FORM 2
THE PATENTS ACT, 1970 (39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
[See section 10, Rule 13]
REDUNDANT SYSTEM AND COMMUNICATION UNIT;
MITSUBISHI ELECTRIC CORPORATION, A CORPORATION ORGANIZED AND EXISTING UNDER THE LAWS OF JAPAN, WHOSE ADDRESS IS 7-3, MARUNOUCHI 2-CHOME, CHIYODA-KU, TOKYO 1008310 JAPAN
THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED.

DESCRIPTION
Field
[0001] The present application relates to a redundant system and a. communication unit. The redundant system is configured to include a first system that includes a control system unit and a first communication unit; a second system that includes a second communication unit connected to the first communication unit and serves as a spare device for the first system; a plurality of dependent communication units connected to the first communication unit and the second communication unit; and a user apparatus connected to the first system.
Background
[0002] For a duplex programmable controller, there is a conventional method in which, when a communication failure occurs, the control system and the standby system are switched and a programmable controller of this standby system that is now operating as a new control system is used to continue communication (for example, see Patent Literature 1) .
[0003] Further, there is a system switching method used in a duplex programmable controller in which a detected communication abnormality includes a trigger requirement for system switching and, if there is no disapproval factor with respect to system switching, the system is switched from an operation programmable controller, which is normally in an operating state, to a standby programmable controller, which is normally in a standby state, (for example, see Patent Literature 2).

Citation List Patent Literature [0004]
Patent Literature 1: Japanese Patent Application Laid-open No. 2009-217358
Patent Literature 2: Japanese Patent Application Laid-open No. 2010-146363
Summary
Technical Problem
[0005] According to the conventional technologies, by utilizing a dedicated control program corresponding to a programmable controller, it is possible to detect a communication abnormality with respect to a specific communication unit and to perform system switching in response to the detection of the communication abnormality. However, this dedicated control program involves a problem in that its creation and correction require a large work burden.
[0006] The present invention has been made in view of the above, and an object of the present invention is to provide a redundant system and a communication unit that can easily achieve system switching that is to be performed in response to the detection of a communication abnormality with respect to a specific communication unit.
Solution to Problem
[0007] In order to solve the above problems and achieve the object, an aspect of the present invention is a redundant system that comprises a first control device; a second control device connected to the first control device and serving as a spare device for the first control device;

and a plurality of dependent communication units connected to the first control device and the second control device, wherein the first control device includes a storage unit that stores parameters including information for specifying a monitoring target unit to be treated as a communication abnormality monitoring target among the dependent communication units, and data for existence confirmation regarding the dependent communication units, and a. control unit that performs system switching from the first control device to the second control device, when it is determined, on a basis of the data for existence confirmation, that there is an existence unconfirmed unit among the dependent communication units and when it is further determined, on a basis of the parameters, that the existence unconfirmed unit is the monitoring target unit.
[0008] Further, in order to solve the above problems and achieve the object, another aspect of the present invention is a redundant system configured to comprise a first system that includes a control system unit and a first communication unit; a second system that includes a second communication unit connected to the first communication unit and serves as a spare device for the first system; and a plurality of dependent communication units connected to the first communication unit and the second communication unit, wherein the first communication unit includes a storage unit that stores parameters including information for specifying a monitoring target unit to be treated as a communication abnormality monitoring target among the dependent communication units, and data for existence confirmation sequentially transmitted from the dependent communication units, and a control unit that determines, on a basis of the data for existence confirmation, whether there is an existence unconfirmed unit that -has not yet had

its existence confirmed for a certain time period or more among the dependent communication units; determines, if, as a result of determination, there is the existence unconfirmed unit, whether the existence unconfirmed unit is the monitoring target unit by referring to the parameters; and notifies, if, as a result of determination, the existence unconfirmed unit is the monitoring target unit, the control system unit of a system switching request for switching from the first system to the second system. [0009] Further, in order to solve the above problems and achieve the object, still another aspect of the present invention is a communication unit provided in a redundant system redundantly configured to comprise a first system and a second system serving as a standby system for the first system, wherein the communication unit is communicatively connected to a plurality of dependent communication units that are connected to the first system and the second system, and the communication unit comprises: a storage unit that stores parameters including information for specifying a monitoring target unit to be treated as a communication abnormality monitoring target among the dependent communication units, and data for existence confirmation sequentially transmitted from the dependent communication units; and a control unit that determines, on a basis of the data for existence confirmation, whether there is an existence unconfirmed unit that has not yet had its existence confirmed for a certain time period or more among the dependent communication units; determines, if, as a result of determination, there is the existence unconfirmed unit, whether the existence unconfirmed unit is the monitoring target unit by referring to the parameters; and determines, if, as a result of determination, the existence unconfirmed

