Patent Publication Number: US-7912558-B2

Title: PLC for distributed control and distributed control system

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a PLC (programmable controller) for distributed control and a distributed control system including a plurality of PLCs for distributed control connected to one another. 
     Recently, in order to control mechanical equipments positioned at a plurality of places for factory automation, a distributed control system including a plurality of PLCs controlling mechanical equipments connected to one another has been known (for example, see Patent Document 1). 
     Such a distributed control system according to the related art uses a method in which adjacent PLCs are connected to each other by a plurality of I/O wiring lines and a plurality of signals are transmitted through the I/O wiring lines. Since safety PLCs for securing the safety of control objects are required to correctly transmit a plurality of control signals, such as interlock signals, according to a plurality of control objects to one another, the above-mentioned method is very effective. 
     Patent Document 1: JP-A-2002-358106 
     However, in the above-mentioned method, an operator is required to manually check the connection between the PLC terminals connected to the individual I/O wiring lines in order to determine whether communication by each wiring line is correctly performed, resulting in a low maintenance property. Further, since the plurality of I/O wiring lines should be provided between the PLCs, the number of PLCs increases, and thus the cost increases. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a PLC for distributed control that improves the maintenance property while securing the safety of the communication with another PLC constituting a distributed control system. 
     Another object of the invention is to provide a distributed control system that improves the maintenance property while securing the safety of the communication between PLCs for distributed control. 
     According to a first aspect of the invention, a PLC for distributed control that constitutes a distributed control system together with another PLC includes: a storage unit that stores common-data specifying information for specifying common data shared by a corresponding PLC and another PLC; a receiving unit that receives the common-data specifying information from another PLC; and a collating unit that collates the common-data specifying information stored in the storage unit with the common-data specifying information received by the receiving unit. In the PLC for distributed control, it is possible to automatically, not manually, determine whether the common data is correctly transmitted between the corresponding PLC for distributed control and another PLC. Therefore, it is possible to secure the safety of communication between the PLC for distributed control and another PLC and to improve a maintenance property. 
     In the PLC for distributed control according to a first aspect, the collating unit may perform the collation of the common-data specifying information when the distributed control system starts or restarts. Therefore, it is possible to secure the safety of the communication between the PLC for distributed control and another PLC whenever the distributed control system starts or restarts. 
     Further, in the PLC for distributed control according to the first aspect, the common-data specifying information may include information on an address where the common data is assigned in a frame used for communication of the distributed control system. For this reason, even though a plurality of common data are assigned to a plurality of addresses of one frame and are simultaneously transmitted thereto, it is possible to secure the safety of the transmission on the basis of the above-mentioned theory. Therefore, it is possible to improve the maintenance property and to reduce the cost by reducing the number of signal lines while securing the safety of the communication between the PLC for distributed control and another PLC. 
     Furthermore, the PLC for distributed control according to the first aspect may constitute the distributed control system together with a plurality of PLCs. In this case, the common-data specifying information may include identification information for identifying the plurality of PLCs. With this construction, it is possible to determine which of the plurality of PLCs the common-data specifying information is received from. Therefore, the PLC for distributed control can accurately perform the collation for the common-data specifying information received from other PLCs by using the collating unit. 
     According to a second aspect of the invention, a distributed control system includes a plurality of PLCs for distributed control according to the first aspect connected to one another. In the distributed control system, the plurality of PLCs for distributed control are mutually cooperated with one another. Therefore, it is possible to secure the safety of the communication among the PLCs for distributed control and to remarkably improve the maintenance property of the overall distributed control system. 
     In the distributed control system according to the second aspect, each of the PLCs for distributed control may be a safety PLC for ensuring the safety of a control object connected to the corresponding PLC for distributed control. Since the safety of the communication among the PLCs for distributed control is secured as described above, it is possible to correctly transmit the common data related to the safety of the control objects among the PLCs for distributed control. Therefore, it is possible to secure the safety of all the control objects connected to each of the PLCs for distributed control with high reliability. 
