes an overview of payloads of a command packet, a response
packet and a data packet.
[0043]
FIG. 2B shows one example of a payload of a command packet. As shown
in FIG. 2B, a payload of a command packet includes the following: an R/W flag 221
indicating a data transfer type, which is one of reading (Read) and writing (Write) of
the data transfer; a data transfer start address (Addr) 222; a data transfer size (Size)
223; and the like. FIG. 2C shows one example of a payload of a response packet. As
shown in FIG. 2C, a payload of a response packet includes a NACK (Negative
Acknowledge) flag 231 indicating whether a command has been received
successfully or not, an error code (Error) 232 associated with a command error, and
the like. FIG. 2D shows one example of a payload of a data packet. As shown in FIG.
2D, a payload of a data packet includes actual data for data transfer (Data) 241 and
the like.
[0044]

With reference to FIGs. 3A to 3C, the following describes payload formats

of a device list packet, a flow control packet and a polling packet that are exchanged
between the communication devices 100a to lOOd shown in FIG. 1.
[0045]
FIG. 3 A shows one example of a payload of a device list packet exchanged
between the communication devices 100a to lOOd shown in FIG. 1. The device list
packet is a packet defined as a message packet by the packet type 211. The device
list packet is used by the master communication device to (i) assign a unique device
ID to each of the non-master communication devices connected to the serial link,
and (ii) obtain the number of such non-master communication devices connected to
the serial link. As shown in FIG. 3 A, the payload of the device list packet includes a
device ID field (Device ID) 261 to which each communication device writes its own
device ID.
[0046]
FIG. 3B shows one example of a payload of a flow control packet
exchanged between the communication devices 100a to lOOd shown in FIG. 1. The
flow control packet is used for performing flow control when a predetermined flow
control condition is satisfied. In the present embodiment, the flow control packet is
further used as a loopback packet for causing a communication device to switch to
loopback mode, or as a standby packet for causing a communication device to
switch to standby mode.
[0047]
As shown in FIG. 3B, the payload of the flow control packet includes a flow
control size (FCSize) 271, to which the number of data packets that can be
transferred is written, and a loopback flag (Lpbk) 272. Here, data having the data
transfer size 223 specified in the command packet is transferred after being divided
into a plurality of data packets that each have a predetermined block size. The flow
control is performed in units of the flow control size 271 encompassing the plurality
of data packets. The loopback flag 272 controls whether or not to cause a

communication device between a source communication device that transmits the
flow control packet and a destination communication device that receives the flow
control packet to switch to loopback mode. The flow control packet is treated as a
loopback packet if the loopback flag 272 has been set (flag on), and as a standby
packet if the loopback flag 272 has not been set (flag off).
[0048]
FIG. 3C shows one example of a payload of a polling packet exchanged
between the communication devices shown in FIG. 1. The polling packet is used for
a status notification, i.e., for indicating whether or not a communication device can
perform communication. In the present embodiment, the polling packet is further
used as a standby packet that causes a communication device to switch to standby
mode. Note that the polling packet is issued after data transfer of the number of data
packets specified by the flow control size 271 of the flow control packet.
[0049]
As shown in FIG. 3C, a payload of a polling packet includes a completion
flag (CPL) 281 and a status field (Status) 282. The completion flag 281 is a flag
indicating that transfer of data having the data transfer size 223 included in the
command packet has been completed. The completion flag 281 is not set in a polling
packet issued before completion of transfer of data having the data transfer size 223
included in the command packet (flag off), and is set in a polling packet issued after
completion of transfer of such data (flag on). The status field 282 is a field used by a
communication device to notify, for example, a status indicating whether or not it
can perform communication.
[0050]
It should be noted that the above-described packet formats may have
different structures depending on the scale of the system, protocols, and the like. The
packet formats may be modified as necessary by, for example, changing the field
lengths, deleting/adding fields, etc.

[0051]

Described below with reference to FIG. 4 are special symbols of the 8b/10b
scheme used by the communication devices 100a to lOOd shown in FIG. 1. FIG. 4
shows one example of how the special symbols of the 8b/10b scheme, which is used
by the communication devices 100a to lOOd shown in FIG. 1, are allocated to
functions.
[0052]
By taking advantage of redundancy arising from conversion of 8-bit data
into 10-bit data, the 8b/10b scheme can make use of special K symbols for control,
in addition to D symbols that express standard 8-bit (byte) data. In the 8b/10b
scheme, there are 12 types of K symbols that can be used. In FIG. 4, the "K code",
"Symbol", "Function", "Original data (hexadecimal)", "Current RD -, "Current RD
+", are illustrated in correspondence. The "Original data (hexadecimal)" denotes
8-bit data before coding based on the 8b/10b scheme. The "Current RD -" and the
"Current RD +" each denote 10-bit data after coding based on the 8b/10b scheme.
Since a method of converting the pre-coding 8-bit (byte) data into 10-bit data of
either "Current RD -" or "Current RD +" is known, the details thereof are omitted.
[0053]
Referring to FIG. 4, SOP (Start of Packet), EOP (End of Packet), RFLB
(Return from Loopback), COM (Comma), SYNC (Synchronization) and EOP (End
of Packet) are allocated to some of the functions.
[0054]
The SOP and EOP indicate, and are appended to, the start and end of a
packet, respectively. They are used to acknowledge positions that delimit packets.
The RFLB (loopback cancellation code) is used to cancel the loopback mode of a
communication device. The SYNC (synchronous code) is used to establish
synchronization between the communication devices. The COM is a unique signal

pattern that cannot be generated by any combination of two other symbols within the
serial data made up of symbol sequences according to the 8b/10b scheme. Hence,
the COM is used as a delimiter for correctly acknowledging positions that delimit
symbol sequences from the serial data and for converting the serial data into parallel
data. It should be noted that special symbols other than the COM may be used not
only independently but also as a symbol set combined with the COM which is a
delimiter.
[0055]

With reference to FIG. 5, the following describes the structure of the
communication devices 100a to lOOd shown in FIG. 1. FIG. 5 is a structural diagram
of the communication devices 100a to lOOd (a communication device 300) shown in
FIG. 1.
[0056]
The communication device 300 is composed of a transmission/reception
processing unit 301, a protocol processing unit 302, and a clock source 303. The
transmission/reception processing unit 301 is the equivalent of each of the
transmission/reception processing units 102a to 102d shown in FIG. 1, and the
protocol processing unit 302 is the equivalent of each of the protocol processing
units 101a to 10Id shown in FIG. 1. The clock source 303 generates a reference
clock and is constituted by, for example, a voltage-controlled crystal oscillator
(VCXO) and the like.
[0057]
[Transmission/Reception Processing Unit]
The transmission/reception processing unit 301 includes a serial reception
subunit (Rx) 311, a decode subunit 312, a code subunit 313, a loopback selector 314,
a serial transmission subunit (Tx) 315, a wakeup detection subunit 316, and a
standby control subunit 317. The serial reception subunit 311 is the equivalent of

device in the ring. Accordingly, the CDR circuit 332 synchronizes the phase of the
data clock with phases of edges of the serial reception data to remove jitter.
[0061]
Here, in a case where "0" or "1" is consecutively input as the serial
reception data, the CDR circuit 332 cannot acknowledge the edges of the serial
reception data. To avoid this problem, the code subunit 313 codes bit strings of a
transmission packet using the 8b/10b scheme, so that a transition from "0" to "1" or
vice versa occurs without fail in a predetermined time period. As a result, generally,
only three bits of "0" or "1" are consecutively input (up to five bits). This enables
the CDR circuit 332 to acknowledge the edges of the serial reception data and to
properly extract information on the edges of the data clock from the serial reception
data.
[0062]
The decode subunit 312 decodes each 10-bit symbol data constituting the
parallel reception data into the original 8-bit (byte) data according to a decode table
for the 8b/10b scheme, and outputs each 8-bit (byte) data to the protocol processing
unit 302 as a reception packet.
[0063]
The code subunit 313 codes each 8-bit (byte) data constituting the
transmission packet input from the protocol processing unit 302 into 10-bit symbol
data using the 8b/10b scheme, and outputs the parallel transmission data constituted
by each 10-bit symbol data to the loopback selector 314.
[0064]
Under control of the standby control subunit 317, the loopback selector 314
selectively outputs one of the parallel transmission data and the parallel loopback
data to the serial transmission subunit 315.
[0065]
The serial transmission subunit 315 includes a parallel-to-serial converter

(P/S converter) 351 and a serial driver 352. The P/S converter 351 converts the
parallel transmission data or the parallel loopback data having a symbol length
conforming to the 8b/10b scheme (a bit width of 10) into the serial transmission data
or the serial loopback data. The serial driver 352 generates a serial signal from the
serial transmission data or the serial loopback data provided by the P/S converter
351, and outputs the serial signal to the serial link 105.
[0066]
The wakeup detection subunit 316 detects, from the signal state of the serial
link 105, that an immediately preceding communication device in the ring switched
from an electrically idle state, where it did not drive the serial link (e.g., high
impedance), to a state where it drives a wakeup signal. The wakeup signal is used to
restore other communication devices to a state where they can transfer packets. Here,
it is necessary to transmit a wakeup signal before the data clock is generated during
the initialization sequence. Thus, data transitioned using a low-speed reference clock,
data fixed as "Low" for a predetermined time period, data fixed as "High" for a
predetermined time period, or the like, is used as a wakeup signal.
[0067]
The standby control subunit 317 performs control for causing each of the
serial reception subunit 311, the decode subunit 312, the code subunit 313 and the
serial transmission subunit 315 included in the transmission/reception processing
unit 301, as well as an entirety of the protocol processing unit 302, to switch
between an active state (running state) and a standby state (stopped state). Note, the
serial reception subunit 311 and the decode subunit 312 relate to reception
processing, and the code subunit 313 and the serial transmission subunit 315 relate
to transmission processing.
[0068]
Also, the standby control subunit 317 causes each communication device to
(i) switch to, for example, loopback mode that is used to measure the BER (Bit Error

Rate) during a reception test and identify the cause of occurrence of failures, and (ii)
cancel the loopback mode. In the present embodiment, the loopback mode is used
not only to measure the BER during the reception test and identify the cause of
occurrence of failures, but also for the purpose of saving power of each
communication device that relays data packets.
[0069]
[Protocol Processing Unit]
The protocol processing unit 302 includes a packet relay subunit 371, a
command processing subunit 372, and a data transfer subunit 373.
[0070]
Based on the destination ID 212 of the reception packet output from the
decode subunit 312, the packet relay subunit 371 makes a judgment on the
destination of the reception packet. In a case where the destination ID 212 of the
reception packet shows another communication device, the packet relay subunit 371
performs the packet relay processing by outputting the reception packet to the code
subunit 313 as a transmission packet. On the other hand, in a case where the
destination ID 212 of the reception packet shows the own device, the packet relay
subunit 371 outputs the reception packet to one of the command processing subunit
372 and the data transfer subunit 373 depending on the packet type 211 of the
reception packet.
[0071]
The command processing subunit 372 performs various types of processing,
such as processing involving exchange of a command packet and a response packet.
The following describes the processing involving exchange of a command packet
and a response packet. Other processing will be explained later with reference to
operational flows and operational sequences.
[0072]
The command processing subunit 372 of a master issues a command packet

to a communication device with which the master is communicating (a slave), waits
for reception of a response packet from the slave, and confirms establishment of
handshaking upon properly receiving the response packet.
[0073]
The command processing subunit 372 of the communication device with
which the master is communicating (the slave) receives the command packet from
the master, and if the data specified by the command packet can be transferred to the
master, establishes handshaking by issuing the response packet in which the NACK
flag 231 is set to ACK (Acknowledge).
[0074]
Depending on the result of handshaking whose establishment is determined
by the command processing subunit 372, the data transfer subunit 373 either reads
(Read) or writes (Write) the data packet.
[0075]
Note that in the present embodiment, the communication device 300 is said
to be in power-saving standby mode when the serial reception subunit 311, the
decode subunit 312, the code subunit 313 and the serial transmission subunit 315
included in the transmission/reception processing unit 301, as well as an entirety of
the protocol processing unit 302, are in the standby state (stopped state). Also, the
communication device 300 is said to be in power-saving loopback mode when the
output from the loopback selector 314 is parallel loopback data and the decode
subunit 312 and the code subunit 313 included in the transmission/reception
processing unit 301, as well as an entirety of the protocol processing unit 302, are in
the standby state (stopped state). It goes without saying that blocks to be placed in
the standby state during the standby mode and the loopback mode may be modified
as necessary in accordance with the structure, etc. of each communication device.
[0076]

FIG. 6 shows one example of connection between a master and a slave in
the communication system shown in FIG. 1.
[0077]
When performing data transfer with the communication device 100c serving
as a slave, the path from the master communication device 100a via the
communication device 100b to the communication device 100c is referred to as a
"downlink" DL. Also, the path from the communication device 100c via the
communication device 1 OOd to the communication device 100a is referred to as an
"uplink" UL.
[0078]
The command processing subunit 372 of the communication device 100a
issues a command packet. The issued command packet is transmitted to the
command processing subunit 372 of the communication device 100c via the
downlink DL. At this time, the command packet is subjected to relay processing in
the packet relay subunit 371 of the communication device 100b. Upon receiving the
command packet, the command processing subunit 372 of the communication
device 100c issues a response packet. The issued response packet is transmitted to
the command processing subunit 372 of the communication device 100a via the
uplink UL. At this time, the response packet is subjected to relay processing in the
packet relay subunit 371 of the communication device lOOd.
[0079]
In a case where the data transfer type specified by the R/W flag 221 of the
command packet indicates data writing, the data transfer subunit 373 of the
communication device 100a issues a data packet, which is then transmitted to the
communication device 100c via the downlink DL. At this time, the data packet is
subjected to relay processing in the communication device 100b. On the other hand,
in a case where the data transfer type specified by the R/W flag 221 of the command
packet indicates data reading, the data transfer subunit 373 of the communication

device 100c issues a data packet, which is then transmitted to the communication
device 100a via the uplink UL. At this time, the data packet is subjected to relay
processing in the communication device lOOd.
[0080]
As set forth above, with relay stations (in the case of FIG. 6, the
communication devices 100b and lOOd) between the master and the slave relaying
packets, the communication system of the present embodiment can perform data
transfer using the same protocol as the protocol used in performing a point-to-point
connection.
[0081]

A description is now given of the operations of the communication system
shown in FIG. 1.
[0082]
[Operations of Communication Devices]
With reference to FIGs. 7 and 8, the following describes the operations of
the master communication device 100a and the operations of the non-master
communication devices 100b to lOOd. Note that FIG. 7 is a flowchart of the
operations of the master communication device 100a shown in FIG. 1, and FIG. 8 is
a flowchart of the operations of the non-master communication devices 100b to
lOOd shown in FIG. 1. Below, for the purpose of simplicity, the flowcharts of FIGs.
7 and 8 will be collectively explained where appropriate.
[0083]
The master communication device 100a performs wakeup processing (step
S101). Each of the non-master communication devices 100b to lOOd also performs
wakeup processing (step S201). It should be noted that the specifics of the wakeup
processing performed by each of the communication devices 100a to lOOd are
omitted here since they will be described later with reference to step S301 in FIG.