unit is the monitoring target unit, to perform system switching from the first system to the second system.
Advantageous Effects of Invention
[0010] According to the present invention, it is possible to easily achieve system switching that is to be performed in response to the detection of a communication abnormality with respect to a specific communication unit.
Brief Description of Drawings
[0011] FIG. 1 is a schematic diagram illustrating a schematic configuration of a redundant system according to a first embodiment.
FIG. 2 is a diagram illustrating an example of a parameter setting screen set by a user apparatus according to the first embodiment.
FIG. 3 is a diagram illustrating an example of an internal configuration of devices included in the redundant system according to the first embodiment.
FIG. 4 is a diagram illustrating a flow of processes in the redundant system according to the first embodiment.
FIG. 5 is a diagram illustrating an example of a communication failure occurrence position.
FIG. 6 is a diagram illustrating an example of the configuration of parameters set by a user apparatus according to a second embodiment.
FIG. 7 is a flowchart illustrating a flow of processes in a master communication unit according to the second embodiment.
FIG. 8 is a diagram illustrating a flow of processes in the redundant system according to a third embodiment.
FIG. 9 is a diagram illustrating an example of the configuration of a diagnostic information screen according

to the third embodiment.
Description of Embodiments
[0012] First embodiment.
Exemplary embodiments of a redundant system and a communication unit according to the present invention will be explained below with reference to the accompanying drawings. The present invention is not limited to the following embodiments.
[0013] FIG. 1 is a schematic diagram illustrating a. schematic configuration of a redundant system 1000 according to the first embodiment. As illustrated in FIG. 1, the redundant system 1000 according to the first-embodiment has a duplex redundant configuration that includes a master system 10 that serves as a control system and a sub-master system 20 that is a standby system and serves as a spare device for the master system 10. The redundant system 1000 according to the first embodiment may be configured to include network units line-connected to each other, or may be configured to include network units line-connected to each other and network units star-connected to each other.
[0014] Further, as illustrated in FIG. 1, the redundant system 1000 according to the first embodiment is configured to include a local system 30, a local system 40, and a local system 50, which are connected to the master system 10 and the sub-master system 20, and a user apparatus 1 connected to the master system 10.
[0015] The master system 10 is configured to include a programmable controller 11 that takes in various inputs and controls the operation of connected devices in accordance with preset conditions; a tracking-use communication unit 12 that synchronizes control information with the sub-

master system 20 serving as a standby system; and a master communication unit 13 that controls communication with the local system 30, the local system 40, and the local system 50. The master system 10 is an example of a first system. The programmable controller 11 is an example of a control system unit. The master communication unit 13 is an example of a first communication unit.
[0016] The sub-master system 20 is configured to include a programmable controller 21 that takes in various inputs and controls the operation of connected devices in accordance with preset conditions; a tracking-use communication unit 22 that synchronizes control information with the master system 10 serving as a control system; and a sub-master communication unit 23 that controls communication with the local system 30, the local system 40, and the local system 50. The sub-master system 20 is an example of a second system. The sub-master communication unit 23 is an example of a second communication unit. [0017] The local system 30 is configured to include a programmable controller 31 that takes in various inputs and controls the operation of connected devices in accordance with preset conditions; and a communication unit 32 that controls communication with the master system 10 or the sub-master system 20. The local system 40 is configured to include a programmable controller 41 that takes in various inputs and controls the operation of connected devices in accordance with preset conditions; and a communication unit 42 that controls communication with the master system 10 or the sub-master system 20. The local system 50 is configured to include a programmable controller 51 that takes in various inputs and controls the operation of connected devices in accordance with preset conditions; and a communication unit 52 that controls communication with