     Further, in the distributed control system according to the second aspect, the common-data specifying information may be input to each of the PLCs for distributed control together with a program to be executed by the corresponding PLC for distributed control. With this construction, the sequence program to be executed by each of the PLCs for distributed control and the common-data specifying information can be freely changed in response to the requirement of the system specification, thereby expanding the versatility of the distributed control system. 
     Furthermore, in the distributed control system according to the second aspect, the common-data specifying information may be associated with each other and may be output on a ladder diagram representing the content of programs to be executed by the PLCs for distributed control. With this construction, an operator of the distributed control system can check the common-data specifying information among the PLCs for distributed control together with the ladder diagram with eyes, thereby further improving the maintenance property. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flow chart illustrating an initial operation according to a first embodiment of the invention. 
         FIG. 2  is a block diagram showing a distributed control system according to the first embodiment and a second embodiment. 
         FIGS. 3A to 3C  are schematic diagrams illustrating the operation of the distributed control systems according to the first and second embodiments. 
       FIG.  4 A 1  to  4 C are schematic diagrams illustrating the operation of the distributed control system according to the second embodiment. 
         FIG. 5  is a flow chart illustrating an initial operation according to the second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereinafter, a plurality of embodiments of the invention will be described with reference to the accompanying drawings. 
     First Embodiment 
       FIG. 2  shows a distributed control system according to a first embodiment of the invention. A distributed control system  1  is a network system including a plurality of safety PLCs connected in a ring topology. The distributed control system  1  performs distributed control on a plurality of output devices  4  on the basis of input information from a plurality of input devices  2  so as to secure the safety of the output devices  4 . 
     In the distributed control system  1 , the maximum number of safety PLCs capable of being connected to the distributed control system  1  is set to 24. In this embodiment, three safety PLCs  10 ,  11 , and  12  are connected to the distributed control system  1 . Each of a plurality of signal lines  14  connecting the safety PLCs  10 ,  11 , and  12  is an optical fiber line or a conductive line. Communication among the safety PLCs  10 ,  11 , and  12  through the signal lines  14  uses frames each of which is composed of a bit string with a predetermined bit length. Here, a bit address is assigned as address information to bit data constituting one frame by the operation of a communication module  30  to be described below. In this embodiment, frame data is received and transmitted between two of the safety PLCs  10 ,  11 , and  12 . In other words, the distributed control system  1  according to the first embodiment is a broadcast network system. 
     Each of the safety PLCs  10 ,  11 , and  12  constituting the distributed control system  1  has a CPU module  20 , a communication module  30 , and an I/O module  40 . 
     The CPU module  20  includes a microcomputer as the main constituent and is connected to the communication module  30  through a bus  21 . A memory  22  of the CPU module  20  stores a sequence program written in a ladder diagram language. The CPU module  20  controls reception and transmission of control instructions or data through the bus  21  by allowing a CPU  24  to execute the sequence program. Further, in this embodiment, the CPU module  20  of the safety PLC  10  instructs the communication module  30  of the safety PLC  10  to output a control instruction for controlling communication between the safety PLCs  11  and  12 , thereby controlling the communication of the whole system. 
     The communication module  30  includes a microcomputer as the main constituent. The communication module  30  is connected to the CPU module  20  through the bus  21 , and is also connected to the I/O module  40  through a bus  31 . The communication module  30  has an interface  32  connected to the signal lines  14  between the communication module  30  and the other PLCs. A memory  34  of the communication module  30  stores a communication program. The communication module  30  executes the communication program by a CPU  36  so as to control reception and transmission of data or control instructions through the buses  21  and  22  and the interface  32  according to a control instruction received from the CPU module  20 . 
     The I/O module  40  is connected to an emergency button, a predetermined number of input devices  2  such as safety sensors, a motor, and a predetermined number of output devices  4  such as robots. The I/O module  40  transmits data indicating the state of the input devices  2  to the communication module  30 . Further, the I/O module  40  performs, for example, power supply control on the output devices  4  according to the control instruction received from the communication module  30 . 