10.
[0084]
After the wakeup processing of steps S101 and S201 is completed, the
master communication device 100a performs device list processing (step SI02), and
each of the non-master communication devices 100b to lOOd also performs device
list processing (step S202). It should be noted that the specifics of the device list
processing performed by each of the communication devices 100a to lOOd are
omitted here since they will be described later with reference to step S302 in FIG.
10.
[0085]
After the above initialization sequence (steps S101 and SI02) is completed,
the master communication device 100a performs command processing, such as
issuing of a command (step SI 03).
[0086]
After the above initialization sequence (steps S201 and S202) is completed,
each of the non-master communication devices 100b to lOOd waits for reception of a
packet. When each of the communication devices 100b to lOOd has received a
packet, the packet relay subunit 371 thereof makes a judgment on the destination of
the reception packet based on the destination ID 212 of the reception packet (step
S203).
[0087]
When the reception packet is addressed to another communication device
(S203: Addressed to another device), each communication device that has received
this packet addressed to another communication device performs packet relay
processing shown in FIG. 9 (step S210).
[0088]
On the other hand, when the reception packet is addressed to the own device
(S203: Addressed to own device), each communication device that has received this

packet addressed to the own device (i.e., the slave communication device) performs
the command processing (step S204).
[0089]
Described below is an overview of the command processing performed by
the master communication device 100a (step SI03) and the command processing
performed by the slave communication device (step S204).
[0090]
The command processing subunit 372 of the master communication device
100a issues a command packet for initiating the data transfer, and waits for reception
of a response packet from the slave communication device specified by the
destination ID212 of the command packet. The command processing subunit 372 of
the slave communication device receives the command packet from the master
communication device 100a, and if the data transfer specified by the command
packet can be performed with the communication device 100a, establishes
handshaking for the command/response by issuing the response packet, in which the
NACK flag 231 is set to ACK. The command processing subunit 372 of the
communication device 100a confirms that the handshaking for the
command/response has been established upon properly receiving the response
packet from the slave communication device.
[0091]
After the handshaking for the command/response has been established in
the above command processing (steps SI03 and S204), the master communication
device 100a performs flow control (step SI04), and the slave communication device
also performs flow control (step S205). In the flow control, the master
communication device 100a and the slave communication device adjust the amount
of each data packet to be transferred and the timing of transferring each data packet,
while confirming each other's buffer state.
[0092]

Described below is an overview of the flow control performed by the master
communication device 100a (step SI04) and the flow control performed by the slave
communication device (step S205).
[0093]
The command processing subunit 372 of the master communication device
100a and the command processing subunit 372 of the slave communication device
each transmit/receive a wakeup signal, and thereafter transmit/receive synchronous
code.
[0094]
Then, the command processing subunit 372 of the master communication
device 100a issues a flow control packet in which (i) the device ID of the slave
communication device is set as the destination ID 212 and (ii) the flow control size
271 contains the number of data packets that can be transferred. The command
processing subunit 372 of the slave communication device receives the flow control
packet from the master communication device 100a, and when the data transfer
indicated by the flow control size 271 of the flow control packet becomes possible,
issues a flow control packet in which the device ID of the master communication
device 100a is set as the destination ID 212. The command processing subunit 372
of the master communication device 100a receives the flow control packet from the
i slave communication device, and confirms that handshaking for the flow control has
been established upon properly receiving the flow control packet.
[0095]
At this time, the data transfer is performed by using one of the downlink and
the uplink. Each of the command processing subunit 372 of the master
communication device 100a and the command processing subunit 372 of the slave
communication device sets a loopback flag 272 in a flow control packet transferred
via a partial link (one of the downlink and the uplink) that is used for the data
transfer (flag on), and does not set a loopback flag 272 in a flow control packet

transferred via a partial link (one of the downlink and the uplink) that is not used for
the data transfer via (flag off).
[0096]
That is to say, when the data transfer type specified by the R/W flag 221
indicates data writing, the command processing subunit 372 of the master
communication device 100a sets the loopback flag 272 in a flow control packet,
whereas the command processing subunit 372 of the slave communication device
does not set the loopback flag 272 in a flow control packet. On the other hand, when
the data transfer type indicates data reading, the command processing subunit 372 of
the slave communication device sets the loopback flag 272 in a flow control packet,
whereas the command processing subunit 372 of the master communication device
100a does not set the loopback flag 272 in a flow control packet.
[0097]
After the handshaking for the flow control has been established in the above
flow control (steps SI04 and S205), the data transfer subunit 373 of the master
communication device 100a performs data transfer (step SI05), and the data transfer
subunit 373 of the slave communication device also performs data transfer (step
S206). At the end of the data transfer, the data transfer subunit 373 of the
communication device that issues the data packet issues loopback cancellation code
(RFLB).
[0098]
In the data communication between the master communication device 100a
and the slave communication device, when the data transfer type indicates data
writing, the data transfer subunit 373 of the master communication device 100a
issues a data packet. This data packet is transmitted as a serial signal via the
downlink, and the data transfer subunit 373 of the slave communication device
receives this data packet. At this time, the uplink and each communication device
connected to the uplink are unused, i.e., not used for the transfer of the data packet.

[0099]
On the other hand, when the data transfer type indicates data reading, the
data transfer subunit 373 of the slave communication device issues a data packet.
This data packet is transmitted as a serial signal via the uplink. Then, the data
transfer subunit 373 of the master communication device 100a receives the data
packet. At this time, the downlink and each communication device connected to the
downlink are unused, i.e., not used for the transfer of the data packet.
[0100]
After completion of data transfer of the number of data packets specified by
the flow control size 271 of the flow control packet (steps SI05 and S206), the
master communication device 100a performs polling processing (step SI06), and the
slave communication device also performs polling processing (step S207).
[0101]
The following describes an overview of the polling processing performed
by the master communication device 100a (step SI06) and the polling processing
performed by the slave communication device (step S207). The following also
describes processing performed with respect to a polling packet by each
communication device that serves as a relay station.
[0102]
The command processing subunit 372 of the master communication device
100a issues a polling packet in which the device ID of the slave communication
device is set as the destination ID 212. Note that the command processing subunit
372 of the master communication device 100a does not set the completion flag 281
before completion of transfer of data having the data transfer size 223 of the
command packet (flag off), and sets the completion flag 281 upon completion of
transfer of such data (flag on).
[0103]
The packet relay subunit 371 of each communication device serving as a

relay station (i) judges that the polling packet is addressed to another communication
device, (ii) additionally writes, to the status field 282 of the polling packet, a status
indicating, for example, whether or not the own device can perform communication,
and (iii) outputs the polling packet to an immediately succeeding communication
device in the ring.
[0104]
The packet relay subunit 371 of the slave communication device judges that
the polling packet is addressed to the own device, and outputs the polling packet to
the command processing subunit 372.
[0105]
The command processing subunit 372 of the slave communication device
issues a polling packet in which (i) the device ID of the master communication
device 100a is set as the destination ID 212 and (ii) the content of the status field
282 of the input polling packet, as well as a status indicating, for example, whether
or not the own device can perform communication, are written to the status field 282.
Note that the command processing subunit 372 of the slave communication device
does not set the completion flag 281 before completion of transfer of data having the
data transfer size 223 of the command packet (flag off), and sets the completion flag
281 upon completion of transfer of such data (flag on).
[0106]
The packet relay subunit 371 of each communication device serving as a
relay station (i) judges that the polling packet is addressed to another communication
device, (ii) additionally writes, to the status field 282 of the polling packet, a status
indicating, for example, whether or not the own device can perform communication,
and (iii) outputs the polling packet to an immediately succeeding communication
device in the ring.
[0107]
The packet relay subunit 371 of the master communication device 100a

judges that the polling packet is addressed to the own device, and outputs the polling
packet to the command processing subunit 372. Then, the command processing
subunit 372 obtains, from the status field 282 of the input polling packet, the statuses
of all of the communication devices that are connected to the serial link 105,
excluding the own device.
[0108]
Note that the command processing subunit 372 of the master
communication device 100a uses the obtained statuses of all of the communication
devices, excluding the own device, to select a slave to which the own device issues
the next command.
[0109]
Until the transfer of the data having the data transfer size 223 of the
command packet is completed (SI07: No), the master communication device 100a
repeats the processing of steps SI04 through SI06. Once the transfer of the data
having the data transfer size 223 of the command packet has been completed (SI 07:
Yes), the processing moves to step SI08. If the communication is to be continued
with issuance of a command packet (SI08: Yes), then the master communication
device 100a performs the processing of step SI03 onward, thus continuing the
communication. If the communication is not to be continued (SI08: No), then the
master communication device 100a completes the communication.
[0110]
Until the transfer of the data having the data transfer size 223 of the
command packet is completed (S208: No), the slave communication device repeats
the processing of steps S205 through S207. Once the transfer of the data having the
data transfer size 223 of the command packet has been completed (S208: Yes), the
processing moves to step S209. If the communication is to be continued with
reception of a command packet (S209: Yes), then the slave communication device
performs the processing of step S203 onward, thus continuing the communication. If

the communication is not to be continued (S209: No), then the slave communication
device completes the communication.
[0111]
(Packet Relay Processing)
FIG. 9 is a flowchart of the operations for the packet relay processing (step
S210) shown in FIG. 8.
[0112]
The packet relay subunit 371 of each communication device that has
received a packet addressed to another communication device judges whether the
reception packet is a standby packet or a loopback packet (step S251).
[0113]
The standby packet is a packet with an attribute that causes another
communication device to switch to standby mode. After the protocol processing unit
302 has received the standby packet, the standby control subunit 317 causes the
serial reception subunit 311 and the decode subunit 312 of the
transmission/reception processing unit 301, which relate to reception processing, to
switch to a standby state. After the standby packet is transmitted, the standby control
subunit 317 causes the code subunit 313 and the serial transmission subunit 315,
which relate to transmission processing, to a standby state. Therefore, when the
protocol processing unit 302 performs relay processing on the standby packet, any
circuit relating to the above-described transmission/reception is placed in a standby
state, and since the transmission/reception is stopped, an entirety of the
communication device, including the protocol processing unit 302, is placed in
standby mode. In the present embodiment, all the packets that are issued to another
communication device after completion of the initialization sequence, excluding a
flow control packet that is issued to another communication device and that has the
loopback flag 272 set therein and a data packet, are considered as standby packets.
[0114]