the master system 10 or the sub-master system 20. The communication unit 32, the communication unit 42, and the communication unit 52 are connected to the master communication unit 13 and the sub-master communication unit 23. Each of the communication unit 32, the communication unit 42, and the communication unit 52 is an example of a dependent communication unit.
[0018] Each of the master communication unit 13. and the sub-master communication unit 23 holds parameters 5 set by a user in the user apparatus 1. The parameters 5 include information for specifying a monitoring target unit to be treated as a communication abnormality monitoring target among the communication unit 32, the communication unit 42, and the communication unit 52; information regarding operation settings for each of the communication units; and the communication area range. The programmable controller
11 downloads the parameters 5 set by the user in the user apparatus 1 and transfers, the parameters 5 to the master communication unit 13. The master communication unit 13 holds the parameters 5 transferred from the programmable controller 11. The master communication unit 13 transfers the parameters 5 through a network to the sub-master communication unit 23. The sub-master communication unit 23 holds the parameters 5 transferred from the master communication unit 13. The tracking-use communication unit
12 and the tracking-use communication unit 22 perform tracking communication for attaining synchronization between control information on the programmable controller 11 and control information on the programmable controller 21.
[0019] Further, as illustrated in FIG. 1, the master system 10 receives data 30a for existence confirmation, data 40a for existence confirmation, and data 50a for

existence confirmation, each of which is for giving notice that the unit is in a communicable state, respectively from the communication unit 32, the communication unit 42, and the communication unit 52. The communication unit 32, the communication unit 42, and the communication unit 52 sequentially transmit the data 30a for existence confirmation, the data 40a for existence confirmation, and the data 50a for existence confirmation, at transmission timing set in their own stations. The communication unit 32, the communication unit 42, and the communication unit 52 transmit the data 30a for existence confirmation, the data 40a for existence confirmation, and the data 50a for existence confirmation, separately from control data. Here, in the example illustrated in FIG. 1, the sub-master communication unit 23 also transmits data 20a for existence confirmation to the master communication unit 13, as in the • communication unit 32, the communication unit 42, and the communication unit 52.
[0020] FIG. 2 is a diagram illustrating an example of a parameter setting screen set by the user apparatus according to the first embodiment. The user apparatus 1 displays a parameter setting screen 60 for setting the parameters 5. The parameter setting screen 60 is configured to include a setting item 61 in which the number of communication units included in the redundant system 1000 is set; a setting item 62 in which the type name of each communication unit is set; a setting item 63 in which the station number of each network unit is set; a setting item 64 in which the station type of each network unit is set; a setting item 65 in which a monitoring target station for system switching is set; and a setting item 66 in which a monitoring priority is set. In the example illustrated in FIG. 2, the local station at the station number "2" is

set as a monitoring target station for system switching.
[0021] The master station at the station number "0" illustrated in FIG. 2 corresponds to the master communication unit 13 illustrated in FIG. 1. The local station at the station number "1" illustrated in FIG. 2 corresponds to the communication unit 32 illustrated in FIG. 1. The local station at the station number "2" illustrated in FIG. 2 corresponds to the communication unit 42 illustrated in FIG. 1. The local station at the station number "3" illustrated in FIG. 2 corresponds to the communication unit 52 illustrated in FIG. 1. The sub-master station at the station number "4" illustrated in FIG. 2 corresponds to the sub-master communication unit 23 illustrated in FIG. 1.
[0022] FIG. 3 is a diagram illustrating an example of an internal configuration of devices included in the redundant system 1000 according to the first embodiment. In this respect, because the master system 10 and the sub-master system 20 basically have the same configuration, an explanation will be given only of the configuration of the master system 10 as an example, hereinafter. [0023] As illustrated in FIG. 3, the programmable controller 11 of the master system 10 includes a microprocessor 11a that performs processes corresponding to various kinds of control in the master system 10; a memory unit lib that stores various programs and data for implementing processes to be performed by the microprocessor 11a; and a bus interface lie connected to an internal bus. The microprocessor 11a performs processes corresponding to various kinds of control in the master system 10. The memory unit lib stores various programs and data for implementing processes to be performed by the microprocessor 11a. The memory unit lib is also utilized