     In the distributed control system  1  according to the first embodiment, I/O devices  50 ,  51 , and  52  are connected to interfaces  26  provided in the CPU modules  20  of the safety PLCs  10 ,  11 , and  12 , respectively. Each of the I/O devices  50 ,  51 , and  52  has a computer  53 , an input device  54 , a monitor  55 , and a printer  56 . 
     An operator inputs, to a computer, exchange numbers of safety PLCs connected to the corresponding computer  53  by operating the input device  54  such as a keyboard or a mouse. Here, the exchange numbers are identification information for identifying the safety PLCs  10 ,  11 , and  12 , and exchange numbers ‘ 00 ’, ‘ 01 ’, and ‘ 02 ’ are set to the safety PLCs  10 ,  11 , and  12  in this embodiment, respectively. 
     Further, the operator inputs, to the computer  53 , a sequence program to be executed by the CPU module  20  of the safety PLC connected to the corresponding computer  53 . Here, the sequence program is a program according to a ladder diagram describing a data reception/transmission sequence by the PLC connected to the computer  53 , for example, a program according to one of  FIGS. 3A to 3C  corresponding to the exchange number of the PLC connected to the computer  53 . 
     In the ladder diagrams shown in  FIGS. 3A to 3C , a parameter ‘d-a’ associated with a coil represents an exchange number ‘d’ (one of  00  to  02 ) of the destination PLC of the transmission data and a bit address ‘a’ (one of L 00  to L 7 F) of the transmission data in the frame. Further, in the ladder diagrams shown in  FIGS. 3A to 3C , a parameter ‘D-A’ associated with a contact point represents an exchange number ‘D’ (one of  00  to  02 ) of the destination PLC of the reception data and a bit address ‘A’ (one of L 00  to L 7 F) of the reception data in the frame. Therefore, the parameters ‘d-a’ and ‘D-A’ can be considered as common-data specifying information for specifying data to be shared by the safety PLCs  10 ,  11 , and  12 . In this embodiment, the operator inputs the common-data specifying information together with the sequence program to the computer  53  by operating the input device  54 . 
     The exchange number, the sequence program, and the common-data specifying information (the parameters ‘d-a’ and ‘D-A’) input to the computer  53  as described above are stored in a memory  57  of the computer  53 . Then, the exchange number, the sequence program, and the common-data specifying information are input from the computer  53  to the CPU module  20  of the safety PLC connected to the corresponding computer in response to the operator&#39;s instruction and are then stored in the memory  22 . The common-data specifying information stored in the memory  22  of the CPU module  20  is transferred to the memory  34  of the communication module  30  through the bus  21  in an initial operation to be described below. Further, in the initial operation, the parameter ‘d-a’ of the transferred information related to the transmission data to be transmitted to another safety PLC is output by the communication module  30  and is transmitted to a safety PLC with the exchange number corresponding to the value ‘d’ of the parameter ‘d-a’. 
     An input operation by the computer  53  and the input device  54  is performed as described above. Now, an output operation of the computer  53  to the monitor  55  and the printer  56  will be described. First, in response to an operator&#39;s instruction, the computer  53  reads the sequence program and the common-data specifying information stored in the memory  57 . Then, the computer  53  associates the common-data specifying information with each other on a ladder diagram representing the content of the sequence program, as shown in  FIGS. 3A to 3C , and then outputs the common-data specifying information from at least one of the monitor  55  and the printer  56 . Here, output from the monitor  55  means displaying, on the monitor  55 , the ladder diagram in which the common-data specifying information are associated with each other. Further, output from the printer  56  means printing the ladder diagram in which the common-data specifying information is associated with each other on a medium such as a sheet. 
     Next, an initial operation that is performed by the distributed control system  1  immediately after starting or immediately after restarting will be described. In this embodiment, the distributed control system  1  starts by turning on a main power supply of the corresponding system. Furthermore, the distributed control system  1  restarts by resetting the main power supply of the corresponding system. 
