DESCRIPTION
Field
[0001] The present invention relates to a communication device and a communication method.
Background
[0002] In an FA (factory automation) network system, for the purpose of wire-saving, a single master device and a plurality of slave devices are connected with a daisy chain in some cases. In the system with a daisy-chain connection, communication between a first device and a second device that are not adjacent to each other is performed by relaying the communication by a device positioned between the first device and the second device. [0003] The master device and the slave devices communicate with each other repeatedly in the same cycle. The mater device transmits command data to the slave devices. The slave devices transmit response data to the master device. As the communication time of the command data and the response data becomes shorter, the master device and the slave devices can exchange these command data and response data more closely and frequently. Therefore, the system can improve the control accuracy. [0004] As a method for reducing the communication time, it is common to reduce the volume of communication data, or reduce the number of frames needed for communication. In the method for reducing the number of frames needed for communication, the volume of data needed for each frame can be reduced. Therefore, the communication time is reduced. As the data needed for each frame, a preamble that

indicates the start of a frame, or a frame check sequence (FCS) that is a code to be added for performing error detection and correction on the frame, is exemplified. [0005] As a related technique, Patent Literature 1 has described a communication system in which a master node and a plurality of terminal nodes are connected with a daisy chain. In this system, each of the terminal nodes relays and forwards the control data, transmitted from the master node, to the subsequent downstream node after the terminal node deletes a command field for its own node in the control data, and then shifts the second and subsequent command fields one by one to shorten the control data (paragraph [0029]).
[0006] Further, Patent Literature 2 has .described a system in which a controller and a plurality of servo amplifiers are connected in a line. In this system, each of the amplifiers receives a frame from a higher amplifier, which includes command-data areas transmitted from the controller to the amplifiers, and then transmits the frame, from which a command-data area addressed to the amplifier of its own has been removed, to a lower amplifier (paragraphs [0039] to [0040]).
[0007] Furthermore, Patent Literature 3 has described a system in which a numerical control device and a plurality of amplifiers are connected with a daisy chain. In this system, each of the amplifiers transmits a transmission-start code to an upstream amplifier, and then transmits its own amplifier data, and subsequently the downstream amplifier data, received from the downstream amplifier, to the upstream amplifier (paragraph [0013]). When each of the amplifiers cannot receive the downstream amplifier data before it has completely transmitted its own amplifier data, the amplifier transmits idle-time data to the upstream

amplifier after the transmission of its own amplifier data and until the reception of the amplifier data from the downstream amplifier, and thereafter transmits this downstream amplifier data to the upstream amplifier (paragraph [0028]).
Citation List Patent Literatures
[0008] Patent Literature 1: Japanese Patent Application Laid-open No. 2009-21700
Patent Literature 2: Japanese Patent Application Laid-open No. 2011-160018
Patent Literature 3: Japanese Patent Application Laid-open No. 2008-135855
Summary
Technical "Problem
[0009] According to the techniques described in Patent Literatures 1 and 2, because the frame size is reduced, the communication time can be reduced. However, in these techniques described in Patent Literatures 1 and 2, each device removes the data addressed to the device of its own, from the frame received from the upstream device. Therefore, the contents and the length of the frame are changed. However, these techniques described in Patent Literatures 1 and 2 do not consider error detection in a frame obtained after each device has removed the data addressed to the device of its own, from the frame received from the upstream device. Therefore, in the techniques described in Patent Literatures 1 and 2, the system cannot detect a data error attributable to a transmission error, and accordingly may not be capable of executing a normal control.

[0010] Further, in the technique described in Patent Literature 3, because the idle-time data is transmitted, this makes the communication time longer. Furthermore, in the technique described in Patent Literature 3, each of the amplifiers transmits its own amplifier data, and subsequently the idle-time data and the downstream amplifier data. However, the technique described in Patent Literature 3 does not consider error detection in the own amplifier data, the idle-time data, or the downstream amplifier data. Therefore, in the technique described in Patent Literature 3, the system cannot detect a data error attributable to a transmission error, and accordingly may not be capable of executing a normal control. [0011] The present invention has been achieved to solve the above problems, and an object of the present invention is to provide a communication device that can reduce the communication time and perform error detection.
Solution to Problem
[0012] In order to solve the above-mentioned problems and achieve the object, there is provided a communication device to transmit and receive a frame including a preamble and a payload, where the payload includes one or a plurality of sub-payloads, each of which includes a main portion including an address portion for storing therein a value that indicates a target-address device, a data portion for storing therein data to be transmitted to the target-address device, and a data-length portion for storing therein a value that indicates a length of the data portion, and an error-detection code portion for storing therein an error-detection code for performing error detection attributable to a transmission error, the communication device including: a frame reception unit to

separate the frame received from one device into a relay sub-payload that is the sub-payload not addressed to a device of its own, and an own-device addressed sub-payload that is the sub-payload addressed to a device of its own; an error detection unit to perform error detection on the own-device addressed sub-payload based on the error-detection code within the own-device addressed sub-payload; a relay sub-payload storage unit to store therein the relay sub-payload; a communication-start notification unit to notify a communication-start timing to start communication simultaneously with other devices; and a frame transmission unit to start, transmitting to another device the frame including the preamble and the relay sub-payload upon being notified of the communication-start timing, when the frame is not being transmitted, and to add the relay sub-payload to an end of the frame being transmitted, so as to continue transmission of the frame, when the'relay sub-payload is stored in the relay sub-payload storage unit while the frame is being transmitted.
Advantageous Effects of Invention
[0013] The communication device according to the present invention has an effect where the communication device can reduce the communication time and perform error detection.
Brief Description of Drawings
[0014] FIG. 1 is a diagram illustrating a configuration
of a communication system according to a first embodiment.
FIG. 2 is a diagram illustrating a frame to be transmitted and received in the communication system according to the first embodiment.
FIG. 3 is a diagram illustrating a sub-payload to be transmitted and received in the communication system