as a work area for temporarily storing process results obtained by the microprocessor 11a. For example, the memory unit lib is configured as a ROM (Read Only Memory), a RAM (Random Access Memory), storage, a detachable memory device, or a combination thereof. The bus interface lie serves as an interface for connecting the tracking-use communication unit 12 and the master communication unit 13. Further, the bus interface lie serves as an interface for connecting to the user apparatus 1 via a cable 200a. [0024] As illustrated in FIG. 3, the tracking-use communication unit 12 includes a tracking memory 12b utilized for tracking communication; and a bus interface 12c connected to the internal bus and a cable 200b. The tracking memory 12b serves as a buffer when communication is performed to attain synchronization between control information on the programmable controller 11 and control information on the programmable controller 21. The bus interface 12c serves as an interface for connecting to the programmable controller 11. Further, the bus interface 12c serves as an interface for connecting to the tracking-use communication unit 22 via the cable 200b. [0025] As illustrated in FIG. 3, the master communication unit 13 includes a microprocessor 13a that performs processes corresponding to various kinds of control in the master communication unit 13; a memory unit 13b that stores various programs and data for implementing processes to be performed by the microprocessor 13a; a bus interface 13c; and a network interface 13d. The master communication unit 13 executes programs corresponding to various kinds of control by use of the microprocessor 13a and the memory unit 13b, which are hardware resources, and thereby achieves processes corresponding to various kinds of control. For example, the master communication unit 13

reads a program stored in the memory unit 13b, loads it into a work area assigned to the memory unit 13b, and causes the microprocessor 13a to execute commands included in the loaded program. Then, the master communication unit 13 performs processes corresponding to various kinds of control in accordance with the results of the commands executed by the microprocessor 13a.
[0026] As illustrated in FIG. 3, the memory unit 13b stores a system switching determination program dl, parameters d2, data d3 for existence confirmation, and system switching information d4. The parameters d2 correspond to the parameters 5 described above. [0027] The system switching determination program dl provides various functions described below that are for achieving processes concerning system switching. Specifically, on the basis of the data d3 for existence confirmation, the system switching determination program dl determines whether there is an existence unconfirmed unit that has not yet had its existence confirmed for a certain time period or more among the communication unit 32, the communication unit 42, and the communication unit 52. If, as a result of the determination, there is an existence unconfirmed unit, the system switching determination program dl refers to the parameters d2 and determines whether this existence unconfirmed unit is a monitoring target station. If, as a result of the determination, the existence unconfirmed unit is a monitoring target station, the system switching determination program dl notifies the programmable controller 11 of a system switching request for switching from the master system 10 to the sub-master system 20. Further, upon making a determination to perform the system switching, the system switching determination program dl stores system switching information d4 regarding

the system switching in the memory unit 13b. [00281 The parameters d2 stored in the memory unit 13b correspond to the parameters 5 set by a user using the user apparatus 1. The data d3 for existence confirmation stored in the memory unit 13b is configured to include the data 30a for existence confirmation, the data 40a for existence confirmation, and the data 50a for existence confirmation sequentially transmitted from the communication unit 32, the communication unit 42, and the communication unit 52. The system switching information d4 is information regarding system switching. The system switching information 4d includes a system switching enforcement time point, a communication abnormal station, a system switching station, and a system switching factor. The system switching information is stored by the microprocessor 13a. [0029] As illustrated in FIG. 3, the user apparatus 1 includes a microprocessor la, a memory unit lb, a display lc, and a bus interface Id. The microprocessor la performs processes corresponding to various kinds of control in the user apparatus .1. The memory unit lb stores various programs and data for implementing processes to be performed by the microprocessor ■ la. The display lc displays various kinds of information.
[0030] The memory unit lb stores a parameter setting program d5. The parameter setting program d5 provides a function for setting the parameters 5 that includes information for specifying a monitoring target unit to be treated as a communication abnormality monitoring target among the communication unit 32, the communication unit 42, and the communication unit 52; information regarding operation settings for each of the communication units; and the communication area range. The parameter setting program d5 displays a parameter setting screen, as