     As shown in  FIG. 1 , when the initial operation starts, each of the safety PLCs,  10 ,  11 , and  12  transfers the common-data specifying information from the memory  22  of the CPU module  20  to the memory  34  of the communication module  30  (Step S 1 ). Subsequently, the communication module  30  of the safety PLC  10  reads the common-data specifying information related to data shared by the safety PLCs  11  and  12  from the memory  34 , and transmits the read common-data specifying information to the safety PLCs  11  and  12  (Step S 2 ). The common-data specifying information transmitted from the safety PLC  10  to the safety PLC  11  is a parameter ‘ 01 - a ’ related to data transmitted from the safety PLC  10  to the safety PLC  11 , for example, a parameter ‘ 01 -L 00 ’ shown in  FIG. 3A . Further, the common-data specifying information transmitted from the safety PLC  10  to the safety PLC  12  is a parameter ‘ 02 - a ’ related to data transmitted from the safety PLC  10  to the safety PLC  12 , for example, a parameter ‘ 02 -L 04 ’ shown in  FIG. 3A . 
     After receiving the common-data specifying information from the safety PLC  10 , the communication module  30  of each of the safety PLCs  11  and  12  collates the received common-data specifying information with the common-data specifying information stored in the memory  34  of the corresponding communication module (Step S 3 ). More specifically, the communication module  30  of the safety PLC  11  checks whether the ‘a’ value of the parameter ‘ 01 - a ’ of the reception information from the safety PLC  10  corresponds to the ‘A’ value of the parameter ‘ 00 -A’ related to data received from the safety PLC  10  of the information stored in the memory  34 . For example, when the parameter ‘ 01 -L 00 ’ shown in  FIG. 3A  is received and the parameter ‘ 00 -L 00 ’ shown in  FIG. 3B  is stored in the memory  34  of the safety PLC  11 , the communication module  30  of the safety PLC  11  determines that the parameters correspond to each other. In the safety PLC  12 , the communication module  30  checks whether the ‘a’ value of the parameter ‘ 02 - a ’ of the reception information from the safety PLC  10  corresponds to the ‘A’ value of the parameter ‘ 00 -A’ related to data received from the safety PLC  10  of the information stored in the memory  34 . For example, when the parameter ‘ 02 -L 04 ’ shown in  FIG. 3A  is received and the parameter ‘ 00 -L 04 ’ shown in  FIG. 3B  is stored in the memory  34  of the safety PLC  11 , the communication module  30  of the safety PLC  12  determines that the parameters correspond to each other. 
     When determining that the reception information and the information stored in the memory  34  correspond to each other in Step S 3 , the communication module  30  of each of the safety PLCs  11  and  12  determines that a normal state in which data is correctly transmitted between the safety PLC  10  and the corresponding safety PLC has been confirmed (Step S 4 ). On the other hand, when determining that the reception information and the information stored in the memory  34  does not correspond to each other, the communication module  30  of each of the safety PLCs  11  and  12  determines that an abnormal state in which data cannot be correctly transmitted between the safety PLC  10  and the corresponding safety PLC has been confirmed (Step S 4 ). 
     After both the safety PLCs  11  and  12  confirm the normal state in Step S 4 , the communication module  30  of the safety PLC  11  reads the common-data specifying information related to data shared by the safety PLCs  11  and  12  from the memory  34 , and transmits the read common-data specifying information to the safety PLCs  10  and  12  (Step S 5 ). Here, the common-data specifying information transmitted from the safety PLC  11  to the safety PLC  10  is a parameter ‘ 00 - a ’ related to data to be transmitted from the safety PLC  11  to the safety PLC  10 , for example, parameters ‘ 00 -L 42 ’ and ‘ 00 -L 43 ’ shown in  FIG. 3B . Also, the common-data specifying information transmitted from the safety PLC  11  to the safety PLC  12  is a parameter ‘ 02 - a ’ related to data to be transmitted from the safety PLC  11  to the safety PLC  12 . 