according to the first embodiment.
FIG. 4 is a diagram illustrating a configuration of a communication unit in a control device according to the first embodiment.
FIG. 5 is a flowchart illustrating processing at the time of reception in the communication unit in the control device according to the first embodiment.
FIG. 6 is a flowchart illustrating processing at the time of transmission in the communication unit in the control device according to the first embodiment.
FIG. 7 is a flowchart illustrating a first transmission process in the communication unit in the control device according to the first embodiment.
FIG. 8 is.a flowchart illustrating a second transmission process in the communication unit in the control device according to the first embodiment.
FIG. 9 is a flowchart illustrating processing at the time of transmission in the communication unit in the control device according to the first embodiment.
FIG. 10 is a flowchart illustrating a third transmission process in the communication unit in the control device according to the first embodiment.
FIG. 11 is a flowchart illustrating a fourth transmission process in the communication unit in the control device according to the first embodiment.
FIG. 12 is a sequence diagram illustrating a communicating operation of the communication system according to the first embodiment.
FIG. 13 is a sequence diagram illustrating a communicating operation of the communication system according to the first embodiment.
Description of Embodiments

[0015] A communication device and a communication method according to an embodiment of the present invention will be described below in detail with reference to the accompanying drawings. The present invention is not limited to the embodiment. [0016] First embodiment.
FIG. 1 is a diagram illustrating a configuration of a communication system according to a first embodiment. A communication system 1 includes control devices M, SI, S2, and S3. The control device M is connected to the control device SI through a cable CI. The control device SI is connected to the control device S2 through a cable C2. The control device S2 is connected to the control device S3 through a cable C3. That is, the control devices M, SI, S2, and S3 are connected with a daisy chain. [0017] The control devices M, SI, S2, and S3 have a period of performing communication in a direction of an arrow 15 from the control device M toward the control device S3, and a period of performing communication in a direction of an arrow 16 from the control device S3 toward the control device M. In the first embodiment, the direction of the arrow 15 is sometimes referred to as "downstream direction", and the direction of the arrow 16 is sometimes referred to as "upstream direction". [0018] The respective transmission-start timings in the control devices M, SI, S2, and S3 are synchronized with each other. The control devices M, SI, S2, and S3 simultaneously start transmission in the downstream direction 15 at their transmission-start timing during the period of performing communication in the downstream direction 15. In contrast, the control devices M, SI, S2, and S3 simultaneously start transmission in the upstream direction 16 at their transmission-start timing during the

period of performing communication in the upstream direction 16.
[0019] The control device M is a master control device that controls the control devices SI, S2, and S3. As the control device M, a programmable controller (JIS B 3502:2011 (PLC)) is exemplified. The control device M includes a communication unit Ma that communicates with the control devices SI, S2, and S3, and a control unit Mb that performs control calculation to control the control devices SI, S2, and S3.
[0020] The control device SI is a slave control device that operates under control of the control device M. As the control device SI, a servo amplifier is exemplified. The control device SI includes a communication unit Sla that communicates with the control devices M, S2, and S3, and a drive unit Sib that drives a motor 11. [0021] The control device S2 is a slave control device that operates under control of the control device M. As the control device S2, a servo amplifier is exemplified. The control device S2 includes a communication unit S2a that communicates with the control devices M, SI, and S3, and a drive unit S2b that drives a motor 12. [0022] The control device S3 is a slave control device that operates under control of the control device M. As the control device S3, a servo amplifier is exemplified. The control device S3 includes a communication unit S3a that communicates with the control devices M, SI, and S2, and a drive unit S3b that drives a motor 13. [0023] FIG. 2 is a diagram illustrating a frame to be transmitted and received in the communication system according to the first embodiment. A frame 20 includes a preamble 21 that is a bit sequence in a predetermined pattern, and that indicates the start of the frame 20, and

a payload 22 in which data is stored. [0024] In the first embodiment, the payload 22 is constituted by one or a plurality of blocks. In the first embodiment, the one block that constitutes the payload 22, or the blocks that constitute the payload 22 are referred to as "sub-payload 23".
[0025] FIG. 3 is a diagram illustrating a sub-payload to be transmitted and received in the communication system according to the first embodiment. The sub-payload 23 includes a main portion 24, and an error-detection code portion 25 for storing therein an error-detection code for performing error detection on the sub-payload 23 attributable to a transmission error. As the error-detection code, a check sum or a cyclic redundancy check (CRC) is exemplified. The error detection code can be an error-correction code for performing error correction in addition to the error detection. As the error-correction code, a cyclic code or a convolutional code is exemplified. [0026] The main portion 24 includes an address portion 26, a data-length portion 27, and a data portion 28. The address portion 26 stores therein the value that indicates a device to which the sub-payload 23 is addressed. The data portion 28 stores therein the data to be transmitted to the device to which the sub-payload 23 is addressed. The data-length portion 27 stores therein the value that indicates the length of the data portion 28. The sub-payload 23 is a unit of the signal to be addressed to a certain device.
[0027] Each of the address portion 26, the data-length portion 27, and the error-detection code portion 25 is fixed in length, while the data portion 28 is variable in length. The length of the data portion 28 is stored in the data-length portion 27. Therefore, by referring to the

data-length portion 27, the communication units Ma, Sla, S2a, and S3a can obtain the length of the sub-payload 23. [0028] Referring back to FIG. 2, the payload 22 includes one or a plurality of sub-payloads 23. It is possible that the one sub-payload 23 included in a single payload 22, or the sub-payloads -23 included in a single, payload 22 have a target address identical to or different from each other. [0029] FIG. 4 is a diagram illustrating a configuration of the communication unit in the control device according to the first embodiment. In the first embodiment, the communication units Ma, Sla, S2a, and S3a have an identical configuration.
[0030] Each of the communication units Ma, Sla, S2a, and S3a includes a frame reception unit 31 that separates the frame 20 received from one device into a relay sub-payload 23 and an own-device addressed sub-payload 23. The relay sub-payload 23 is a sub-payload that is not addressed to the device of its own. The own-device addressed sub-payload 23 is a sub-payload that is addressed to the device of its own.
[0031] The frame reception unit 31 stores one or a plurality of relay sub-payloads 23 in a relay sub-payload storage unit 33.
[0032] Each of the communication units Ma, Sla, S2a, and S3a includes an error detection unit 32 that performs error detection on the own-device addressed sub-payload 23 based on an error-detection code within the own-device addressed sub-payload 23. The error detection unit 32 transmits a result: of the error detection, and the main portion 24 within the own-device addressed sub-payload 23, to the control unit Mb, or to the drive unit Sib, S2b, or S3b. [0033] Each of the communication units Ma, Sla, S2a, and S3a includes the relay sub-payload storage unit 33 that