illustrated in FIG. 2, on the display 1c, so that a user can set the parameters 5.
[0031] Next, with reference to FIG, 4, an explanation will be given of the flow of processes in the redundant system 1000 according to the first embodiment. FIG. 4 is a diagram illustrating a flow of processes in the redundant system 1000 according to the first embodiment. [0032] As illustrated in FIG. 4, the master communication unit 13 performs reception confirmation of data for existence confirmation (step Sll). Subsequently, the master communication unit 13 determines whether there is an existence unconfirmed unit that has not yet had its existence confirmed for a certain time period or more (step S12) .
[0033] If, as a result of the determination, there is no existence unconfirmed unit that has not yet had its existence confirmed for a certain time period or more (No at step S12), the master communication unit 13 returns to the process sequence of the above step Sll and continues reception confirmation of data for existence confirmation. In contrast, if, as a result of the determination, there is an existence unconfirmed unit that has not yet had its existence confirmed for a certain time period or more (Yes at step S12), the master communication unit 13 determines whether this communication unit is a monitoring target station (step S13).
[0034] If, as a result of the determination, the existence unconfirmed unit detected in the step S12 is not a monitoring target station (No at step S13), the master communication unit 13 returns to the process sequence of the above step Sll and continues reception confirmation of data for existence confirmation. In contrast, if, as a result of the determination, the existence unconfirmed unit

detected in the step S12 is a monitoring target station (Yes at step S13), the master communication unit 13 gives notice of a system switching request to each of the tracking-use communication unit 12 and the programmable controller 11 (step S14). For example, the system switching request sent from the master communication unit 13 is sent to each of an interruption accepting area pre-assigned to the tracking memory 12b of the tracking-use communication unit 12 and an interruption accepting area assigned to the memory unit lib of the programmable controller 11. The tracking-use communication unit 12 detects the system switching request by reading the interruption accepting area of the tracking memory 12b. The programmable controller 11 detects the system switching request by reading the interruption accepting area of the memory unit lib.
[0035] Subsequently, the master communication unit 13 stores system switching information (step S15). [0036] Upon receiving the system switching request from the master communication unit 13, the tracking-use communication unit 12 transmits a system switching notice to the tracking-use communication unit 22 of the sub-master system 20 (step S16).
[0037] Upon receiving the system switching request from the master communication unit 13, the programmable controller 11 performs system switching to change the sub-master system 20 serving as a spare device from a standby state into an operating state (step S17).
[0038] Upon receiving the system switching notice from the tracking-use communication unit 22, the tracking-use communication unit 22 sends the system switching notice to each of the programmable controller 21 and the sub-master communication unit 23 (step S18).

[0039] Upon receiving the system switching notice from the tracking-use communication unit 22, the programmable controller 21 performs system switching to shift from a standby state into an operating state (step S19). [0040] As described above, in the redundant system 1000 according to the first embodiment, when detecting a communication abnormality with respect to the communication unit 42 set as a monitoring target station, the master communication unit 13 automatically gives notice of a system switching request. Consequently, in the first embodiment, it is possible to easily achieve the system switching that is to be performed in response to the detection of a communication abnormality with respect to a specific communication unit without creating a dedicated control program for performing system switching by detecting a communication abnormality with respect to a communication unit treated as a monitoring target station, and then executing this dedicated control program by using the programmable controller 11.
[0041] FIG. 5 is a diagram illustrating an example of a communication failure occurrence position. In the example illustrated in FIG. 5, a communication failure CF1 has occurred due to a cable disconnection or the like between the master system 10 and the local system 42. In this case, the data 40a for existence confirmation transmitted from the communication unit 42 is not received by the master communication unit 13. Consequently, the master communication unit 13 cannot confirm the existence of the communication unit 42 treated as a monitoring target station, and so it is determined to perform system switching to continue communication with the communication unit 42 by using the sub-master communication unit 23. In this way, according to the first embodiment, it is possible

to continue communication with a specific communication unit treated as a monitoring target station on the basis of parameters set by the user apparatus 1.
[0042] Further, according to the first embodiment, the processing capacity of the programmable controller 11 is not utilized for performing monitoring of the communication unit 42 treated as a monitoring target station. Consequently, when compared with a case where the programmable controller 11 is used for performing monitoring of the communication unit 42 treated as a monitoring target station, it is possible to reduce the load on the programmable controller 11 and thereby to increase the processing capacity of the master system 10 as a whole. Further, according to the first embodiment, along with notice of a system switching request, the master communication unit 13 stores a system switching enforcement time point, a communication abnormal station, a system switching station, and a system switching factor in the memory unit 13b. Consequently, it is possible to utilize them for system maintenance and troubleshooting. [0043] Second embodiment.
Hereinafter, in the second embodiment, an explanation will be given of the processes of the master communication unit 13 in a case where a plurality of communication units are set as monitoring target stations.
[0044] FIG. 6 is a diagram illustrating an example of a parameter setting screen set by a user apparatus according to the second embodiment. In the second embodiment, a plurality of communication units are set as monitoring target stations on the parameter setting screen 60. In the example illustrated in FIG. 6, each of the local station at the station number "1" and the local station at the station number "2" is set as a monitoring target station. Further,