A loopback packet is a packet with an attribute that causes each
communication device connected to a partial link (one of the uplink and the
downlink) that is used for data transfer to switch to loopback mode. After the
protocol processing unit 302 has performed relay processing on the loopback packet,
the standby control subunit 317 causes the loopback selector 314 to switch to
loopback mode where it outputs parallel loopback data to the serial transmission
subunit 315. In the present embodiment, a flow control packet that is addressed to
another communication device and that has the loopback flag 272 set therein is
considered as a loopback packet.
[0115]
When the reception packet is a standby packet (S251: Standby), the packet
relay subunit 371 of each communication device relays the reception packet as a
transmission packet to an immediately succeeding communication device in the ring.
In each communication device that has relayed the standby packet, the standby
control subunit 317 causes the own device to switch to the power-saving standby
mode by placing the serial reception subunit 311, the decode subunit 312, the code
subunit 313 and the serial transmission subunit 315 included in the
transmission/reception processing unit 301, as well as an entirety of the protocol
processing unit 302, in a standby state (step S255). After switching to the standby
mode, each communication device waits for detection of a wakeup signal. When the
wakeup detection unit 316 has detected the wakeup signal, the standby control
subunit 317 cancels the standby mode of the own device by placing the serial
reception subunit 311, the decode subunit 312, the code subunit 313 and the serial
transmission subunit 315 included in the transmission/reception processing unit 301,
as well as an entirety of the protocol processing unit 302, in an active state (step
S256).
[0116]
When the reception packet is a loopback packet (S251: Loopback), the

packet relay subunit 371 of each communication device relays the reception packet
as a transmission packet to an immediately succeeding communication device in the
ring. In each communication device that has relayed the loopback packet, the
standby control subunit 317 causes the own device to switch to the power-saving
loopback mode by (i) switching the output from the loopback selector 314 from
parallel transmission data to parallel loopback data, and (ii) placing the decode
subunit 312 and the code subunit 313 included in the transmission/reception
processing unit 301, as well as an entirety of the protocol processing unit 302, in a
standby state (step S252). Note that after switching to the loopback mode, each
communication device transfers the input from the serial link 105 to the serial
reception subunit 311, the standby control subunit 317, the loopback selector 314
and the serial transmission subunit 315, in this order, and thereafter outputs the input
to the serial link 105, until the loopback mode is cancelled.
[0117]
After switching to the loopback mode, each communication device waits for
cancellation of the loopback mode. When detecting the loopback cancellation code
(RFLB) from the parallel loopback data, the standby control subunit 317 switches
the output from the loopback selector 314 from the parallel loopback data to parallel
transmission data, and cancels the loopback mode of the own device by placing the
decode subunit 312 and the code subunit 313 included in the transmission/reception
processing unit 301, as well as an entirety of the protocol processing unit 302, in an
active state (step S253).
[0118]
After the loopback mode has been cancelled, the packet relay subunit 371 of
each communication device waits for reception of a polling packet. The packet relay
subunit 371 additionally writes, to the status field 282 of the polling packet, a status
indicating whether or not the own device can perform communication, and relays
the polling packet, to which the status of the own device has been additionally

written, to an immediately succeeding communication device in the ring (step S254).
In each communication device that has relayed the polling packet, the standby
control subunit 317 causes the own device to switch to standby mode by placing the
serial reception subunit 311, the decode subunit 312, the code subunit 313 and the
serial transmission subunit 315 included in the transmission/reception processing
unit 301, as well as an entirety of the protocol processing unit 302, in a standby state
(step S255). After switching to the standby mode, each communication device waits
for detection of a wakeup signal. When the wakeup detection unit 316 has detected
the wakeup signal, the standby control subunit 317 cancels the standby mode of the
own device by placing the serial reception subunit 311, the decode subunit 312, the
code subunit 313 and the serial transmission subunit 315 included in the
transmission/reception processing unit 301, as well as an entirety of the protocol
processing unit 302, in an active state (step S256).
[0119]
[Operational Sequences for Entirety of Communication System]
(Initialization Sequence and Operational Sequence Relating to Command
Processing)
FIG. 10 shows the initialization sequence and an operational sequence
relating to command processing for an entirety of the communication system shown
in FIG. 1.
[0120]
In the communication device 100a, the command processing subunit 372
starts transmission of a wakeup signal from the serial driver 352 via the serial link
105, which is in an electrically idle state that occurs immediately after the power is
turned on, and the PLL circuit 336 starts generating a data clock. Each of the
communication devices 100b to lOOd is in a state where it is waiting for the wakeup
signal. In the communication devices 100b to lOOd, the wakeup detection subunits
316 detect the wakeup signal from the signal state of the serial link 105 in order, the

serial drivers 352 output the wakeup signal to the serial link 105 thereby relaying the
wakeup signal, and the PLL circuits 336 start generating a data clock. In the
communication device 100a, the wakeup detection subunit 316 detects the looped
wakeup signal from the signal state of the serial link 105.
[0121]
In the communication device 100a, when the PLL circuit 336 completes the
generation of the data clock as a predetermined time period elapses, the command
processing subunit 372 starts issuing the synchronous code (SYNC). After the
generation of the data clock is completed, each of the packet relay subunits 371 of
the communication devices 100b to lOOd relays the synchronous code to an
immediately succeeding communication device in the ring, in order. The command
processing subunit 372 of the communication device 100a receives the synchronous
code that has been looped via the serial link 105.
[0122]
This concludes the description of processing performed in step S301 by the
master communication device 100a and the non-master communication devices
100b to lOOd of the communication system.
[0123]
The command processing subunit 372 of the communication device 100a
issues a device list packet in which (i) the device ID "15" for broadcasting is set as
the destination ID 212 and (ii) the device ID "0" of the own device is included in the
device ID field 261.
[0124]
Each of the packet relay subunits 371 of the communication devices 100b to
lOOd receives the device list packet in order, and considers that a value obtained by
incrementing the value of the device ID field 261 included in the payload 202 of the
device list packet by "1" as the device ID of the own device. Then, each of the
packet relay subunits 371 writes the device ID of the own device to the device ID

field 261, and relays the device list packet to which the device ID of the own device
has been written to an immediately succeeding communication device in the ring.
[0125]
Once the device IDs have been respectively allocated to all of the
communication devices in the above manner, the device list packet arrives at the
communication device 100a. Based on the value "3" of the device ID field 261
included in the payload 202 of the device list packet, the command processing
subunit 372 of the communication device 100a acknowledges that other than the
own device, three communication devices 100b to lOOd are connected to the serial
link 105.
[0126]
This concludes the description of processing performed in step S302 by the
master communication device 100a and the non-master communication devices
100b to lOOd of the communication system.
[0127]
The command processing subunit 372 of the communication device 100a
issues a command packet (CMD) to the communication device 100c. This command
packet is input to the code subunit 313 via the packet relay subunit 371, and a serial
signal corresponding to the command packet is output from the serial transmission
subunit 315 to the serial link 105.
[0128]
In the communication device 100a, the command processing subunit 372,
which has issued the command packet that serves as a standby packet, outputs a
control signal to the standby control subunit 317 via the packet relay subunit 371,
the control signal being for causing the transmitting side of the
transmission/reception processing unit 301 (the code subunit 313 and the serial
transmission subunit 315) to switch to a standby state. In response to the control
signal, after the serial signal corresponding to the command packet is output from

the serial transmission subunit 315 to the serial link 105, the standby control subunit
317 places the code subunit 313 and the serial transmission subunit 315 in the
standby state.
[0129]
In the communication device 100b, the serial signal corresponding to the
command packet is input to the serial reception subunit 311, and the command
packet is input to the packet relay subunit 371. The packet relay subunit 371 makes a
judgment on the destination of the command packet, judges that the command
packet is addressed to another communication device, and outputs the command
packet to the code subunit 313. The serial signal corresponding to this command
packet is output from the serial transmission subunit 315 to the serial link 105.
[0130]
In the communication device 100b, the packet relay subunit 371 judges the
command packet, which is addressed to another communication device, as a standby
packet, and outputs to the standby control subunit 317 a control signal for causing
the communication device 100b to switch to standby mode. In response to this
control signal, after the serial signal corresponding to the command packet is output
from the serial transmission subunit 315 to the serial link 105, the standby control
subunit 317 causes the own communication device 100b to switch to the standby
mode.
[0131]
In the communication device 100c, the serial signal corresponding to the
command packet is input to the serial reception subunit 311, and the command
packet is input to the packet relay subunit 371. The packet relay subunit 371 makes a
judgment on the destination of the command packet, judges that the command
packet is addressed to the own device, and outputs the command packet to the
command processing subunit 372.
[0132]

In the communication device 100c, the packet relay subunit 371, which has
received the command packet that serves as a standby packet, outputs to the standby
control subunit 317 a control signal for causing the receiving side of the
transmission/reception processing unit 301 (the serial reception subunit 311 and the
decode subunit 312) to switch to a standby state. In response to this control signal,
the standby control subunit 317 places the serial reception subunit 311 and the
decode subunit 312 of the transmission/reception processing unit 301 in the standby
state.
[0133]
Through the above sequence of operations, the downlink is placed in a
standby state and the communication device 100b connected to the downlink is
placed in standby mode.
[0134]
This concludes the description of processing performed in step S303 by the
master communication device 100a and the non-master communication devices
100b to lOOd of the communication system.
[0135]
In response to the command packet, the command processing subunit 372 of
the destination communication device (slave communication device) 100c, to which
the command packet is addressed, issues a response packet (RES) addressed to the
communication device 100a. This response packet is input to the code subunit 313
via the packet relay subunit 371, and a serial signal corresponding to the response
packet is output from the serial transmission subunit 315 to the serial link 105.
[0136]
In the communication device 100c, the command processing subunit 372,
which has issued the response packet that serves as a standby packet, outputs a
control signal to the standby control subunit 317 via the packet relay subunit 371,
the control signal being for causing the transmitting side of the

transmission/reception processing unit 301 to switch to a standby state. In response
to the control signal, after the serial signal corresponding to the response packet is
output to the serial link 105, the standby control subunit 317 places the code subunit
313 and the serial transmission subunit 315 in the standby state.
[0137]
In the communication device lOOd, the serial signal corresponding to the
response packet is input to the serial reception subunit 311, and the response packet
is input to the packet relay subunit 371. The packet relay subunit 371 makes a
judgment on the destination of the response packet, judges that the response packet
is addressed to another communication device, and outputs the response packet to
the code subunit 313. The serial signal corresponding to this response packet is
output from the serial transmission subunit 315 to the serial link 105.
[0138]
In the communication device lOOd, the packet relay subunit 371 judges the
response packet, which is addressed to another communication device, as a standby
packet, and outputs to the standby control subunit 317 a control signal for causing
the own communication device lOOd to switch to standby mode. In response to this
control signal, after the serial signal corresponding to the response packet is output
from the serial transmission subunit 315 to the serial link 105, the standby control
subunit 317 causes the own communication device lOOd to switch to the standby
mode.
[0139]
In the communication device 100a, the serial signal corresponding to the
response packet is input to the serial reception subunit 311, and the response packet
is input to the packet relay subunit 371. The packet relay subunit 371 makes a
judgment on the destination of the response packet, judges that the response packet
is addressed to the own device, and outputs the response packet to the command
processing subunit 372. With this response packet, the command processing subunit

372 confirms that handshaking for the command/response has been established.
[0140]
In the communication device 100a, the packet relay subunit 371, which has
received the response packet that serves as a standby packet, outputs to the standby
control subunit 317 a control signal for causing the receiving side of the
transmission/reception processing unit 301 to switch to a standby state. In response
to this control signal, the standby control subunit 317 places the serial reception
subunit 311 and the decode subunit 312 in the standby state.
[0141]
Through the above sequence of operations, the uplink is placed in a standby
state and the communication device lOOd connected to the uplink is placed in
standby mode.
[0142]
This concludes the description of processing performed in step S304 by the
master communication device 100a and the non-master communication devices
100b to lOOd of the communication system.
[0143]
(Operational Sequences Relating to Flow Control, Data Transfer and
Polling in Case of Data Writing)
The operational sequences shown in FIGs. 11 and 12 relate to flow control,
data transfer and polling, and follow the command processing shown in FIG. 10 in a
case where the R/W flag 221 of the command packet indicates data writing.
[0144]
The command processing subunit 372 of the communication device 100a
outputs a control signal for cancelling the standby state of the transmitting side of
the transmission/reception processing unit 301 to the standby control subunit 317 via
the packet relay subunit 371. In response to this control signal, the standby control
subunit 317 places the code subunit 313 and the serial transmission subunit 315 in

an active state. Then, the command processing subunit 372 of the communication
device 100a causes the serial transmission subunit 315 to output a wakeup signal to
the serial link 105 so that the wakeup signal will be transmitted to the
communication device 100c.
[0145]
In the communication device 100b, the wakeup detection subunit 316
detects the wakeup signal, and outputs to the standby control subunit 317 a control
signal for cancelling the standby mode of the own communication device 100b. In
response to this control signal, the standby control subunit 317 cancels the standby
mode of the own communication device 100b. Then, the communication device
100b relays the wakeup signal to an immediately succeeding communication device
in the ring by causing the serial transmission subunit 315 to output the wakeup
signal to the serial link 105.
[0146]
In the communication device 100c, the wakeup detection subunit 316
detects the wakeup signal and outputs to the standby control subunit 317 a control
signal for cancelling the standby state of the serial reception subunit 311 and the
decode subunit 312. In response to this control signal, the standby control subunit
317 places the serial reception subunit 311 and the decode subunit 312 in an active
state.
[0147]
Through the above sequence of operations, the standby state of the
downlink is cancelled, and the standby mode of the communication device 100b
connected to the downlink is cancelled.
[0148]
Next, the command processing subunit 372 of the communication device
100a issues a predetermined number of synchronous codes (S YNCs), and thereafter
issues a flow control packet (FCTL) which is addressed to the communication
A/1

device 100c and in which the loopback flag 272 is set. This flow control packet is
input to the code subunit 313 via the packet relay subunit 371, and a serial signal
corresponding to the flow control packet is output from the serial transmission
subunit 315 to the serial link 105.
[0149]
In the communication device 100b, the S/P converter 333 of the serial
reception subunit 311 properly acknowledges positions that delimit symbols from
the serial reception data corresponding to the synchronous codes, and converts the
serial reception data into parallel data. As a result, succeeding serial reception data is
received as a flow control packet and input to the packet relay subunit 371. The
packet relay subunit 371 makes a judgment on the destination of the flow control
packet, judges that the flow control packet is addressed to another communication
device, and after performing the relay output of the received synchronous codes,
outputs the flow control packet to the code subunit 313. The serial signal
corresponding to this flow control packet is output from the serial transmission
subunit 315 to the serial link 105.
[0150]
In the communication device 100b, the packet relay subunit 371 judges the
flow control packet, which is addressed to another communication device and in
which the loopback flag 272 is set, as a loopback packet, and outputs to the standby
control subunit 317 a control signal for causing the own communication device 100b
to switch to loopback mode. In response to this control signal, after the serial signal
corresponding to the flow control packet is output from the serial transmission
subunit 315 to the serial link 105, the standby control subunit 317 causes the own
communication device 100b to switch to the loopback mode.
[0151]
Similarly, in the communication device 100c also, the flow control packet,
which has been properly converted to parallel data as a result of the S/P converter