stores therein the relay sub-payload 23.
[0034] Each of the communication units Ma, Sla, S2a, and S3a includes a communication-start notification unit 34 that notifies the communication-start timing to start communication simultaneously with other devices. [0035] Each of the communication units Ma, Sla, S2a, and S3a includes a preamble storage unit 35 that stores therein the preamble 21.
[0036] Each of the communication units Ma, Sla, S2a, and S3a includes a transmission-data storage unit 36 that stores therein the main portion 24 created by the control unit Mb or the drive unit Sib, S2b, or S3b of the device of its own.
[0037] It is preferable for the control unit Mb or the drive unit Sib, S2b, or S3b to store one or a plurality of main portions 24 in the transmission-data storage unit 36 in the order of the furthest address to the closest address from the device of its own. Due to this operation, only by reading the data in the transmission-data storage unit 36 sequentially from the beginning, a frame transmission unit 38 can create an own-device issued sub-payload 23 that is issued by the device of its own to another device in the order of the furthest address to the closest address from the device of its own.
[0038] Each of the control devices M, SI, S2, and S3 transmits the frame 20 in which the own-device issued sub-payloads 23 are combined in the order of the furthest address to the closest address from the device of its own. Therefore, the following effects are obtained. [0039] It is assumed that the control device M transmits to the control device SI the frame 20 in which the own-device issued sub-payload 23 addressed to the control device SI, the own-device issued sub-payload 23 addressed

to the control device S3, and the own-device issued sub-payload 23 addressed to the control device S2, are combined in the described order. Under this assumption, the control device SI cannot immediately transmit the relay sub-payload 23 addressed to the control device S3. That is, the control device SI cannot start transmitting the relay sub- " payload 23 addressed to the control device S3 until the control device SI has received this relay sub-payload 23. Therefore, completion of the communication among the control devices M, SI, S2, and S3 is delayed, and this increases the communication time.
[0040] Meanwhile, the control device M transmits to the control device SI the frame 20 in which the own-device issued sub-payload 23 addressed to the control device S3, the own-device issued sub-payload 23 addressed to the control device S2, and the own-device issued sub-payload 23 addressed to the control device SI are combined in the described order.
[0041] Therefore, upon receiving the relay sub-payload 23 issued by the control device M and addressed to the control device S3, the control device SI can transmit to the control device S2 this relay sub-payload 23 issued by the control device M and addressed to the control device S3. Due to this operation, the completion of the communication among the control devices M, SI, S2, and S3 can be advanced forward, and this can reduce the communication time. [0042] Each of the communication units Ma, Sla, S2a, and S3a includes an error-detection-code generation unit 37 that generates the error-detection code portion 25 to be added to the main portion 24. After the main portion 24 is stored in the transmission-data storage unit 36, the error-detection-code generation unit 37 refers to the stored main portion 24 to generate the error-detection code portion 25.

[0043] Each of the communication units Ma, Sla, S2a, and S3a includes the frame transmission unit 38. When the frame transmission unit 38 is notified of the communication-start timing, and when the frame 20 is not being transmitted,, this frame transmission unit 38 starts transmitting to" another device the frame 20 including the preamble 21 and the relay sub-payload 23. When the relay sub-payload 23 is stored in the relay sub-payload storage unit 33 while the frame 20 is"being transmitted, the frame transmission unit 38 adds this relay sub-payload 23 to the end of the frame 20 being transmitted, so as to continue the transmission of the frame 20.
[0044] FIG. 5 is a flowchart illustrating processing at the time of reception in the communication unit in the control device according to the first embodiment. At Step S100, the frame reception unit 31 separates one or a plurality of sub-payloads 23 within the payload 22 of a received frame 20 into the own-device addressed sub-payload 23 that is addressed to the device of its own, and the relay sub-payload 23 that is not addressed to the device of its own.
[0045] By referring to the address portion 26 of the one sub-payload 23 or each of the sub-payloads 23, the frame reception unit 31 can obtain a target address of the one sub-payload 23 or each of the sub-payloads 23. Further, by referring to the data-length portion 27 of the one sub-payload 23 or each of the sub-payloads 23, the frame reception unit 31 can obtain the length of the one sub-payload 23 or each of the sub-payloads 23. [0046] Therefore, based on the address portion 26 and the data-length portion 27 of the one sub-payload 23 or the sub-payloads 23, the frame reception unit 31 can separate the one sub-payload 23 or the sub-payloads 23 into the own-

device addressed sub-payload 23 and the relay sub-payload 23. The frame reception unit 31 transmits the own-device addressed sub-payload 23 to the error detection unit 25, and stores the relay sub-payload 23 in the relay sub-payload storage unit 33.
[0047] Each of the control devices M, SI,' S2, and S3 transmits the frame 20 in which the own-device issued sub-payloads 23 are combined in the order of the furthest address to the closest address from the device of its own. Therefore, the following effects are obtained. [0048] It is assumed that the control device M transmits to the control device SI the frame 20 in which the own-device issued sub-payload 23 addressed to the control device SI, the own-device issued sub-payload 23 addressed to the control device S3, and the own-device issued sub-• payload 23 addressed to the control device S2, are combined in the described order. Under this assumption, the control device SI cannot immediately transmit the relay sub-payload 23 addressed to the control device S3. That is, the control device SI cannot start transmitting the relay sub-payload 23 addressed to the control device S3 until the
0
control device SI has received this relay sub-payload 23. Therefore, completion of the communication among the control devices M, SI, S2, and S3 is delayed, and this increases the communication time.
[0049] Meanwhile, the control device M transmits to the control device SI the frame 20 in which the own-device issued sub-payload 23 addressed to' the control device S3, the own-device issued sub-payload 23 addressed to the control device S2, and the own-device issued sub-payload 23 addressed to the control device SI are combined in the described order. [0050] Therefore, upon receiving the relay sub-payload