in the example illustrated in FIG. 6, the local station at the station number "2" is set with a monitoring priority "1", and the local station at the station number "1" is set with a monitoring priority "2".
[0045] A system switching determination program provides ai function for performing processes corresponding to a plurality of monitoring target stations in addition to the function described in the first embodiment. Specifically, the system switching determination program provides a function such that, when an existence unconfirmed unit is a monitoring target station, it determines whether there are a plurality of monitoring target stations and, when there are a plurality of monitoring target stations, it makes a determination to perform system switching on the monitoring target stations starting from one having a higher monitoring priority.
[0046] Next, with reference to FIG. 7, an explanation will be given of the flow of processes in the master communication unit according to the second embodiment. FIG. 7 is a flowchart illustrating the flow of processes in the master communication unit according to the second embodiment. The processes illustrated in FIG. 7 are achieved by the microprocessor 13a executing a system switching program dl stored in the memory unit 13b. [0047] As illustrated in FIG. 7, the master communication unit 13 performs reception confirmation of data for existence confirmation (step S101). Subsequently, the master communication unit 13 determines whether there is an existence unconfirmed unit that has not yet had its existence confirmed for a certain time period or more (step S102) .
[0048] If, as a result of the determination, there is no existence unconfirmed unit that has not yet had its

existence confirmed for a certain time period or more (No at step S102), the master communication unit 13 returns to the process sequence of the above step S101 and continues reception confirmation of data for existence confirmation. In contrast, if, as a result of the determination, there is an existence unconfirmed unit that has not yet had its existence confirmed for a certain time period or more (Yes at step S102), the master communication unit 13 determines whether this communication unit is a monitoring target station (step 3103).
[0049] If, as a result of the determination, the existence unconfirmed unit detected in the step S102 is not a monitoring target station (No at step S103), the master communication unit 13 returns to the process sequence of the above step S101 and continues reception confirmation of data for existence Confirmation. In contrast, if, as a result of the determination, the existence unconfirmed unit detected in the step S102 is a monitoring target station (Yes at step S103), the master communication unit 13 determines whether there are a plurality of existence unconfirmed units corresponding to monitoring target stations (step S104). Specifically, the master communication unit 13 determines whether there are a plurality of units each of which corresponds to an existence unconfirmed unit detected in the step S102 and also corresponds to a monitoring target station. [0050] If, as a result of the determination, there are a plurality of units each of which corresponds to an existence unconfirmed unit and also corresponds to a monitoring target station (Yes at step S104), the master communication unit 13 gives notice of a system switching request regarding a communication unit having the highest monitoring priority among the communication units each of

which is an existence unconfirmed unit and also a monitoring target station (step S105), and it stores system switching information corresponding to the system switching request made in the step S105 (step S106). [0051] Subsequently, the master communication unit 13 gives notice of a system, switching request regarding a communication unit having the second highest monitoring priority (step S107), and it stores system switching information corresponding to the system switching request made in the step S107 (step S108) .
[0052] Subsequently, the master communication unit 13 determines whether a communication unit that needs system switching is left (step S109). In other words, the master communication unit 13 determines, for all the communication units detected as a result of the determination in the step S104, whether it has finished giving notice of a system switching request.
[0053] If, as a result of the determination, a communication unit that needs system switching is left (Yes at step S109), the master communication unit 13 returns to the process sequence of the above step S107. In contrast, if, as a result of the determination, a communication unit that needs system switching is not left (No at step S109), the master communication unit 13 ends the processes illustrated in FIG. 7.
[0054] In the above step S104, if, as a result of the determination, there, are no two or more units each of which corresponds to an existence unconfirmed unit and also corresponds to a monitoring target station, i.e., when there is only one such unit (No at step S104), the master communication unit 13 gives notice of a system switching request regarding a communication unit that is an existence unconfirmed unit detected in the step S102 (step S110),