333 of the serial reception subunit 311 receiving the synchronous codes, is input to
the packet relay subunit 371. The packet relay subunit 371 makes a judgment on the
destination of the flow control packet, judges that the flow control packet is
addressed to the own device, and outputs the flow control packet to the command
processing subunit 372.
[0152]
This concludes the description of processing performed in step S331 by the
master communication device 100a and the non-master communication devices
100b and 100c of the communication system.
[0153]
In the communication device 100c, the command processing subunit 372
outputs a control signal to the standby control subunit 317 via the packet relay
subunit 371, the control signal being for cancelling the standby state of the
transmitting side of the transmission/reception processing unit 301. In response to
this control signal, the standby control subunit 317 places the code subunit 313 and
the serial transmission subunit 315 in an active state. Then, the command processing
subunit 372 of the communication device 100c causes the serial transmission
subunit 315 to output a wakeup signal to the serial link 105 so that the wakeup
signal will be transmitted to the communication device 100a.
[0154]
In the communication device lOOd, the wakeup detection subunit 316
detects the wakeup signal, and outputs to the standby control subunit 317 a control
signal for cancelling the standby mode of the own communication device lOOd. In
response to this control signal, the standby control subunit 317 cancels the standby
mode of the own communication device lOOd. Then, the communication device
lOOd relays the wakeup signal to an immediately succeeding communication device
in the ring by causing the serial transmission subunit 315 to output the wakeup
signal to the serial link 105.

[0155]
In the communication device 100a, the wakeup detection subunit 316
detects the wakeup signal and outputs to the standby control subunit 317 a control
signal for cancelling the standby state of the serial reception subunit 311 and the
decode subunit 312. In response to this control signal, the standby control subunit
317 places the serial reception subunit 311 and the decode subunit 312 in an active
state.
[0156]
Through the above sequence of operations, the standby state of the uplink is
cancelled, and the standby mode of the communication device lOOd connected to the
uplink is cancelled.
[0157]
Next, the command processing subunit 372 of the communication device
100c issues a predetermined number of synchronous codes (SYNCs), and thereafter
issues a flow control packet (FCTL) which is addressed to the communication
device 100a and in which the loopback flag 272 is not set. This flow control packet
is input to the code subunit 313 via the packet relay subunit 371, and a serial signal
corresponding to the flow control packet is output from the serial transmission
subunit 315 to the serial link 105.
[0158]
In the communication device 100c, the command processing subunit 372
that has issued the flow control packet, which serves as a standby packet and in
which the loopback flag 272 is not set, outputs a control signal to the standby control
subunit 317 via the packet relay subunit 371, the control signal being for causing the
transmitting side of the transmission/reception processing unit 301 to switch to a
standby state. In response to this control signal, after the serial signal corresponding
to the flow control packet is output from the serial transmission subunit 315 to the
serial link 105, the standby control subunit 317 places the code subunit 313 and the

serial transmission subunit 315 in the standby state.
[0159]
In the communication device lOOd, the S/P converter 333 of the serial
reception subunit 311 properly acknowledges positions that delimit symbols from
the serial reception data corresponding to the synchronous codes, and converts the
serial reception data into parallel data. As a result, succeeding serial reception data is
received as a flow control packet and input to the packet relay subunit 371. The
packet relay subunit 371 makes a judgment on the destination of the flow control
packet, judges that the flow control packet is addressed to another communication
device, and after performing the relay output of the received synchronous codes,
outputs the flow control packet to the code subunit 313. The serial signal
corresponding to this flow control packet is output from the serial transmission
subunit 315 to the serial link 105.
[0160]
In the communication device lOOd, the packet relay subunit 371 judges the
flow control packet, which is addressed to another communication device and in
which the loopback flag 272 is not set, as a standby packet, and outputs to the
standby control subunit 317 a control signal for causing the own communication
device 1 OOd to switch to standby mode. In response to this control signal, after the
serial signal corresponding to the flow control packet is output from the serial
transmission subunit 315 to the serial link 105, the standby control subunit 317
causes the own communication device 1OOd to switch to the standby mode.
[0161]
Similarly, in the communication device 100a also, the flow control packet,
which has been properly converted to parallel data as a result of the S/P converter
333 of the serial reception subunit 311 receiving the synchronous codes, is input to
the packet relay subunit 371. The packet relay subunit 371 makes a judgment on the
destination of the flow control packet, judges that the flow control packet is

addressed to the own device, and outputs the flow control packet to the command
processing subunit 372. With this flow control packet, the command processing
subunit 372 confirms that handshaking for the flow control has been established.
[0162]
In the communication device 100a, the packet relay subunit 371 that has
received the flow control packet, which serves as a standby packet and in which the
loopback flag 272 is not set, outputs to the standby control subunit 317 a control
signal for causing the receiving side of the transmission/reception processing unit
301 to switch to a standby state. In response to this control signal, the standby
control subunit 317 places the serial reception subunit 311 and the decode subunit
312 in the standby state.
[0163]
Through the above sequence of operations, the uplink is placed in a standby
state and the communication device lOOd connected to the uplink is placed in
standby mode.
[0164]
This concludes the description of processing performed in step S332 by the
master communication device 100a and the non-master communication devices
100c and lOOd of the communication system.
[0165]
The data transfer subunit 373 of the communication device 100a issues a
data packet (W-DAT) to the communication device 100c. This data packet is input
to the code subunit 313 via the packet relay subunit 371, and a serial signal
corresponding to the data packet is output from the serial transmission subunit 315
to the serial link 105.
[0166]
In the communication device 100b, an input signal corresponding to the
data packet from the serial link 105 arrives at the serial reception subunit 311, the

standby control subunit 317, the loopback selector 314 and the serial transmission
subunit 315, in this order, and then is output to the serial link 105.
[0167]
In the communication device 100c, the serial signal corresponding to the
data packet is input to the serial reception subunit 311, and the data packet is input
to the packet relay subunit 371. The packet relay subunit 371 makes a judgment on
the destination of the data packet, judges that the data packet is addressed to the own
device, and outputs the data packet to the data transfer subunit 373.
[0168]
The above sequence of processing is repeatedly performed until the writing
of the number of data packets specified by the flow control packet is completed.
[0169]
Once the writing of the number of data packets specified by the flow control
packet has been completed, the data transfer subunit 373 of the communication
device 100a issues loopback cancellation code (RFLB). This loopback cancellation
code is input to the code subunit 313 via the packet relay subunit 371, and a serial
signal corresponding to the loopback cancellation code is output from the serial
transmission subunit 315 to the serial link 105.
[0170]
In the communication device 100b, the input signal corresponding to the
loopback cancellation code from the serial link 105 arrives at the serial reception
subunit 311, the standby control subunit 317, the loopback selector 314 and the
serial transmission subunit 315, in this order, and then is output to the serial link 105.
At this time, the standby control subunit 317 detects the loopback cancellation code,
and after the input signal corresponding to the loopback cancellation code is output
to the serial link 105, cancels the loopback mode of the own communication device
100b.
[0171]

each of the serial reception subunits 104a to 104d shown in FIG. 1. The serial
transmission subunit 315 is the equivalent of each of the serial transmission subunits
103a to 103d shown in FIG. 1.
[0058]
The serial reception subunit 311 includes a serial receiver 331, a clock data
recovery (CDR) circuit 332, and a serial-to-parallel converter (S/P converter) 333.
[0059]
The serial receiver 331 generates serial reception data from a serial signal
input from the serial link 105. The CDR circuit 332 generates a data clock and
synchronizes the serial reception data based on the reference clock, which is
provided by the external clock source 303, and the serial reception data. The S/P
converter 333 detects the first bit position of a symbol by detecting a delimiter such
as the aforementioned COM symbol from a bit string of the serial reception data
from the CDR circuit 332, and converts the serial reception data into parallel
reception data having a symbol length conforming to the 8b/10b scheme (a bit width
of 10). The parallel reception data output from the S/P converter 333 is input to the
decode subunit 312. The parallel reception data is also branched by a loopback path
318, input to the standby control subunit 317, and thereafter input from the standby
control subunit 317 to the loopback selector 314. Hereinafter, the parallel reception
data branched by the loopback path 318 is referred to as "parallel loopback data".
[0060]
The following is a further description of the CDR circuit 332. The CDR
circuit 332 includes a PLL (Phase-Locked Loop) circuit 336. At the time of
initialization, the PLL circuit 336 generates the data clock by multiplying the
frequency of the reference clock provided by the clock source 303 so that the
reference clock has a desired data clock frequency and by maintaining that desired
data clock frequency. In general, jitter (displacement in a time axis direction) is
present in serial reception data input from an immediately preceding communication

In the communication device 100c, the serial signal corresponding to the
loopback cancellation code is input to the serial reception subunit 311, and the
loopback cancellation code is input to the packet relay subunit 371. The packet relay
subunit 371 outputs the loopback cancellation code to the data transfer subunit 373.
Upon receiving the loopback cancellation code, the data transfer subunit 373
completes reception of data packets specified by the flow control size.
[0172]
This concludes the description of processing performed in step S333 by the
master communication device 100a and the non-master communication devices
100b and 100c of the communication system.
[0173]
After the completion of data transfer such as issuance of the number of data
packets specified by the flow control size 271 of the flow control packet, the
command processing subunit 372 of the communication device 100a issues a polling
packet (POL) to the communication device 100c. This polling packet is input to the
code subunit 313 via the packet relay subunit 371, and a serial signal corresponding
to the polling packet is output from the serial transmission subunit 315 to the serial
link 105.
[0174]
In the communication device 100a, the command processing subunit 372,
which has issued the polling packet that serves as a standby packet, outputs a control
signal to the standby control subunit 317 via the packet relay subunit 371, the
control signal being for causing the transmitting side of the transmission/reception
processing unit 301 to switch to a standby state. In response to this control signal,
after the serial signal corresponding to the polling packet is output from the serial
transmission subunit 315 to the serial link 105, the standby control subunit 317
places the code subunit 313 and the serial transmission subunit 315 in the standby
state.

[0175]
In the communication device 100b, the serial signal corresponding to the
polling packet is input to the serial reception subunit 311, and the polling packet is
input to the packet relay subunit 371. The packet relay subunit 371 makes a
judgment on the destination of the polling packet, judges that the polling packet is
addressed to another communication device, additionally writes a status of the own
communication device 100b to the status field 282 of the polling packet, and outputs
to the code subunit 313 the polling packet to which the status of the own
communication device 100b has been additionally written. The serial signal
corresponding to this polling packet is output from the serial transmission subunit
315 to the serial link 105.
[0176]
In the communication device 100b, the packet relay subunit 371 judges that
the polling packet, which is addressed to another communication device, as a
standby packet, and outputs to the standby control subunit 317 a control signal for
causing the communication device 100b to switch to standby mode. In response to
this control signal, after the serial signal corresponding to the polling packet is
output from the serial transmission subunit 315 to the serial link 105, the standby
control subunit 317 causes the own communication device 100b to switch to the
standby mode.
[0177]
In the communication device 100c, the serial signal corresponding to the
polling packet is input to the serial reception subunit 311, and the polling packet is
input to the packet relay subunit 371. The packet relay subunit 371 makes a
judgment on the destination of the polling packet, judges that the polling packet is
addressed to the own device, and outputs the polling packet to the command
processing subunit 372.
[0178]