23 issued by the control device M and addressed to the control device S3, the control device SI can transmit to the control device S2 this relay sub-payload 23 issued by the control device M and addressed to the control device S3. - Due to this operation, the completion of the communication among the control devices M, SI, S2, and S3 can be advanced forward, and this can reduce the communication time. [0051] At Step S102, the error detection unit 32 performs error detection on the own-device addressed sub-payload 23. The error detection unit 32 transmits a result of the error detection, and the main portion 24 within the own-device addressed sub-payload 23, to the control unit Mb, or to the drive unit Sib, S2b, or S3b.
[0052] FIG. 6 is a flowchart illustrating processing at the time of transmission in the communication unit in the control device according to the first embodiment. The flowchart in FIG. 6 illustrates the processing at the time of transmission in the communication units Ma, Sla, S2a, and S3a in the control devices M, SI, S2, and S3 when each of the control devices M, SI, S2, and S3 transmits the own-device issued sub-payload 23 issued by the device of its own to another device.
[0053] Upon receiving a notification of the communication-start timing from the communication-start notification unit 34, the frame transmission unit 38 performs the processing illustrated in FIG. 6 if the main portion 24 has been stored in the transmission-data storage unit 36.
[0054] At Step S200, the frame transmission unit 38 determines whether the frame 20 is being transmitted. When the frame transmission unit 38 determines that the frame 20 is not being transmitted at Step S200 (NO), the frame transmission unit 38 advances the process to Step S202.

When the frame transmission unit 38 determines that the frame 20 is being transmitted at Step S200 (YES), the frame transmission unit 38 advances the process to Step S204. [0055] It is also possible that the frame transmission unit 38 determines whether the frame 20 is being transmitted, based on the remaining length of the sub-payload 23 being transmitted. The frame transmission unit 38 reads the main portion 24 from the transmission-data storage unit 36, and reads the error-detection code portion 25 from the error-detection-code generation unit 37, to create the own-device issued sub-payload 23. The processing time required for the frame transmission unit 38 to create the own-device issued sub-payload 23 is represented as TO. When a transmission remaining time Tl is longer than the processing time TO, where the transmission remaining time Tl is obtained by dividing the remaining length of the sub-payload 23 being transmitted by a predetermined communication speed, the frame transmission unit 38 can determine that the frame 20 is being transmitted. Due to this operation, the frame transmission unit 38 can determine that the frame 20 is being transmitted, taking into account the processing time required to create the own-device issued sub-payload 23. [0056] It is also possible that the frame transmission unit 38 determines whether the frame 20 is being transmitted, based on the transmission-start time-point and the length of the sub-payload 23 being transmitted. The frame transmission unit 38 can determine that the frame 20 is being transmitted when a scheduled transmission-completion time-point T4 is later than the time-point obtained by adding the processing time TO to the present time-point, where the scheduled transmission-completion time-point T4 is obtained by adding a transmission time T2

to a transmission-start time-point T3 for the sub-payload 23 being transmitted, and where the transmission time T2 is obtained by dividing the length of the sub-payload 23 being transmitted by the communication speed. Due to this operation, the frame transmission unit 3.8 can determine that the frame 20 is being transmitted, taking into account the processing time required to create the own-device issued sub-payload 23.
[0057] At Step S202, the frame transmission unit 38 performs a first transmission-process sub-routine. [0058] FIG. 7 is a flowchart illustrating a first transmission process in the communication unit in the control device according to the first embodiment. [0059] At Step S210, the frame transmission unit 38 reads the preamble 21 from the preamble storage unit 35. [0060] At Step S212, the frame transmission unit 38 reads one or a plurality of main portions 24 from the transmission-data storage unit 36, and reads one or a plurality of error-detection code portions 25 from the error-detection-code generation unit 37, sequentially in the order of the furthest address to the closest address from the device of its own. The frame transmission unit 38 then creates one or a plurality of own-device issued sub-payloads 23 in the order of the furthest address to the closest address from the device of its.own. [0061] It is preferable for the control unit Mb or the drive unit Sib, S2b, or S3b to store one or a plurality of main portions 24 in the transmission-data storage unit 36 in the order of the furthest address to the closest address from the device of its own. Due to this operation, only by reading the data in the transmission-data storage unit 36 sequentially from the beginning, the frame transmission unit 38 can create the own-device issued sub-payload 23

that is issued by the device of its own to another device in the order of the furthest address to the closest address from the device of its own.
[0062] Each of the control devices M, SI, S2, and S3 transmits the frame 20 in which the own-device issued sub-payloads 23 are combined in the order of the furthest address to the closest address from the device of its own. Therefore, the following effects are obtained. [0063] It is assumed that the control device M transmits to the control device SI the frame 20 in which the own-device issued sub-payload 23 addressed to the control device SI, the own-device issued sub-payload 23 addressed to the control device S3, and the own-device issued sub-payload 23 addressed to the control device S2, are combined in the described order. Under this assumption, the control device SI cannot immediately transmit the relay sub-payload 23 addressed to the control device S3. That is, the control device SI cannot start transmitting the relay sub-payload 23 addressed to the control device S3 until the control device SI has received this relay sub-payload 23. Therefore, completion of the communication among the control devices M, SI, S2, and S3 is delayed, and this increases the communication time.
[0064] Meanwhile, the control device M transmits to the control device SI the frame 20 in which the own-device issued sub-payload 23 addressed to the control device S3, the own-device issued sub-payload 23 addressed to the control device S2, and the own-device issued sub-payload 23 addressed to the control device SI are combined in the described order.
[0065] Therefore, upon receiving the relay sub-payload 23 issued by the control device M and addressed to the control device S3, the control device SI can transmit to