stores system switching information (step Sill), and then ends the processes illustrated in FIG. 7. [0055] As described above, according to the second embodiment, when there are a plurality of communication units each of which is an existence unconfirmed unit and also a monitoring target station, the master communication unit 13 gives notice of a system switching request regarding each of the communication units starting sequentially from one having a higher monitoring priority. Thus, according to the second embodiment, it is possible to treat a plurality of communication units as monitoring targets. Further, according to the second embodiment, among a plurality of communication units treated as monitoring targets, it is possible to recover the communication abnormality with respect to each of the communication units starting sequentially from one having a higher monitoring priority. [0056] Third embodiment.
Hereinafter, in the third embodiment, an explanation will be given of processes for displaying system switching information on the user apparatus 1. FIG. 8 is a diagram illustrating the flow of processes•in the redundant system 1000 according to the third embodiment. [0057] As illustrated in FIG. 8, upon receiving a request for displaying a diagnostic information screen (step S51), the user apparatus 1 transmits a- request for acquiring system switching information to the master system 10 (step S52).
[0058] Upon receiving the request for acquiring system switching information from the user apparatus 1, the programmable controller 11 of the master system 10 sends the request for acquiring system switching information to the master communication unit 13 (step S53).

[0059] Upon receiving the request for acquiring system switching information from the programmable controller 11, the master communication unit 13 reads system switching information d4 from the memory unit 13b (step S54). Subsequently, the master communication unit 13 sends the system switching information d4, which has been read, to the programmable controller 11 (step S55).
[0060] Upon receiving the system switching information d4 from the master communication unit 13, the programmable controller 11 transmits the system switching information d4 to the user apparatus 1 (step S56).
[0061] Upon receiving the system switching information d4 from the master system 10, the user apparatus 1 creates diagnostic information on the basis of the received system switching information d4, and outputs and displays a diagnostic information screen onto the display lc (step S57) .
[0062] .. FIG. 9 is a diagram illustrating an example of the configuration of a diagnostic information screen according to the third embodiment. As illustrated in FIG. 9, diagnostic information screen 90 is configured to include a display area 91 for displaying configuration information regarding the redundant system 1000 at the time of occurrence of a communication abnormality and configuration information regarding the current redundant system 1000; and a display area 92 for displaying the operational state of a communication unit selected by a user in the display area 91. In the example illustrated in FIG. 9, the system configuration is shown by using a model formed of icons representing communication units and straight lines representing cables in the display area 91 and the display area 92. Further, as illustrated in FIG. 9, the diagnostic information screen 90 has a multi-window

configuration that utilizes a pop-up screen 93 to display detailed information about a communication unit corresponding to an icon selected by a user in the display area 91 or the display area 92. The pop-up screen 93 displays the cause of a communication abnormality with respect to the selected communication unit. [0063] As described above, according to the third embodiment, because the diagnostic information screen 90 is displayed on the user apparatus 1, it is possible to allow a user to efficiently perform system maintenance and troubleshooting.
[0064] In the redundant system 1000 according to the above embodiments, the master communication unit 13 has been described by taking an example that executes programs corresponding to various kinds of control by using the microprocessor 13a and the memory unit 13b, which are hardware resources, and thereby achieves processes corresponding to various kinds of control. However, the way of achieving various kinds of control is not limited to this example. Specifically, a wired logic including a plurality of process circuits linked to each other, such, as an ASIC (Application Specific Integrated Circuit) and an FPGA (Field Programmable Gate Array) that correspond to programs stored in the memory unit 13b, may be used to achieve these various kinds of control.
[0065] In the above embodiments, the master system 10 has been described by taking an example in which the programmable controller 11 and the master communication unit 13 are configured such that they are functionally distributed. However, this is not a limitation, and the function to be achieved by the master communication unit 13 may be implemented in the programmable controller 11. Specifically, the programmable controller 11 may detect a