In the communication device 100c, the packet relay subunit 371, which has
received the polling packet that serves as a standby packet, outputs to the standby
control subunit 317 a control signal for causing the receiving side of the
transmission/reception processing unit 301 to switch to a standby state. In response
to this control signal, the standby control subunit 317 places the serial reception
subunit 311 and the decode subunit 312 in the standby state.
[0179]
Through the above sequence of operations, the downlink is placed in a
standby state and the communication device 100b connected to the downlink is
placed in standby mode.
[0180]
This concludes the description of processing performed in step S334 by the
master communication device 100a and the non-master communication devices
100b and 100c of the communication system.
[0181]
In the communication device 100c, the command processing subunit 372
outputs a control signal to the standby control subunit 317 via the packet relay
subunit 371, the control signal being for cancelling the standby state of the
transmitting side of the transmission/reception processing unit 301. In response to
this control signal, the standby control subunit 317 places the code subunit 313 and
the serial transmission subunit 315 in an active state. Then, the command processing
subunit 372 of the communication device 100b causes the serial transmission
subunit 315 to output a wakeup signal to the serial link 105 so that the wakeup
signal will be transmitted to the communication device 100a.
[0182]
In the communication device lOOd, the wakeup detection subunit 316
detects the wakeup signal, and outputs to the standby control subunit 317a control
signal for cancelling the standby mode of the own communication device 1 OOd. In

response to this control signal, the standby control subunit 317 cancels the standby
mode of the own communication device lOOd. Then, the communication device
lOOd relays the wakeup signal to an immediately succeeding communication device
in the ring by causing the serial transmission subunit 315 to output the wakeup
signal to the serial link 105.
[0183]
In the communication device 100a, the wakeup detection subunit 316
detects the wakeup signal and outputs to the standby control subunit 317 a control
signal for cancelling the standby state of the serial reception subunit 311 and the
decode subunit 312. In response to this control signal, the standby control subunit
317 places the serial reception subunit 311 and the decode subunit 312 in an active
state.
[0184]
Through the above sequence of operations, the standby state of the uplink is
cancelled, and the standby mode of the communication device lOOd connected to the
uplink is cancelled.
[0185]
Next, after issuing a predetermined number of synchronous codes (SYNC),
the command processing subunit 372 of the communication device 100c issues to
the communication device 100a a polling packet (POL), in which the content of the
status field 282 of the polling packet input in step S334, as well as a status indicating,
for example, whether the own communication device 100c can perform
communication, are written to the status field 282. This polling packet is input to the
code subunit 313 via the packet relay subunit 371, and a serial signal corresponding
to the polling packet is output from the serial transmission subunit 315 to the serial
link 105.
[0186]
In the communication device 100c, the command processing subunit 372,

which has issued the polling packet that serves as a standby packet, outputs a control
signal to the standby control subunit 317 via the packet relay subunit 371, the
control signal being for causing the transmitting side of the transmission/reception
processing unit 301 to switch to a standby state. In response to this control signal,
after the serial signal corresponding to the polling packet is output from the serial
transmission subunit 315 to the serial link 105, the standby control subunit 317
places the code subunit 313 and the serial transmission subunit 315 in the standby
state.
[0187]
In the communication device lOOd, the S/P converter 333 of the serial
reception subunit 311 properly acknowledges positions that delimit symbols from
the serial reception data corresponding to the synchronous codes, and converts the
serial reception data into parallel data. As a result, succeeding serial reception data is
properly received as a polling packet and input to the packet relay subunit 371. The
packet relay subunit 371 (i) makes a judgment on the destination of the polling
packet, (ii) judges that the polling packet is addressed to another communication
device, (iii) after performing the relay output of the received synchronous codes,
additionally writes a status indicating, for example, whether or not the own device
can perform communication to the status field 282 of the polling packet, and (iv)
outputs to the code subunit 313 the polling packet to which the status of the own
communication device lOOd has been additionally written. The serial signal
corresponding to this polling packet is output from the serial transmission subunit
315 to the serial link 105.
[0188]
In the communication device lOOd, the packet relay subunit 371 judges the
polling packet, which is addressed to another communication device, as a standby
packet, and outputs to the standby control subunit 317 a control signal for causing
the communication device lOOd to switch to standby mode. In response to this

control signal, after the serial signal corresponding to the polling packet is output
from the serial transmission subunit 315 to the serial link 105, the standby control
subunit 317 causes the own communication device lOOd to switch to the standby
mode.
[0189]
Similarly, in the communication device 100a also, the polling packet, which
has been properly converted to parallel data as a result of the S/P converter 333 of
the serial reception subunit 311 receiving the synchronous codes, is input to the
packet relay subunit 371. The packet relay subunit 371 makes a judgment on the
destination of the polling packet, judges that the polling packet is addressed to the
own device, and outputs the polling packet to the command processing subunit 372.
[0190]
In the communication device 100a, the packet relay subunit 371, which has
received the polling packet that serves as a standby packet, outputs to the standby
control subunit 317 a control signal for causing the receiving side of the
transmission/reception processing unit 301 to switch to a standby state. In response
to this control signal, the standby control subunit 317 places the serial reception
subunit 311 and the decode subunit 312 in the standby state.
[0191]
Through the above sequence of operations, the uplink is placed in a standby
state and the communication device lOOd connected to the uplink is placed in
standby mode.
[0192]
This concludes the description of processing performed in step S335 by the
master communication device 100a and the non-master communication devices
100c and lOOd of the communication system.
[0193]
(Operational Sequences Relating to Flow Control, Data Transfer and

Polling in Case of Data Reading)
The operational sequences shown in FIGs. 13 and 14 relate to flow control,
data transfer and polling, and follow the command processing shown in FIG. 10 in a
case where the R/W flag 221 of the command packet indicates data reading.
[0194]
The communication devices 100a to 100c perform processing that is
substantially the same as processing of step S331 shown in FIG. 11, namely,
processing ranging from transmission of the wakeup signal by the communication
device 100a to reception of the synchronous codes (SYNCs) by the communication
device 100c.
[0195]
The command processing subunit 372 of the communication device 100a
issues a flow control packet (FCTL) which is addressed to the communication
device 100c and in which the loopback flag 272 is not set. This flow control packet
is input to the code subunit 313 via the packet relay subunit 371, and a serial signal
corresponding to this flow control packet is output from the serial transmission
subunit 315 to the serial link 105.
[0196]
In the communication device 100a, the command processing subunit 372
that has issued the flow control packet, in which the loopback flag 272 is not set and
which thus serves as a standby packet, outputs a control signal to the standby control
subunit 317 via the packet relay subunit 371, the control signal being for causing the
transmitting side of the transmission/reception processing unit 301 to switch to a
standby state. In response to this control signal, after the serial signal corresponding
to the flow control packet is output from the serial transmission subunit 315 to the
serial link 105, the standby control subunit 317 places the code subunit 313 and the
serial transmission subunit 315 in the standby state.
[0197]

In the communication device 100b, the serial signal corresponding to the
flow control packet is input to the serial reception subunit 311, and the flow control
packet is input to the packet relay subunit 371. The packet relay subunit 371 makes a
judgment on the destination of the flow control packet, judges that the flow control
packet is addressed to another communication device, and outputs the flow control
packet to the code subunit 313. The serial signal corresponding to this flow control
packet is output from the serial transmission subunit 315 to the serial link 105.
[0198]
In the communication device 100b, the packet relay subunit 371 judges the
flow control packet, which is addressed to another communication device and in
which the loopback flag 272 is not set, as a standby packet, and outputs to the
standby control subunit 317 a control signal for causing the own communication
device 100b to switch to standby mode. In response to this control signal, after the
serial signal corresponding to the flow control packet is output to the serial link 105,
the standby control subunit 317 causes the own communication device 100b to
switch to the standby mode.
[0199]
In the communication device 100c, the serial signal corresponding to the
flow control packet is input to the serial reception subunit 311, and the flow control
packet is input to the packet relay subunit 371. The packet relay subunit 371 makes a
judgment on the destination of the flow control packet, judges that the flow control
packet is addressed to the own device, and outputs the flow control packet to the
command processing subunit 372.
[0200]
In the communication device 100c, the packet relay subunit 371 that has
received the flow control packet, in which the loopback flag 272 is not set and which
thus serves as a standby packet, outputs to the standby control subunit 317 a control
signal for causing the receiving side of the transmission/reception processing unit
zrrv

301 to switch to a standby state. In response to this control signal, the standby
control subunit 317 places the serial reception subunit 311 and the decode subunit
312 in the standby state.
[0201]
Through the above sequence of operations, the downlink is placed in a
standby state and the communication device 100b connected to the downlink is
placed in standby mode.
[0202]
This concludes the description of processing performed in step S351 by the
master communication device 100a and the non-master communication devices
100b and 100c of the communication system.
[0203]
The communication devices 100c, 1 OOd, and 100a perform processing that
is substantially the same as processing of step S332 shown in FIG. 11, namely,
processing ranging from transmission of the wakeup signal by the communication
device 100c to reception of the synchronous codes (SYNCs) by the communication
device 100a.
[0204]
The command processing subunit 372 of the communication device 100c
issues a flow control packet (FCTL) which is addressed to the communication
device 100a and in which the loopback flag 272 is set. This flow control packet is
input to the code subunit 313 via the packet relay subunit 371, and a serial signal
corresponding to this flow control packet is output from the serial transmission
subunit 315 to the serial link 105.
[0205]
In the communication device lOOd, the serial signal corresponding to the
flow control packet is input to the serial reception subunit 311, and the flow control
packet is input to the packet relay subunit 371. The packet relay subunit 371 makes a

judgment on the destination of the flow control packet, judges that the flow control
packet is addressed to another communication device, and outputs the flow control
packet to the code subunit 313. The serial signal corresponding to this flow control
packet is output from the serial transmission subunit 315 to the serial link 105.
[0206]
In the communication device lOOd, the packet relay subunit 371 judges the
flow control packet, which is addressed to another communication device and in
which the loopback flag 272 is set, as a loopback packet, and outputs to the standby
control subunit 317 a control signal for causing the own communication device lOOd
to switch to loopback mode. In response to this control signal, after the serial signal
corresponding to the flow control packet is output to the serial link 105, the standby
control subunit 317 causes the own communication device lOOd to switch to the
loopback mode.
[0207]
In the communication device 100a, the serial signal corresponding to the
flow control packet is input to the serial reception subunit 311, and the flow control
packet is input to the packet relay subunit 371. The packet relay subunit 371 makes a
judgment on the destination of the flow control packet, judges that the flow control
packet is addressed to the own device, and outputs the flow control packet to the
command processing subunit 372. With this flow control packet, the command
processing subunit 372 confirms that handshaking for the flow control has been
established.
[0208]
This concludes the description of processing performed in step S352 by the
master communication device 100a and the non-master communication devices
100c and lOOd of the communication system.
[0209]
The data transfer subunit 373 of the communication device 100c issues a

data packet (R-DAT) to the communication device 100a. This data packet is input to
the code subunit 313 via the packet relay subunit 371, and a serial signal
corresponding to this data packet is output from the serial transmission subunit 315
to the serial link 105.
[0210]
In the communication device lOOd, an input signal corresponding to the
data packet from the serial link 105 arrives at the serial reception subunit 311, the
standby control subunit 317, the loopback selector 314 and the serial transmission
subunit 315, in this order, and then is output to the serial link 105.
[0211]
In the communication device 100a, the serial signal corresponding to the
data packet is input to the serial reception subunit 311, and the data packet is input
to the packet relay subunit 371. The packet relay subunit 371 makes a judgment on
the destination of the data packet, judges that the data packet is addressed to the own
device, and outputs the data packet to the data transfer subunit 373.
[0212]
The above sequence of processing is repeatedly performed until the reading
of the number of data packets specified by the flow control packet is completed.
[0213]
Once the reading of the number of data packets specified by the flow
control packet has been completed, the data transfer subunit 373 of the
communication device 100c issues loopback cancellation code (RFLB). This
loopback cancellation code is input to the code subunit 313 via the packet relay
subunit 371, and a serial signal corresponding to this loopback cancellation code is
output from the serial transmission subunit 315 to the serial link 105.
[0214]
In the communication device lOOd, the input signal corresponding to the
loopback cancellation code from the serial link 105 arrives at the serial reception

subunit 311, the standby control subunit 317, the loopback selector 314 and the
serial transmission subunit 315, in this order, and then is output to the serial link 105.
At this time, the standby control subunit 317 detects the loopback cancellation code,
and after the serial signal corresponding to the loopback cancellation code is output
to the serial link 105, cancels the loopback mode of the own communication device
lOOd.
[0215]
In the communication device 100a, the serial signal corresponding to the
loopback cancellation code is input to the serial reception subunit 311, and the
loopback cancellation code is input to the packet relay subunit 371. The packet relay
subunit 371 outputs the loopback cancellation code to the data transfer subunit 373.
Upon receiving the loopback cancellation code, the data transfer subunit 373
completes reception of data packets specified by the flow control size.
[0216]
This concludes the description of processing performed in step S353 by the
master communication device 100a and the non-master communication devices
100c and lOOd of the communication system.
[0217]
After completion of data transfer such as issuance of the number of data
packets specified by the flow control size 271 of the flow control packet, the
communication devices 100a to 100c perform (i) processing that is substantially the
same as processing of step S331 in FIG. 11, namely, processing ranging from
transmission of the wakeup signal by the communication device 100a to reception of
the synchronous codes (SYNCs) by the communication device 100c, and (ii)
processing that is substantially the same as processing of step S334 shown in FIG.
12 onward, the processing of step S334 being the issuance of the polling packet by
the communication device 100a (step S354).
[0218]
64

The communication devices 100c, lOOd and 100a perform processing that is
substantially the same as processing of step S3 3 5 shown in FIG. 12 onward, the
processing of step S335 being the issuance of the polling packet by the
communication device 100c.
[0219]
«First Modification Example»
The following describes the first modification example with reference to the
drawings. The above-described embodiment and the first modification example are
different from each other in types of packets handled as standby packets, types of
packets handled as loopback packets, and types of special symbols for cancelling
loopback mode.
[0220]

Described below with reference to FIG. 15 are special symbols of the
8b/10b scheme used by communication devices 100a to lOOd pertaining to the first
modification example. FIG. 15 shows one example of how the special symbols of
the 8b/10b scheme, which is used by the communication devices 100a to lOOd
pertaining to the first modification example, are allocated to functions.
[0221]
Referring to FIG. 15, SDB (Start of DATA Burst), SOP, EDB (End of
DATA Burst), COM, LIDL (Logical Idle), and EOP are allocated to some of the
functions. SDB and EDB respectively indicate the start and the end of data transfer
corresponding to the flow control size. LIDL is used to maintain synchronization
between the communication devices.
[0222]

[Loopback Packet]
In the first modification example, a communication device that is to issue a

data packet issues a flow control request packet addressed to another communication
device before issuing the data packet. The flow control request packet requests the
destination communication device, to which the data packet is addressed, to perform
flow control. The flow control request packet is the equivalent of a loopback packet.
Here, the loopback flag 272 shown in FIG. 3B is not necessary, because the transfer
path of the flow control request packet is also going to be the transfer path of the
later-issued data packet. The flow control size 271 is not necessary, either, if its
value is shared in advance.
[0223]
[Standby Packet]
In the first modification example, all the packets that are issued to another
communication device after completion of the initialization sequence, excluding a
flow control request packet addressed to another communication device and a data
packet, are considered as standby packets.
[0224]

[Operational Sequences for Entirety of Communication System]
The initialization sequence for an entirety of the communication system and
the operational sequence relating to command processing pertaining to the first
modification example are substantially the same as the operational sequence of FIG.
10 explained in the above embodiment.
[0225]
(Operational Sequences Relating to Flow Control, Data Transfer and Reception
Status Notification in Case of Data Writing)
The operational sequences shown in FIGs. 16 and 17 relate to flow control,
data transfer and reception status notification, and follow the command processing
shown in FIG. 10 in a case where the R/W flag 221 of the command packet indicates
data writing.