the control device S2 this relay sub-payload 23 issued by the control device M and addressed to the control device S3. Due to this operation, the completion of the communication among the control devices M, SI, S2, and S3 can be advanced forward, and this can reduce the communication time. [0066] At Step S214, the frame transmission unit 38 transmits the frame 20 in which the preamble 21 is combined with one or a plurality of own-device issued sub-payloads 23 in the order of the furthest address to the closest address from the device of its own.
[0067] Referring back to FIG. 6, the frame transmission unit 38 performs a second transmission-process sub-routine at Step S204.
[0068] FIG. 8 is a flowchart illustrating a second transmission process in the communication unit in the control device according to the first embodiment. [0069] At Step S220, the frame transmission unit 38 reads one or a plurality of main portions 24 from the transmission-data storage unit 36, and reads one or a plurality of error-detection code portions 25 from the error-detection-code generation unit 37, sequentially in the order of the furthest address to the closest address from the device of its own. The frame transmission unit 38 then creates one or a plurality of own-device issued sub-payloads 23 in the order of the furthest address to the closest address from the device of its own.
[0070] At Step S222, the frame transmission unit 38 adds one or a plurality of own-device issued sub-payloads 23 to the end of the frame 20 being transmitted in the order of the furthest address to the closest address from the device of its own, so as to continue the transmission of the frame 20. This increases the length of the frame 20 being transmitted.

[0071] FIG. 9 is a flowchart illustrating processing at the time of transmission in the communication unit in the control device according to the first embodiment. The flowchart in FIG. 9 illustrates the processing at the time of transmission in the communication units Ma, Sla, S2a, and S3a in the control devices M, SI., S2, and S3 when each of the control devices.M, SI, S2, and S3 forwards the relay sub-payload 23.
[0072] Upon receiving a notification of the communication-start timing from the communication-start notification unit 34, the frame transmission unit 38 performs the processing illustrated in FIG. 9 if one or a plurality of relay sub-payloads 23 have been stored in the relay sub-payload storage unit 33.
[0073] At Step S230, the frame transmission unit 38 determines whether the frame 20 is being transmitted. When the frame transmission unit 38 determines that the frame 20 is not being transmitted at Step S230 (NO), the frame transmission unit 38 advances the process to Step S232. When the frame transmission unit 38 determines that the frame 20 is being transmitted at Step S230 (YES), the frame transmission unit 38 advances the process to Step S234. [0074] At Step S232, the frame transmission unit 38 performs a third transmission-process sub-routine. [0075] FIG. 10 is a flowchart illustrating a third transmission process in the communication unit in the control device according to the first embodiment. [0076] At Step S240, the frame transmission unit 38 reads the preamble 21 from the preamble storage unit 35. [0077] At Step S242, the frame transmission unit 38 reads one or a plurality of relay sub-payloads 23 from the relay sub-payload storage unit 33. [0078] At Step S244, the frame transmission unit 38

transmits the frame 20 in which the preamble 21 is combined with one or a plurality of relay sub-payloads 23 in the order of the furthest address to the closest address from the device of its own.
[0079] Referring back to FIG. 9, the frame transmission unit 38 performs a fourth transmission-process sub-routine at Step S234.
[0080] FIG. 11 is a flowchart illustrating a fourth transmission process in the communication unit in the control device according to the first embodiment. [0081] At Step S250, the frame transmission unit 38 reads one or a plurality of relay sub-payloads 23 from the relay sub-payload storage unit 33.
[0082] At Step S252, the frame transmission unit 38 adds one or a plurality of relay sub-payloads 23 to the end of the frame 20 being transmitted in the order of the furthest address to the closest address from the device of its own, so as to continue the transmission of the frame 20. This increases the length of the frame 20 being transmitted. [0083] FIG. 12 is a sequence diagram illustrating a communicating operation of the communication system according to the first embodiment. The sequence diagram in FIG. 12 illustrates the operation of the communication system 1 during the period of performing the communication in the downstream direction 15.
[0084] At a timing tO, the control devices M, SI, S2, and S3 simultaneously start the communication in the downstream direction 15.
[0085] At the timing tO, the control device M starts transmitting the frame 20 to the control device SI, where the frame 20 includes the own-device issued sub-payload 23 addressed to the control device S3, the own-device issued sub-payload 23 addressed to the control device S2, and the

own-device issued sub-payload 23 addressed to the control device SI.
[0086] The control device M transmits the frame 20 to the control device SI, in which the own-device issued sub-payload 23 addressed to the control device S3, the own-device issued sub-payload 23 addressed to the control device S2, and the own-device issued sub-payload 23 addressed to the control device SI are combined in the order of the furthest address to the closest address from the control device M, that is, in the order of the control device S3, the control device S2, and the control device SI. [0087] At the timing tO, the control device SI starts transmitting to the control device S2 the frame 20 including the own-device issued sub-payload 23 addressed to the control device S2.
[0088] At the timing tO, the control device S2 starts transmitting to the control device S3 the frame 20 including the own-device issued sub-payload 23 addressed to the control device S3.
[0089] At a timing tl, the control device SI finishes transmitting the own-device issued sub-payload 23 addressed to the control device S2. At this point in time, the control device SI has started receiving the main portion 24 of the relay sub-payload 23 issued by the control device M and addressed to the control device S3. Therefore, the control device SI adds this relay sub-payload 23, issued by the control device M and addressed to the control device S3, to the end of the frame 20 being transmitted, so as to increase the length of the frame 20 being transmitted, thereby continuing the transmission of the frame 20. [0090] At a timing t2, the control device SI finishes transmitting the relay sub-payload 23 issued by the control device M and addressed to the control device S3. At this