communication abnormality with respect to the communication unit 42 set as a monitoring target station and proactively determine whether to perform system switching. In this case, the programmable controller 11 is regarded as an example of a first control device. Further, the master communication unit 13 described in the above embodiments may be configured in a detachable state in an arbitrary redundant system redundantly configured to include an operation system and a standby system. In this case, the master communication unit 13 is regarded as an example of a communication unit.
[0066] The configurations illustrated in the above embodiments are mere examples of the content of the present invention, and they may be combined with other known technologies. Further, the configurations may be partly omitted or changed without departing from the spirit of the present invention.
Reference Sighs List
[0067] 1 user apparatus, 10 local system, 11 programmable controller, 12 tracking-use communication unit, 13 master communication unit, 20 sub-master system, 21 programmable controller, 22 tracking-use communication unit, 23 sub-master communication unit, 30 local system, 31 programmable controller, 32 communication unit, 40 local system, 41 programmable controller, 42 communication unit, 50 local system, 51 programmable controller, 52 communication unit, 100 network, 200a, 200b cable.

We Claim :
1. A redundant system that comprises a first control device; a second control device connected to the first control device and serving as a spare device for the first ) control device; and a plurality of dependent communication units connected to the first control device and the second control device, wherein
the first control device includes
a storage unit that stores parameters including information for specifying a monitoring target unit to be treated as a communication abnormality monitoring target among the dependent communication units, and data for existence confirmation regarding the dependent communication units, and
a control unit that performs system switching from the first control device to the second control device, when it is determined, on a basis of the data for existence confirmation, that there is an existence unconfirmed unit among the dependent communication units and when it is further determined, on a basis of the parameters, that the existence unconfirmed unit is the monitoring target unit.
2. A redundant system configured to comprise a first system that includes a control system unit and a first communication unit; a second system that includes a second communication unit connected to the first communication unit and serves as a spare device for the first system; and a plurality of dependent communication units connected to the first communication unit and the second communication unit, wherein
the first communication unit includes
a storage unit that stores parameters including information for specifying a monitoring target unit to be

treated as a communication abnormality monitoring target among the dependent communication units, and data for existence confirmation sequentially transmitted from the dependent communication units, and
a control unit that determines, on a basis of the data for existence confirmation, whether there is an existence unconfirmed unit that has not yet had its existence confirmed for a certain time period or more among the dependent communication units; determines, if, as a result of determination, there is the existence unconfirmed unit, whether the existence unconfirmed unit is the monitoring target unit by referring to the parameters; and notifies, if, as a result of determination, the existence unconfirmed unit is the monitoring target unit, the control system unit of a system, switching request for switching from the first system to the second system.
3. The redundant system according to claim 2, wherein
the storage unit stores, as the parameters, parameters
including monitoring priorities corresponding to the monitoring target units, and
when there are a plurality of existence unconfirmed units and the existence unconfirmed units are respectively the monitoring target units, the control unit determines to perform, on a basis of the monitoring priorities included in the parameters, system switching on the monitoring target units starting from a monitoring target unit having a higher monitoring priority.
4. The redundant system according to claim 2 or 3,
wherein, when making a determination to perform the system
switching, the control unit stores system switching
information regarding the system switching in the storage

unit.
5. The redundant system according to claim 4, further
comprising a user apparatus connected to the first system,
wherein
upon receiving a transmission request for the system switching information from the user apparatus, the control unit transmits the system switching information to the user apparatus, and
the user apparatus displays the system switching information received from the first system on a display.
6. A communication unit provided in a redundant system
redundantly configured to comprise a first system and a
second system serving as a standby system for the first
system, wherein
the communication unit is communicatively connected to a plurality of dependent communication units that are connected to the first system and the second system, and
the communication unit comprises:
a storage unit that stores parameters including information for specifying a monitoring target unit to be treated as a communication abnormality monitoring target among the dependent communication units, and data for existence confirmation sequentially transmitted from the dependent communication units; and
a control unit that determines, on a basis of the data for existence confirmation, whether there is an existence unconfirmed unit that has not yet had its existence confirmed for a certain time period or more among the dependent communication units; determines, if, as a result of determination, there is the existence unconfirmed unit, whether the existence unconfirmed unit is the

monitoring target unit by referring to the parameters; and determines, if, as a result of determination, the existence unconfirmed unit is the monitoring target unit, to perform system switching from the first system to the second system.