[0226]
The communication devices 100a to 100c perform processing that is
substantially the same as processing of step S331 shown in FIG. 11, namely,
processing ranging from transmission of the wakeup signal by the communication
device 100a to reception of the synchronous codes (SYNCs) by the communication
device 100c.
[0227]
The command processing subunit 372 of the communication device 100a
issues a flow control request packet (FCREQ) to the communication device 100c.
This flow control request packet is input to the code subunit 313 via the packet relay
subunit 371, and a serial signal corresponding to this flow control request packet is
output from the serial transmission subunit 315 to the serial link 105.
[0228]
In the communication device 100b, the serial signal corresponding to the
flow control request packet is input to the serial reception subunit 311, and the flow
control request packet is input to the packet relay subunit 371. The packet relay
subunit 371 makes a judgment on the destination of the flow control request packet,
judges that the flow control request packet is addressed to another communication
device, and outputs the flow control request packet to the code subunit 313. The
serial signal corresponding to this flow control request packet is output from the
serial transmission subunit 315 to the serial link 105.
[0229]
In the communication device 100b, the packet relay subunit 371 judges the
flow control request packet addressed to another communication device as a
loopback packet, and outputs to the standby control subunit 317a control signal for
causing the own communication device 100b to switch to loopback mode. In
response to this control signal, after the serial signal corresponding to the flow
control request packet is output from the serial transmission subunit 315 to the serial

link 105, the standby control subunit 317 causes the own communication device
100b to switch to the loopback mode.
[0230]
In the communication device 100c, the serial signal corresponding to the
flow control request packet is input to the serial reception subunit 311, and the flow
control request packet is input to the packet relay subunit 371. The packet relay
subunit 371 makes a judgment on the destination of the flow control request packet,
judges that the flow control request packet is addressed to the own device, and
outputs the flow control request packet to the command processing subunit 372.
[0231]
This concludes the description of processing performed in step S501 by the
master communication device 100a and the non-master communication devices
100b and 100c of the communication system.
[0232]
It should be noted that even after the communication device 100a has issued
the flow control request packet to the communication device 100c and the
communication device 100c has received this flow control request packet, the
communication device 100a keeps transmitting the LIDL, the communication device
100b keeps relaying the LIDL, and the communication device 100c keeps receiving
the LIDL.
[0233]
The communication devices 100c, lOOd, and 100a perform processing that
is substantially the same as processing of step S332 shown in FIG. 11, namely,
processing ranging from transmission of the wakeup signal by the communication
device 100c to reception of the synchronous codes (SYNCs) by the communication
device 100a.
[0234]
When the command processing subunit 372 of the communication device
zro

100c becomes ready to write the data packet corresponding to the flow control
request packet received in step S501, it issues a flow control ready packet (FCRDY)
to the communication device 100a. This flow control ready packet is input to the
code subunit 313 via the packet relay subunit 371, and a serial signal corresponding
to this flow control ready packet is output from the serial transmission subunit 315
to the serial link 105.
[0235]
In the communication device 100c, the command processing subunit 372,
which has issued the flow control ready packet that serves as a standby packet,
outputs a control signal to the standby control subunit 317 via the packet relay
subunit 371, the control signal being for causing the transmitting side of the
transmission/reception processing unit 301 to switch to a standby state. In response
to this control signal, after the serial signal corresponding to the flow control ready
packet is output from the serial transmission subunit 315 to the serial link 105, the
standby control subunit 317 places the code subunit 313 and the serial transmission
subunit 315 in the standby state.
[0236]
In the communication device lOOd, the serial signal corresponding to the
flow control ready packet is input to the serial reception subunit 311, and the flow
control ready packet is input to the packet relay subunit 371. The packet relay
subunit 371 makes a judgment on the destination of the flow control ready packet,
judges that the flow control ready packet is addressed to another communication
device, and outputs the flow control ready packet to the code subunit 313. The serial
signal corresponding to this flow control ready packet is output from the serial
transmission subunit 315 to the serial link 105.
[0237]
In the communication device lOOd, the packet relay subunit 371 judges the
flow control ready packet addressed to another communication device as a standby

packet, and outputs to the standby control subunit 317 a control signal for causing
the own communication device lOOd to switch to standby mode. In response to this
control signal, after the serial signal corresponding to the flow control ready packet
is output from the serial transmission subunit 315 to the serial link 105, the standby
control subunit 317 causes the own communication device lOOd to switch to the
standby mode.
[0238]
In the communication device 100a, the serial signal corresponding to the
flow control ready packet is input to the serial reception subunit 311, and the flow
control ready packet is input to the packet relay subunit 371. The packet relay
subunit 371 makes a judgment on the destination of the flow control ready packet,
judges that the flow control ready packet is addressed to the own device, and outputs
the flow control ready packet to the command processing subunit 372. With this
flow control ready packet, the command processing subunit 372 confirms that
handshaking for the flow control has been established.
[0239]
In the communication device 100a, the packet relay subunit 371, which has
received the flow control ready packet that serves as a standby packet, outputs to the
standby control subunit 317 a control signal for causing the receiving side of the
transmission/reception processing unit 301 to switch to a standby state. In response
to this control signal, the standby control subunit 317 places the serial reception
subunit 311 and the decode subunit 312 in the standby state.
[0240]
Through the above sequence of operations, the uplink is placed in a standby
state and the communication device lOOd connected to the uplink is placed in
standby mode.
[0241]
This concludes the description of processing performed in step S 5 02 by the

master communication device 100a and the non-master communication devices
100c and lOOd of the communication system.
[0242]
Prior to transferring of data having the flow control size, the data transfer
subunit 373 of the communication device 100a issues an SDB symbol. This SDB
symbol is input to the code subunit 313 via the packet relay subunit 371, and a serial
signal corresponding to the SDB symbol is output from the serial transmission
subunit 315 to the serial link 105.
[0243]
In the communication device 100b, the input signal corresponding to the
SDB symbol from the serial link 105 arrives at the serial reception subunit 311, the
standby control subunit 317, the loopback selector 314 and the serial transmission
subunit 315, in this order, and then is output to the serial link 105.
[0244]
In the communication device 100c, the serial signal corresponding to the
SDB symbol is input to the serial reception subunit 311, and the SDB symbol is
input to the packet relay subunit 371. The packet relay subunit 371 outputs the SDB
symbol to the data transfer subunit 373.
[0245]
Until the writing of the number of the data packets specified by the flow
control request packet is completed, the communication devices 100a to 100c
perform processing that is substantially the same as processing of step S333 shown
in FIG. 12, namely, processing ranging from transmission of the data packet
(W-DAT) by the communication device 100a to reception of the data packet by the
communication device 100c.
[0246]
Once the writing of the number of data packets specified by the flow control
request packet has been completed, the data transfer subunit 373 of the

communication device 100a issues an EDB symbol. This EDB symbol is input to the
code subunit 313 via the packet relay subunit 371, and a serial signal corresponding
to the EDB symbol is output from the serial transmission subunit 315 to the serial
link 105.
[0247]
In the communication device 100b, the input signal corresponding to the
EDB symbol from the serial link 105 arrives at the serial reception subunit 311, the
standby control subunit 317, the loopback selector 314 and the serial transmission
subunit 315, in this order, and then is output to the serial link 105. At this time, the
standby control subunit 317 detects the EDB and cancels the loopback mode of the
own communication device 100b.
[0248]
In the communication device 100c, the serial signal corresponding to the
EDB symbol is input to the serial reception subunit 311, and the EDB symbol is
input to the packet relay subunit 371. The packet relay subunit 371 outputs the EDB
symbol to the data transfer subunit 373.
[0249]
This concludes the description of processing performed in step S503 by the
master communication device 100a and the non-master communication devices
100b and 100c of the communication system.
[0250]
It should be noted that even after the transmission/reception of the EDB
symbol, the communication device 100a keeps transmitting the LIDL, the
communication device 100b keeps relaying the LIDL, and the communication
device 100c keeps receiving the LIDL.
[0251]
The communication devices 100c, lOOd, and 100a perform processing that
is substantially the same as processing of step S335 shown in FIG. 12, namely,

processing ranging from transmission of the wakeup signal by the communication
device 100c to reception of the synchronous codes (SYNCs) by the communication
device 100a.
[0252]
The command processing subunit 372 of the communication device 100c
issues a status packet (STAT) that is addressed to the communication device 100a to
notify the result of receiving data packets. This status packet is input to the code
subunit 313 via the packet relay subunit 371, and a serial signal corresponding to
this status packet is output from the serial transmission subunit 315 to the serial link
105.
[0253]
In the communication device 100c, the command processing subunit 372,
which has issued the status packet that serves as a standby packet, outputs a control
signal to the standby control subunit 317 via the packet relay subunit 371, the
control signal being for causing the transmitting side of the transmission/reception
processing unit 301 to switch to a standby state. In response to this control signal,
after the serial signal corresponding to the status packet is output to the serial link
105, the standby control subunit 317 places the code subunit 313 and the serial
transmission subunit 315 in the standby state.
[0254]
In the communication device lOOd, the serial signal corresponding to the
status packet is input to the serial reception subunit 311, and the status packet is
input to the packet relay subunit 371. The packet relay subunit 371 makes a
judgment on the destination of the status packet, judges that the status packet is
addressed to another communication device, and outputs the status packet to the
code subunit 313. A serial signal corresponding to this status packet is output from
the serial transmission subunit 315 to the serial link 105.
[0255]

In the communication device lOOd, the packet relay subunit 371 judges the
status packet addressed to another communication device as a standby packet, and
outputs to the standby control subunit 317 a control signal for causing the own
communication device lOOd to switch to standby mode. In response to this control
signal, after the serial signal corresponding to the status packet is output from the
serial transmission subunit 315 to the serial link 105, the standby control subunit 317
causes the own communication device lOOd to switch to the standby mode.
[0256]
In the communication device 100a, the serial signal corresponding to the
status packet is input to the serial reception subunit 311, and the status packet is
input to the packet relay subunit 371. The packet relay subunit 371 makes a
judgment on the destination of the status packet, judges that the status packet is
addressed to the own device, and outputs the status packet to the command
processing subunit 372.
[0257]
In the communication device 100a, the packet relay subunit 371, which has
received the status packet that serves as a standby packet, outputs to the standby
control subunit 317 a control signal for causing the receiving side of the
transmission/reception processing unit 301 to switch to a standby state. In response
to this control signal, the standby control subunit 317 places the serial reception
subunit 311 and the decode subunit 312 in the standby state.
[0258]
On the occasion of reception of the status packet that serves as a standby
packet, the relay processing subunit 371 of the communication device 100a further
outputs to the standby control subunit 317 a control signal for causing the
transmitting side of the transmission/reception processing unit 301 in a standby state.
In response to this control signal, the standby control subunit 317 places the code
subunit 313 and the serial transmission subunit 315 in a standby state.