point in time, the control device SI has started receiving the main portion 24 of the sub-payload 23 issued by the control device M and addressed to the control device S2. Therefore, the control device SI adds this relay sub-payload 23, issued by the control device M and addressed to the control device S2, to the end of the frame 20 being transmitted, so as to increase the length of the frame 20 being transmitted, thereby continuing the transmission of the frame 20.
[0091] At a timing t3, the control device S2 finishes transmitting the own-device issued sub-payload 23 addressed to the control device S3. At this point in time, the control device S2 has not yet finished receiving the main portion 24 of the relay sub-payload 23 issued by the control device M and addressed to the control device S3. Therefore, the control device S2 does not perform the communication until it finishes receiving the main portion 24 of the relay sub-payload 23 issued by the control device M and addressed to the control device S3. It is also possible that while not performing the communication, the control device S2 transmits a low-level or high-level signal to the control device S3.
[0092] At a timing t4, the control device S2 starts receiving the main portion 24 of the relay sub-payload 23 issued by the control device M and addressed to the control device S3. Therefore, at a timing t5, the control device S2 starts transmitting the frame 20 including the relay sub-payload 23 issued by the control device M and addressed to the control device S3.
[0093] As described above, according to the first embodiment, the control device SI can transmit a single frame 20 including the own-device issued sub-payload 23 addressed to the control device S2, the relay sub-payload

23 issued by the control device M and addressed to the control device S3, and the relay sub-payload 23 issued by the control device M and addressed to the control device S2. That is, the control device SI can reduce the number of frames 20 to be transmitted. In other words, the control device SI can reduce the number of preambles. Therefore, the control device SI can reduce the communication time by an amount corresponding to the reduction in number of preambles.
[0094] Further, according to the first embodiment, the control device M transmits the frame 20 to the control device SI, in which the own-device issued sub-payload 23 addressed to the control device S3, the own-device issued sub-payload 23 addressed to the control device S2, and the own-device issued sub-payload 23 addressed to the control device SI are combined in the order of the furthest address to the closest address from the control device M, that is, in the order of the control device S3, the control device S2, and the control device SI. Therefore, the following effects are obtained.
[0095] It is assumed that the control device M transmits to the control device SI the frame 20 in which the own-device issued sub-payload 23 addressed to the control device SI, the own-device issued sub-payload 23 addressed to the control device S3, and the own-device issued sub-payload 23 addressed to the control device S2, are combined in the described order. Under this assumption, the control device SI cannot transmit the relay sub-payload 23 addressed to the control device S3 at the timing tl. That is, the control device SI cannot start transmitting the relay sub-payload 23 addressed to the control device S3 until the control device SI has received this relay sub-payload 23. Therefore, completion of the communication

.among the control devices M, SI, S2, and S3 is delayed, and this increases the communication time.
[0096] Meanwhile, according to the first embodiment, the control device M transmits to the control device SI the frame 20 in which the own-device issued sub-payload 23 addressed to the control device S3, the own-device issued sub-payload 23 addressed to the control device S2, and the own-device issued sub-payload 23 addressed to the control device SI are combined in the described order. [0097] Therefore, according to the first embodiment, upon receiving the relay sub-payload 23 issued by the control device M and addressed to the control device S3, at the timing tl, the control device SI can transmit to the control device S2 this relay sub-payload 23 issued by the control device M and addressed to the control device S3. Due to this operation, the completion of the communication among the control devices M, SI, S2, and S3 can be advanced forward, and this can reduce the communication time. [0098] Further, it is assumed that the control device M transmits to the control device SI the frame 20 in which the own-device issued sub-payload 23 addressed to the control device S2, the own-device issued sub-payload 23 addressed to the control device S3, and the own-device issued sub-payload 23 addressed to the control device SI, are combined in the described order. Under this assumption, the control device S2 cannot transmit the relay sub-payload 23 addressed to the control device S3 at the timing t4. That is, the control device S2 cannot start transmitting the relay sub-payload 23 addressed to the control device S3 until the control device S2 has received this relay sub-payload 23. Therefore, the completion of the communication among the control devices M, SI, S2, and S3 is delayed, and this increases the communication time.

[0099] Meanwhile, according to the first embodiment, the control device M transmits to the control device SI the frame 20 in which the own-device issued sub-payload 23 addressed to the control device S3, the owri-device issued sub-payload 23 addressed to the control device S2, and the own-device issued sub-payload 23 addressed to the control device SI are combined in the described order. The control device SI then transmits to the control device S2 the frame 20 including the own-device issued sub-payload 23 addressed to the control device S2, the relay sub-payload 23 addressed to the control device S3, and the relay sub-payload 23 addressed to the control device S2. [0100] Therefore, according to the first embodiment, upon receiving the relay sub-payload 23 issued by the control device M and addressed to the control device S3, at the timing t4, the control device S2 can transmit to the control device S3 this relay sub-payload 23 issued by the control device M and addressed to the control device S3. Due to this operation, the completion of the communication among the control devices M, SI, S2, and S3 can be advanced forward, and this can reduce the communication time. [0101] Further, according to the first embodiment, the control devices M, SI, S2, and S3 add the error-detection code portion 25 within the own-device issued sub-payload 23, thereby obtaining the following effects.
[0102] When the control devices SI and S2 receive the relay sub-payload 23 from another device, while transmitting the frame 20 including the own-device issued sub-payload 23, the control devices SI and S2 add this relay sub-payload 23 to the end of the frame 20 being transmitted. That is, the contents and the length of the frame 20 are dynamically changed. [0103] In order to detect an error attributable to a