[0259]
Although the communication device 100a has kept transmitting the LIDL
even after the transmission/reception of the EDB symbol, the communication device
100a stops transmission of the LIDL once the transmitting side of the
transmission/reception processing unit 301 has been placed in the standby state.
When the communication device 100b no longer receives the LIDL—for instance,
when the wakeup detection subunit 316 detects that the serial link 105 is in an
electrically idle state, the wakeup detection subunit 316 outputs to the standby
control subunit 317 a control signal for causing the own communication device 100b
to switch to standby mode. In response to this control signal, the standby control
subunit 317 causes the own communication device 100b to switch to the standby
mode.
[0260]
When the transmission of the LIDL from the communication device 100b
has stopped and the communication device 100c no longer receives the LIDL—for
instance, when the wakeup detection subunit 316 detects that the serial link 105 is in
an electrically idle state, the wakeup detection subunit 316 outputs to the standby
control subunit 317 a control signal for causing the receiving side of the
transmission/reception processing unit 301 to switch to a standby state. In response
to this control signal, the standby control subunit 317 places the serial reception
subunit 311 and the decode subunit 312 in the standby state.
[0261]
As such, on the occasion of the issuance of the status packet by the
command processing subunit 372 of the communication device 100c, the downlink
switches to the standby state and the communication device 100b connected to the
downlink switches to the standby mode.
[0262]
Through the above sequence of operations, the uplink is placed in a standby

state and the communication device lOOd connected to the uplink is placed in
standby mode. Also, the downlink is placed in a standby state and the
communication device 100b connected to the downlink is placed in standby mode.
[0263]
This concludes the description of processing performed in step S 5 04 by the
master communication device 100a and the non-master communication devices
100b to lOOd of the communication system.
[0264]
(Operational Sequences Relating to Flow Control, Data Transfer and
Reception Status Notification in Case of Data Writing)
The operational sequences shown in FIGs. 18 and 19 relate to flow control,
data transfer and reception status notification, and follow the command processing
shown in FIG. 10 in a case where the R/W flag 221 of the command packet indicates
data reading.
[0265]
The communication devices 100c, lOOd, and 100a perform processing that
is substantially the same as processing of step S332 shown in FIG. 11, namely,
processing ranging from transmission of the wakeup signal by the communication
device 100c to reception of the synchronous codes (SYNCs) by the communication
device 100a.
[0266]
The command processing subunit 372 of the communication device 100c
issues a flow control request packet (FCREQ) to the communication device 100a.
This flow control request packet is input to the code subunit 313 via the packet relay
subunit 371, and a serial signal corresponding to this flow control request packet is
output from the serial transmission subunit 315 to the serial link 105.
[0267]
In the communication device lOOd, the serial signal corresponding to the

flow control request packet is input to the serial reception subunit 311, and the flow
control request packet is input to the packet relay subunit 371. The packet relay
subunit 371 makes a judgment on the destination of the flow control request packet,
judges that the flow control request packet is addressed to another communication
device, and outputs the flow control request packet to the code subunit 313. The
serial signal corresponding to this flow control request packet is output from the
serial transmission subunit 315 to the serial link 105.
[0268]
In the communication device lOOd, the packet relay subunit 371 judges the
flow control request packet addressed to another communication device as a
loopback packet, and outputs to the standby control subunit 317 a control signal for
causing the own communication device lOOd to switch to loopback mode. In
response to this control signal, after the serial signal corresponding to the flow
control request packet is output from the serial transmission subunit 315 to the serial
link 105, the standby control subunit 317 causes the own communication device
1 OOd to switch to the loopback mode.
[0269]
In the communication device 100a, the serial signal corresponding to the
flow control request packet is input to the serial reception subunit 311, and the flow
control request packet is input to the packet relay subunit 371. The packet relay
subunit 371 makes a judgment on the destination of the flow control request packet,
judges that the flow control request packet is addressed to the own device, and
outputs the flow control request packet to the command processing subunit 372.
[0270]
This concludes the description of processing performed in step S551 by the
master communication device 100a and the non-master communication devices
100c and lOOd of the communication system.
[0271]

It should be noted that even after the communication device 100c has issued
the flow control request packet to the communication device 100a and the
communication device 100a has received this flow control request packet, the
communication device 100c keeps transmitting the LIDL, the communication device
lOOd keeps relaying the LIDL, and the communication device 100a keeps receiving
the LIDL.
[0272]
The communication devices 100a to 100c perform processing that is
substantially the same as processing of step S331 shown in FIG. 11, namely,
processing ranging from transmission of the wakeup signal by the communication
device 100a to reception of the synchronous codes (SYNCs) by the communication
device 100c.
[0273]
When the command processing subunit 372 of the communication device
100a becomes ready to read data packets corresponding to the flow control request
packet received in step S551, it issues a flow control ready packet (FCRDY) to the
communication device 100c. This flow control ready packet is input to the code
subunit 313 via the packet relay subunit 371, and a serial signal corresponding to
this flow control ready packet is output from the serial transmission subunit 315 to
the serial link 105.
[0274]
In the communication device 100a, the command processing subunit 372,
which has issued the flow control ready packet that serves as a standby packet,
outputs a control signal to the standby control subunit 317 via the packet relay
subunit 371, the control signal being for causing the transmitting side of the
transmission/reception processing unit 301 to switch to a standby state. In response
to this control signal, after the serial signal corresponding to the flow control ready
packet is output from the serial transmission subunit 315 to the serial link 105, the

standby control subunit 317 places the code subunit 313 and the serial transmission
subunit 315 in the standby state.
[0275]
In the communication device 100b, the serial signal corresponding to the
flow control ready packet is input to the serial reception subunit 311, and the flow
control ready packet is input to the packet relay subunit 371. The packet relay
subunit 371 makes a judgment on the destination of the flow control ready packet,
judges that the flow control ready packet is addressed to another communication
device, and outputs the flow control ready packet to the code subunit 313. The serial
signal corresponding to this flow control ready packet is output from the serial
transmission subunit 315 to the serial link 105.
[0276]
In the communication device 100b, the packet relay subunit 371 judges the
flow control ready packet addressed to another communication device as a standby
packet, and outputs to the standby control subunit 317 a control signal for causing
the own communication device 100b to switch to standby mode. In response to this
control signal, after the serial signal corresponding to the flow control ready packet
is output from the serial transmission subunit 315 to the serial link 105, the standby
control subunit 317 causes the own communication device 100b to switch to the
standby mode.
[0277]
In the communication device 100c, the serial signal corresponding to the
flow control ready packet is input to the serial reception subunit 311, and the flow
control ready packet is input to the packet relay subunit 371. The packet relay
subunit 371 makes a judgment on the destination of the flow control ready packet,
judges that the flow control ready packet is addressed to the own device, and outputs
the flow control ready packet to the command processing subunit 372. With this
flow control ready packet, the command processing subunit 372 confirms that

handshaking for the flow control has been established.
[0278]
In the communication device 100c, the packet relay subunit 371, which has
received the flow control ready packet that serves as a standby packet, outputs to the
standby control subunit 317 a control signal for causing the receiving side of the
transmission/reception processing unit 301 to switch to a standby state. In response
to this control signal, the standby control subunit 317 places the serial reception
subunit 311 and the decode subunit 312 in the standby state.
[0279]
Through the above sequence of operations, the downlink is placed in a
standby state and the communication device 100b connected to the downlink is
placed in standby mode.
[0280]
This concludes the description of processing performed in step S552 by the
master communication device 100a and the non-master communication devices
100b and 100c of the communication system.
[0281]
Prior to transferring of data having the flow control size, the data transfer
subunit 373 of the communication device 100c issues an SDB symbol. This SDB
symbol is input to the code subunit 313 via the packet relay subunit 371, and a serial
signal corresponding to this SDB symbol is output from the serial transmission
subunit 315 to the serial link 105.
[0282]
In the communication device lOOd, an input signal corresponding to the
SDB symbol from the serial link 105 arrives at the serial reception subunit 311, the
standby control subunit 317, the loopback selector 314 and the serial transmission
subunit 315, in this order, and then is output to the serial link 105.
[0283]

In the communication device 100a, the serial signal corresponding to the
SDB symbol is input to the serial reception subunit 311, and the SDB symbol is
input to the packet relay subunit 371. The packet relay subunit 371 outputs the SDB
symbol to the data transfer subunit 373.
[0284]
Until completion of the reading of the number of data packets specified by
the flow control ready packet, the communication devices 100c, lOOd and 100a
perform processing that is substantially the same as processing of step S353 shown
in FIG. 14, namely, processing ranging from transmission of the data packet
(R-DAT) by the communication device 100c to reception of the data packet by the
communication device 100a.
[0285]
Once the reading of the number of data packets specified by the flow
control request packet has been completed, the data transfer subunit 373 of the
communication device 100c issues an EDB symbol. This EDB symbol is input to the
code subunit 313 via the packet relay subunit 371, and a serial signal corresponding
to this EDB symbol is output from the serial transmission subunit 315 to the serial
link 105.
[0286]
In the communication device lOOd, an input signal corresponding to the
EDB symbol from the serial link 105 arrives at the serial reception subunit 311, the
standby control subunit 317, the loopback selector 314 and the serial transmission
subunit 315, in this order, and then is output to the serial link 105. At this time, the
standby control subunit 317 detects the EDB and cancels the loopback mode of the
own communication device 1 OOd.
[0287]
In the communication device 100a, the serial signal corresponding to the
EDB symbol is input to the serial reception subunit 311, and the EDB symbol is

input to the packet relay subunit 371. The packet relay subunit 371 outputs the EDB
symbol to the data transfer subunit 373.
[0288]
This concludes the description of processing performed in step S553 by the
master communication device 100a and the non-master communication devices
100c and lOOd of the communication system.
[0289]
It should be noted that even after the transmission/reception of the EDB
symbol, the communication device 100c keeps transmitting the LIDL, the
communication device lOOd keeps relaying the LIDL, and the communication
device 100a keeps receiving the LIDL.
[0290]
The communication devices 100a to 100c perform processing that is
substantially the same as processing of step S331 shown in FIG. 11, namely,
processing ranging from transmission of the wakeup signal by the communication
device 100a to reception of the synchronous codes (SYNCs) by the communication
device 100c.
[0291]
The command processing subunit 372 of the communication device 100a
issues a status packet (STAT) to the communication device 100c. This status packet
is input to the code subunit 313 via the packet relay subunit 371, and a serial signal
corresponding to this status packet is output from the serial transmission subunit 315
to the serial link 105.
[0292]
In the communication device 100a, the command processing subunit 372,
which has issued the status packet that serves as a standby packet, outputs a control
signal to the standby control subunit 317 via the packet relay subunit 371, the
control signal being for causing the transmitting side of the transmission/reception

processing unit 301 to switch to a standby state. In response to this control signal,
after the serial signal corresponding to the status packet is output to the serial link
105, the standby control subunit 317 places the code subunit 313 and the serial
transmission subunit 315 in the standby state.
[0293]
In the communication device 100b, the serial signal corresponding to the
status packet is input to the serial reception subunit 311, and the status packet is
input to the packet relay subunit 371. The packet relay subunit 371 makes a
judgment on the destination of the status packet, judges that the status packet is
addressed to another communication device, and outputs the status packet to the
code subunit 313. A serial signal corresponding to this status packet is output from
the serial transmission subunit 315 to the serial link 105.
[0294]
In the communication device 100b, the packet relay subunit 371 judges the
status packet addressed to another communication device as a standby packet, and
outputs to the standby control subunit 317 a control signal for causing the own
communication device 100b to switch to standby mode. In response to this control
signal, after the serial signal corresponding to the status packet is output from the
serial transmission subunit 315 to the serial link 105, the standby control subunit 317
causes the own communication device 100b to switch to the standby mode.
[0295]
In the communication device 100c, the serial signal corresponding to the
status packet is input to the serial reception subunit 311, and the status packet is
input to the packet relay subunit 371. The packet relay subunit 371 makes a
judgment on the destination of the status packet, judges that the status packet is
addressed to the own device, and outputs the status packet to the command
processing subunit 372.
[0296]

In the communication device 100c, the packet relay subunit 371, which has
received the status packet that serves as a standby packet, outputs to the standby
control subunit 317 a control signal for causing the receiving side of the
transmission/reception processing unit 301 to switch to a standby state. In response
to this control signal, the standby control subunit 317 places the serial reception
subunit 311 and the decode subunit 312 in the standby state.
[0297]
On the occasion of reception of the status packet that serves as a standby
packet, the relay processing subunit 371 of the communication device 100c further
outputs to the standby control subunit 317 a control signal for causing the
transmitting side of the transmission/reception processing unit 301 in a standby state.
In response to this control signal, the standby control subunit 317 places the code
subunit 313 and the serial transmission subunit 315 in a standby state.
[0298]
Although the communication device 100c has kept transmitting the LIDL
even after the transmission/reception of the EDB symbol, the communication device
100c stops transmission of the LIDL once the transmitting side of the
transmission/reception processing unit 301 has been placed in the standby state.
When the communication device lOOd no longer receives the LIDL—for instance,
when the wakeup detection subunit 316 detects that the serial link 105 is in an
electrically idle state, the wakeup detection subunit 316 outputs to the standby
control subunit 317 a control signal for causing the own communication device lOOd
to switch to standby mode. In response to this control signal, the standby control
subunit 317 causes the own communication device lOOd to switch to the standby
mode.
[0299]
When the transmission of the LIDL from the communication device lOOd
has stopped and the communication device 100a no longer receives the LIDL—for

instance, when the wakeup detection subunit 316 detects that the serial link 105 is in
an electrically idle state, the wakeup detection subunit 316 outputs to the standby
control subunit 317 a control signal for causing the receiving side of the
transmission/reception processing unit 301 to switch to a standby state. In response
to this control signal, the standby control subunit 317 places the serial reception
subunit 311 and the decode subunit 312 in the standby state.
[0300]
As such, on the occasion of the issuance of the status packet by the
command processing subunit 372 of the communication device 100a, the uplink
switches to the standby state and the communication device lOOd connected to the
uplink switches to the standby mode.
[0301]
Through the above sequence of operations, the downlink is placed in a
standby state and the communication device 100b connected to the downlink is
placed in standby mode. Also, the uplink is placed in a standby state and the
communication device lOOd connected to the downlink is placed in standby mode.
[0302]
This concludes the description of processing performed in step S554 by the
master communication device 100a and the non-master communication devices
100b to lOOd of the communication system.
[0303]
«Second Modification Example»
The following describes the second modification example with reference to
the drawings. In the above embodiment, each of the communication devices in the
communication system includes the clock source 303 constituted by, for example, a
voltage-controlled crystal oscillator (VCXO). As opposed to this, in the second
modification example, one of communication devices in the communication system
includes the clock source. Alternatively, one or more of communication devices in

the communication system may each include the clock source.
[0304]

FIG. 20 shows a system structure of a communication system pertaining to
the second modification example.
[0305]
In the communication system shown in FIG. 20, four communication
devices 400a through 400d are connected with one another via a serial link 402.
Here, the communication device 400a is a master communication device, and the
communication devices 400b to 400d are slave communication devices. It should be
noted that, except for the structure of the clock source, the communication device
400a is structured the same as the communication device 100a, and the
communication devices 400b to 400d are structured the same as the communication
devices 100b to lOOd, respectively.
[0306]
The communication device 400a includes a clock source 401, and each of
the communication device 400a to 400d is structured such that it can relay a
reference clock. The communication system has a common clock structure in which
the reference clock output from the clock source 401 is propagated through a clock
communication channel 403 that coexists with the serial link 402 for data transfer.
[0307]
«Supplementary Notes»
The present invention is not limited to the contents explained in the above
embodiment, and can be implemented in any embodiment that can achieve the aim
of the present invention and other aims that relate to or accompany the aim of the
present invention. For example, the following cases are possible.
[0308]
(1) In the above embodiment and modification examples, the number of

non-master communication devices connected to each of the serial links 105 and 402
is three. However, the present invention is not limited to this. The number of
non-master communication devices may be two, or four or more.
[0309]
(2) In the above embodiment and modification examples, the decode
subunit 312 and the code subunit 313 use the 8b/10b scheme for data conversion.
However, the present invention is not limited to this. Instead, the 64b/66b scheme or
other schemes may be used for data conversion.
[0310]
(3) The above embodiment and modification examples have explained the
structure in which data output from the S/P converter 333 is routed in loopback.
However, the present invention is not limited to this. The present invention may
have the structure in which data output from the CDR circuit 332 is routed in
loopback.
[0311]
(4) According to the above embodiment and modification examples, a
packet designed for a certain use is also used for a different purpose in the form of a
standby packet and a loopback packet. However, the present invention is not limited
to this. It is permissible to prepare a packet for exclusive use as a standby packet and
a loopback packet.
[0312]
(5) A method for flow control is not limited to the ones explained in the
above embodiment and modification examples.
[0313]
(6) The polling packet used in the above embodiment and modification
examples may be broadcast.
[0314]
(7) The communication devices of the above embodiment and modification

examples may each be realized as, for example, an LSI which is an integrated circuit.
Each of constituent elements of the communication devices may be mounted on one
chip. Alternatively, all or part of such constituent elements may be mounted on one
chip. Although referred to here as an LSI, depending on the degree of integration,
the terms IC, system LSI, super LSI, or ultra LSI are used. In addition, the method
for assembling integrated circuits is not limited to the LSI, and a dedicated circuit or
a general-purpose processor may be used. An FPGA (Field Programmable Gate
Array), which is programmable after the LSI is manufactured, or a reconfigurable
processor, which allows reconfiguration of the connection and setting of circuit cells
inside the LSI, may be used. Furthermore, if technology for forming integrated
circuits that replaces LSIs emerges, owing to advances in semiconductor technology
or to another derivative technology, the integration of function blocks may naturally
be accomplished using such technology. The application of biotechnology or the like
is possible.
[0315]
(8) The contents of the above embodiment and the contents of the above
supplementary notes may be combined as necessary.
[Industrial Applicability]
[0316]
The present invention is useful in a communication system or the like that
aims to reduce the amount of power consumed in a case where data transfer is
performed between a plurality of communication devices and the like connected in a
ring via a serial link.
[Reference Signs List]
[0317]
lOOatolOOd communication device

101 a to 101d protocol processing unit
102a to 102d transmission/reception processing unit
103 a to 103 d serial transmission subunit
104a to 104d serial reception subunit
105 serial link
300 communication device
301 transmission/reception processing unit
302 protocol processing unit
303 clock source

311 serial reception subunit
312 decode subunit
313 code subunit
314 loopback selector
315 serial transmission subunit
316 wakeup detection subunit
317 standby control subunit
318 loopback path

331 serial receiver
332 CDR circuit
333 serial-to-parallel converter (S/P converter)
336 PLL circuit

351 parallel-to-serial converter (P/S converter)
352 serial driver

371 packet relay subunit
372 command processing subunit
373 data transfer subunit

WE CLAIM:
1. A communication system including a first communication device and a plurality
of second communication devices, the first communication device and the second
communication devices being connected with one another in a ring via a serial link,
wherein
the first communication device comprises:
a first command processing unit operable to, when a downlink is not used
for data packet transfer, issue a standby packet to a partner device that is one of the
second communication devices with which the first communication device performs
communication, the downlink being a part of the serial link extending from the first
communication device to the partner device; and
a first data transfer unit operable to (i) transmit a write data packet to be
written via the downlink, and (ii) receive a read data packet to be read via an uplink,
which is a part of the serial link extending from the partner device to the first
communication device, and
each of the second communication devices comprises:
a second command processing unit operable to, when the uplink is not used
for the data packet transfer and the own device is the partner device, issue the
standby packet to the first communication device;
a second data transfer unit operable to receive the write data packet via the
downlink and transmit the read data packet via the uplink;
a packet relay unit operable to, in accordance with a destination of a packet
input to the own device, relay the input packet to one of the first and second
communication devices that immediately succeeds the own device in the ring; and
a standby control unit operable to, when the input packet relayed by the
packet relay unit of the own device is the standby packet, cause the own device to
switch to standby mode.

2. The communication system of Claim 1, wherein
the first command processing unit issues a loopback packet to the partner
device when the downlink is used for the data packet transfer,
each of the second command processing units issues the loopback packet to
the first communication device when the uplink is used for the data packet transfer
and the own device is the partner device, and
each of the standby control units causes the own device to switch to
loopback mode when the input packet relayed by the packet relay unit of the own
device is the loopback packet.
3. The communication system of Claim 2, wherein
each of the first command processing unit and the second command
processing units performs transmission and reception of a flow control packet via
the downlink and the uplink,
the flow control packet is considered as the standby packet when
transmitted via one of the downlink and the uplink that is not used for the data
packet transfer, and
the flow control packet is considered as the loopback packet when
transmitted via one of the downlink and the uplink that is used for the data packet
transfer.
4. The communication system of Claim 2, wherein
the first command processing unit issues a flow control request packet to the
partner device in order to write the write data packet, and in response to the flow
control request packet, the second command processing unit of the partner device
issues a flow control ready packet to the first communication device,
the second command processing unit of the partner device issues the flow

control request packet to the first communication device in order for the read data
packet to be read, and in response to the flow control request packet, the first
command processing unit issues the flow control ready packet to the partner device,
and
the flow control request packet is considered as the loopback packet, and the
flow control ready packet is considered as the standby packet.
5. The communication system of Claim 2, wherein
when the own device is a transmitter of the write data packet or the read
data packet, each of the first data transfer unit and the second data transfer units
transmits a loopback cancellation signal at an end of transfer of data having a
predetermined size, the loopback cancellation signal cancelling the loopback mode
of one or more of the second communication devices that have switched to the
loopback mode, and
each of the standby control units cancels the loopback mode of the own
device in accordance with detection of the loopback cancellation signal.
6. The communication system of Claim 5, wherein
when the own device is a receiver of the write data packet or the read data
packet, each of the first command processing unit and the second command
processing units transmits a wakeup signal after the transfer of the data having the
predetermined size, the wakeup signal cancelling the standby mode of one or more
of the second communication devices that have switched to the standby mode,
each of the first command processing unit and the second command
processing units issues a polling packet to which each of the second communication
devices writes a status of the own device,
each of the second communication devices further comprises a wakeup
detection unit operable to detect the wakeup signal, and

each of the standby control units cancels the standby mode of the own
device in accordance with the detection of the wakeup signal, and when the input
packet relayed by the packet relay unit of the own device is the polling packet,
causes the own device to switch to the standby mode.
7. The communication system of Claim 2, wherein
when the own device is a transmitter of the write data packet or the read
data packet, each of the first data transfer unit and the second data transfer units
transmits a data burst end signal at an end of transfer of data having a predetermined
size, and
each of the standby control units cancels the loopback mode of the own
device in accordance with detection of the data burst end signal.
8. (Amended) The communication system of Claim 7, wherein
when the own device is a receiver of the write data packet or the read data
packet, each of the first command processing unit and the second command
processing units (i) transmits a wakeup signal after the transfer of the data having
the predetermined size, the wakeup signal cancelling the standby mode of one or
more of the second communication devices that have switched to the standby mode,
and (ii) after transmitting the wakeup signal, issues a status packet for notifying a
result of the reception of the write data packet or the read data packet,
each of the second communication devices further comprises a wakeup
detection unit operable to detect the wakeup signal, and
each of the standby control units cancels the standby mode of the own
device in accordance with the detection of the wakeup signal, and when the input
packet relayed by the packet relay unit of the own device is the status packet, causes
the own device to switch to the standby mode.

9. The communication system of Claim 1, wherein
the first command processing unit issues a command packet,
each of the second command processing units issues a response packet, and
each of the standby control units causes the own device to switch to the
standby mode when the input packet relayed by the packet relay unit of the own
device is the command packet or the response packet.
10. A communication device included in a plurality of communication devices that
are connected with one another in a ring via a serial link and that constitute a
communication system, the serial link being made up of (i) a downlink, which is a
part of the serial link that extends from a master communication device to a slave
communication device, the master communication device and the slave
communication device being included in the plurality of communication devices,
and (ii) an uplink, which is a part of the serial link that extends from the slave
communication device to the master communication device, the communication
device comprising:
a command processing unit operable to issue, to a partner device that is one
of the plurality of communication devices with which the own device performs
communication, (i) a standby packet for causing one or more of the plurality of
communication devices connected to one of the downlink and the uplink that is not
used for data packet transfer to switch to standby mode, and (ii) a loopback packet
for causing one or more of the plurality of communication devices connected to one
of the downlink and the uplink that is used for the data packet transfer to switch to
loopback mode;
a data transfer unit operable to transfer a write data packet to be written via
the downlink, and to transfer a read data packet to be read via the uplink;
a packet relay unit operable to, in accordance with a destination of a packet
input to the own device, relay the input packet to one of the plurality of

communication devices that immediately succeeds the own device in the ring; and
a standby control unit operable to (i) when the input packet relayed by the
packet relay unit is the standby packet, cause the own device to switch to the
standby mode, and (ii) when the input packet relayed by the packet relay unit is the
loopback packet, cause the own device to switch to the loopback mode.
11. An integrated circuit used by a communication device included in a plurality of
communication devices that are connected with one another in a ring via a serial link
and that constitute a communication system, the serial link being made up of (i) a
downlink, which is a part of the serial link that extends from a master
communication device to a slave communication device, the master communication
device and the slave communication device being included in the plurality of
communication devices, and (ii) an uplink, which is a part of the serial link that
extends from the slave communication device to the master communication device,
the integrated circuit comprising:
a command processing circuit operable to issue, to a partner device that is
one of the plurality of communication devices with which the own device performs
communication, (i) a standby packet for causing one or more of the plurality of
communication devices connected to one of the downlink and the uplink that is not
used for data packet transfer to switch to standby mode, and (ii) a loopback packet
for causing one or more of the plurality of communication devices connected to one
of the downlink and the uplink that is used for the data packet transfer to switch to
loopback mode;
a data transfer circuit operable to transfer a write data packet to be written
via the downlink, and to transfer a read data packet to be read via the uplink;
a packet relay circuit operable to, in accordance with a destination of a
packet input thereto, relay the input packet to one of the plurality of communication
devices that immediately succeeds the own device in the ring; and

a standby control circuit operable to (i) when the input packet relayed by the
packet relay circuit is the standby packet, cause the own device to switch to the
standby mode, and (ii) when the input packet relayed by the packet relay circuit is
the loopback packet, cause the own device to switch to the loopback mode.
12. A communication method used in a communication system including a first
communication device and a plurality of second communication devices, the first
communication device and the second communication devices being connected with
one another in a ring via a serial link, wherein
the communication method causes the first communication device to
perform the steps of:
when a downlink is not used for data packet transfer, issuing a standby
packet to a partner device that is one of the second communication devices with
which the first communication device performs communication, the downlink being
a part of the serial link extending from the first communication device to the partner
device; and
transmitting a write data packet to be written via the downlink and receiving
a read data packet to be read via an uplink, which is a part of the serial link
extending from the partner device to the first communication device, and
the communication method causes each of the second communication
devices to perform the steps of:
issuing the standby packet to the first communication device when the
uplink is not used for the data packet transfer and the own device is the partner
device;
receiving the write data packet via the downlink and transmitting the read
data packet via the uplink;
in accordance with a destination of a packet input to the own device,
relaying the input packet to one of the first and second communication devices that

immediately succeeds the own device in the ring; and
causing the own device to switch to standby mode when the input packet
that has been relayed in the own device is the standby packet.

A communication system includes communication devices that are
connected with one another in a ring via a serial link. In the communication system,
one communication device issues a standby packet for causing each communication
device connected to a part of the link that is not involved with data transfer to switch
to standby mode. Each communication device connected to this part of the link
relays the standby packet from an immediately preceding communication device in
the link to an immediately succeeding communication device in the link, and after
relaying the standby packet, causes the own device to switch to standby mode.

Further, a communication device that performs communication with said one
communication device issues a loopback packet for causing each communication
device connected to a part of the link that is involved with data transfer to switch to
loopback mode. Each communication device connected to this part of the link relays
the loopback packet from an immediately preceding communication device in the
link to an immediately succeeding communication device in the link, and after
relaying the loopback packet, causes the own device to switch to loopback mode.