transmission error, it is common to add a frame check sequence to the end of a frame at the time of creating the frame. However, before a certain device receives the frame to the end, this device completes transmission of the frame from its own station. Therefore, even though there is an error in the frame check sequence of the received frame, the device cannot notify the error.
[0104] Meanwhile, according to the first embodiment, the control devices M, SI, S2, and S3 add the error-detection code portion 25 within the own-device issued sub-payload 23. Therefore, a target-address device can refer to the error-detection code portion 25, and detect an error in the own-device addressed sub-payload 23.
[0105] It is also possible that the frame transmission unit 38 determines whether the frame 20 is being transmitted, based on the remaining length of the sub-payload 23 being transmitted. The frame transmission unit 38 reads the main portion 24 from the transmission-data storage unit 36, and reads the error-detection code portion 25 from the error-detection-code generation unit 37, to create the own-device issued sub-payload 23. The processing time required for the frame transmission unit 38 to create the own-device issued sub-payload 23 is represented as TO. When the transmission remaining time Tl is longer than the processing time TO, where the transmission remaining time Tl is obtained by dividing the remaining length of the sub-payload 23 being transmitted by a predetermined communication speed, the frame transmission unit 38 can determine that the frame 20 is being transmitted. Due to this operation, the frame transmission unit 38 can determine that the frame 20 is being transmitted, taking into account the processing time required to create the own-device issued sub-payload 23.

[0106] It is also possible that the frame transmission unit 38 determines whether the frame 20 is being transmitted, based on the transmission-start time-point and the length of the sub-payload 23 being transmitted. The frame transmission unit 38 can determine that the frame 20 is being transmitted when the scheduled transmission-completion time-point T4 is later than the time-point obtained by adding the processing time TO to the present time-point, where the scheduled transmission-completion time-point T4 is obtained by adding the transmission time T2 to the transmission-start time-point T3 for the sub-payload 23 being transmitted, and where the transmission time T2 is obtained by dividing the length of the sub-payload 23 being transmitted by the communication speed. Due to this operation, the frame transmission unit 38 can determine that the frame 20 is being transmitted, taking into account the processing time required to create the own-device issued sub-payload 23.
[0107] Further, according to the first embodiment, the frame transmission unit 38 can determine whether the frame 20 is being transmitted, based on the remaining length of the sub-payload 23 being transmitted, or based on the transmission-start time-point and the length of the sub-payload 23 being transmitted.
[0108] When the transmission remaining time Tl is longer than the processing time TO for creating the own-device issued sub-payload 23, where the transmission remaining time Tl is obtained by dividing the remaining length of the sub-payload 23 being transmitted by the communication speed, the frame transmission unit 38 can determine that the frame 20 is being transmitted.
[0109] Further, the frame transmission unit 38 can determine that the frame 20 is being transmitted when the

scheduled transmission-completion time-point T4 is later than the time-point obtained by adding the processing time TO to the present time-point, where the scheduled transmission-completion time-point T4 is obtained by adding the transmission time T2 to the transmission-start time-point T3 for the sub-payload 23 being transmitted, and where the transmission time T2 is obtained by dividing the length of the sub-payload 23 being transmitted by the communication speed.
[0110] Due to this operation, the frame transmission unit 38 can appropriately determine whether to add the sub-payload 23 to the end of the frame 20 being transmitted, taking into account the processing time for creating the own-device issued sub-payload 23.
[0111] FIG. 13 is a sequence diagram illustrating a communicating operation of the communication system according to the first embodiment. The sequence diagram in FIG. 13 illustrates the operation of the communication system 1 during the period of performing the communication in the upstream direction 16.
[0112] At a timing tlO, the control devices SI, S2, and S3 simultaneously start the communication in the upstream direction 16.
[0113] At the timing tlO, the control device SI starts transmitting to the control device M the frame 20 including the own-device issued sub-payload 23 addressed to the control device M.
[0114] At the timing tlO, the control device S2 starts transmitting to the control device SI the frame 20 including the own-device issued sub-payload 23 addressed to the control device M.
[0115] At the timing tlO, the control device S3 starts transmitting to the control device S2 the frame 20

including the own-device issued sub-payload 23 addressed to the control device M.
[0116] At a timing til, the control device SI finishes transmitting the own-device issued sub-payload 23 addressed to the control device M. At this point in time, the control device SI has started receiving the main portion 24 of the relay sub-payload 23 issued by"the control device S2 and addressed to the control device M. Therefore, the control device SI adds this relay sub-payload 23 issued by the control device S2 and addressed to the control device M, to the end of the frame 20 being transmitted, so as to increase the length of the frame 20 being transmitted, thereby continuing the transmission of the frame 20. [0117] At a timing tl2, the control device S2 finishes transmitting to the control device SI the frame 20 including the own-device issued sub-payload 23 addressed to the control device M.
[0118] At a timing tl3, the control device SI finishes transmitting the frame 20 to the control device M, where the frame 20 includes, the own-device issued sub-payload 23 addressed to the control device M, and the relay sub-payload 23 issued by the control device S2 and addressed to the control device M.
[0119] At a timing tl4, the control device S2 starts receiving the relay sub-payload 23 issued by the control device S3 and addressed to the control device M. At a timing tl5, the control device S2 starts transmitting a new frame 20 to the control device SI, where the frame 20 includes this relay sub-payload 23 issued by the control device S3 and addressed to the control device M. [0120] At a timing tl6, the control device SI starts receiving the relay sub-payload 23 issued by the control device S3 and addressed to the control device M. At a

timing tl7,. the control device SI starts transmitting a new frame 20 to the control device M, where the frame 20 includes this relay sub-payload 23 issued by the control device S3 and addressed to the control device M. [0121] As described above, according to the first embodiment, the following effects are obtained. [0122] It is common that the maximum frame length is defined with respect to the communication physical layer. It is assumed that the control device SI combines the own-device issued sub-payload 23 addressed to the control device M, the relay sub-payload 23 issued by the control device S2 and addressed to the control device M, and the relay sub-payload 23 issued by the control device S3 and addressed to the control device M, into a single frame 20. Under this assumption, the length of the frame 20 may exceed the maximum frame length.
[0123] Meanwhile, according to the first embodiment, when the control device SI receives the relay sub-payload 23 issued by the control device S2 and addressed to the control device M, while transmitting the frame 20 including the own-device issued sub-payload 23 addressed to the control device M, the control device SI adds this relay sub-payload 23 issued by the control device S2 and addressed to the control device M, to the end of the frame 20 being transmitted, so as to increase the length of the frame 20 being transmitted, thereby continuing the transmission of the frame 20.
[0124] On the other hand, even if the control device SI receives the frame 20 including the relay sub-payload 23 issued by the control device S3 and addressed to the control device M, while transmitting the relay sub-payload 23 issued by the control device S2 and addressed to the control device M and when this length of the frame 20 being

transmitted exceeds the maximum frame length, the control device SI does not increase the length of the frame 20 being transmitted, but the control device SI transmits a new frame 20 to the control device M, where the frame 20 includes the relay sub-payload 23 issued by the control device S3 and addressed to the control device M. [0125] Therefore, according to the first embodiment, the control device SI can achieve both reduction in the communication time by an amount corresponding to the reduction in number of preambles, and prevention of the frame 2 0 from exceeding the maximum frame length. The reduction in the communication time is achieved by means of adding the relay sub-payload 23 issued by the control device S2 and addressed to the control device M, to the end of the frame 20 including the own-device issued sub-payload 23 addressed to the control device M. The prevention of the frame 20 from exceeding the maximum frame length is achieved by means of transmitting a new frame 20 including the relay sub-payload 23 issued by the control device S3 and addressed to the control device M.
[0126] Further, according to the first embodiment, the control device SI does not transmit the idle-time data from the timing tl3 to a timing tl7, differently from the
•technique described in Patent Literature 3. Therefore, an effect is obtained where the control device SI does not need to be configured to create the idle-time data. [0127] The configuration described in the embodiment described above is an example of the contents of the
present invention. The configuration can be combined with other publicly known techniques, and a part of the configuration can be omitted or modified without departing from the scope of the present invention.

Reference Signs List
[0128] 1 communication system
M, Si, S2, S3 control device.
Ma, Sla, S2a, S3a communication unit
31 frame reception unit
32 error detection unit
33 relay sub-payload storage unit
34 communication-start notification unit
35 preamble storage unit
36 transmission-data storage unit
37 error-detection-code generation unit
38 frame transmission unit

We Claim :
1. A communication device to transmit and receive a frame including a preamble and a payload, where the payload includes one or a plurality of sub-payloads, each of which includes a main portion including an address portion for storing therein a value that indicates a target-address device, a data portion for storing therein data to be transmitted to the target-address device, and a data-length portion for storing therein a value that indicates a length of the data portion, and an error-detection code portion for storing therein an error-detection code for performing error detection attributable to a transmission error, the communication device comprising:
a frame reception unit to separate the frame received from one device into a relay sub-payload that is the sub-payload not addressed to a device of its own, and an own-device addressed sub-payload that is the sub-payload addressed to a device of its own;
an error detection unit to perform error detection on the own-device addressed sub-payload based on the error-detection code within the own-device addressed sub-payload;
a relay sub-payload storage unit to store therein the relay sub-payload;
a communication-start notification unit to notify a communication-start timing to start communication simultaneously with other devices; and
a frame transmission unit to start transmitting to another device the frame including the preamble and the relay sub-payload upon being notified of the communication-start timing, when the frame is not being transmitted, and to add the relay sub-payload to an end of the frame being transmitted, so as to continue transmission of the frame, when the relay sub-payload is stored in the relay sub-

payload storage unit while the frame is being transmitted.
2. The communication device according to claim 1, further comprising:
a transmission-data storage unit to store therein the main portion created by a device of its own; and
an error-detection-code generation unit to generate the error-detection code to be added to the main portion, wherein
when the frame transmission unit is notified of the communication-start timing, and when the frame is not being transmitted, the frame transmission unit starts transmitting to another device the frame in which the preamble is combined with one or a plurality of own-device issued sub-payloads in order of the furthest address to the closest address from a device of its own, where the one own-device issued sub-payload or the own-device issued sub-payloads respectively include the main portion and the error-detection code portion, and when the own-device issued sub-payload is stored in the transmission-data storage unit while the frame is being transmitted, the frame transmission unit adds the own-device issued sub-payload to an end of the frame being transmitted, so as to continue transmission of the frame.
3. The communication device according to claim 1, wherein the frame transmission unit determines whether the frame is being transmitted, based on a remaining length of the sub-payload being transmitted.
4. The communication device according to claim 1, wherein the frame transmission unit determines whether the frame is being transmitted, based on a transmission-start time-point

and a length of the sub-payload being transmitted.
5. The communication device according to claim 1, wherein based on the address portion and the data-length portion of each of the one sub-payload or the sub-payloads, the frame reception unit separates the one sub-payload or the sub-payloads into-the relay sub-payload and the own-device addressed sub-payload.
6. A communication method for transmitting and receiving a frame including a preamble and a payload, where the
..payload includes one or a plurality of sub-payloads, each of which includes a main portion including an address portion for storing therein a value that indicates a target-address device, a data portion for storing therein data to be transmitted to a target-address device, and a data-length portion for storing therein a value that indicates a length of the data portion, and an error-detection code portion for storing therein an error-detection code for performing error detection attributable to a transmission error, the communication method comprising:
a frame receiving step of separating the frame received from one device into a relay sub-payload that is the sub-payload not addressed to a device of its own, and an own-device addressed sub-payload that is the sub-payload addressed to a device of its own;
an error detecting step of performing error detection on the own-device addressed sub-payload based on the error-detection code within the own-device addressed sub-payload;
a relay sub-payload storing step of storing the relay sub-payload;
a communication-start notifying step of notifying a

communication-start timing to start communication simultaneously with other devices; and
a frame transmitting step of starting transmitting to another device the frame including the preamble and the relay sub-payload upon being notified of the communication-start timing, when the frame is not being transmitted, and adding the relay sub-payload to an end of the frame being transmitted, so as to continue transmission of the frame, when the relay sub-payload is stored in the relay sub-payload storage unit while the frame is being transmitted.