     When having received the common-data specifying information from the safety PLC  11 , the communication module  30  of each of the safety PLCs  10  and  12  collates the received information with the common-data specifying information stored in the memory  34  of the corresponding module (Step S 6 ). More specifically, the communication module  30  of the safety PLC  10  checks whether the ‘a’ value of the parameter ‘ 00 - a ’ of the information received from the safety PLC  11  corresponds to the ‘A’ value of the parameter ‘ 01 -A’ related to data received from the safety PLC  11  of the information stored in the memory  34 . For example, when the parameter ‘ 00 -L 42 ’ shown in  FIG. 3B  is received and the parameter ‘ 01 -L 42 ’ shown in  FIG. 3A  is stored in the memory  34  of the safety PLC  10 , or when the parameter ‘ 00 -L 43 ’ shown in  FIG. 3B  is received and the parameter ‘ 01 -L 43 ’ shown in  FIG. 3A  is stored in the memory  34  of the safety PLC  10 , the communication module  30  of the safety PLC  10  determines that the parameters correspond to each other. In the safety PLC  12 , the communication module  30  checks whether the ‘a’ value of the parameter ‘ 02 - a ’ of the information received from the safety PLC  11  corresponds to the ‘A’ value of the parameter ‘ 01 -A’ related to data received from the safety PLC  11  of the information stored in the memory  34 . 
     When determining that the received information and the information stored in the memory  34  correspond to each other in Step S 6 , the communication module  30  of each of the safety PLCs  10  and  12  determines that the normal state in which data is correctly transmitted between the safety PLC  11  and the corresponding safety PLC has been confirmed (Step S 7 ). On the other hand, when determining that the received information and the information stored in the memory  34  does not correspond to each other, the communication module  30  of each of the safety PLCs  10  and  12  determines that the abnormal state in which data cannot be correctly transmitted between the safety PLC  11  and the corresponding safety PLC has been confirmed (Step S 7 ). 
     After both the safety PLCs  10  and  12  confirm the normal state in Step S 7 , the communication module  30  of the safety PLC  12  reads the common-data specifying information related to data shared by the safety PLCs  10  and  11  from the memory  34 , and transmits the read common-data specifying information to the safety PLCs  10  and  11  (Step S 8 ). Here, the common-data specifying information transmitted from the safety PLC  12  to the safety PLC  10  is the parameter ‘ 00 - a ’ related to data to be transmitted from the safety PLC  12  to the safety PLC  10 , for example, parameters ‘ 00 -L 44 ’ and ‘ 00 -L 45 ’ shown in  FIG. 3C . Also, the common-data specifying information transmitted from the safety PLC  12  to the safety PLC  11  is the parameter ‘ 01 - a ’ related to data to be transmitted from the safety PLC  12  to the safety PLC  11 . 
     The communication module  30  of each of the safety PLCs  10  and  11  having received the common-data specifying information from the safety PLC  12  collates the received information with the common-data specifying information stored in the memory  34  of the corresponding communication module (Step S 9 ). More specifically, the communication module  30  of the safety PLC  10  checks whether the ‘a’ value of the parameter ‘ 00 - a ’ of the information received from the safety PLC  12  corresponds to the ‘A’ value of the parameter ‘ 02 -A’ related to data received from the safety PLC  12  of the information stored in the memory  34 . For example, when the parameter ‘ 00 -L 44 ’ shown in  FIG. 3C  is received and the parameter ‘ 02 -L 44 ’ shown in  FIG. 3A  is stored in the memory  34  of the safety PLC  10 , or when the parameter ‘ 00 -L 45 ’ shown in  FIG. 3B  is received and the parameter ‘ 02 -L 45 ’ shown in  FIG. 3A  is stored in the memory  34  of the safety PLC  10 , the communication module  30  of the safety PLC  10  determines that the parameters correspond to each other. In the safety PLC  11 , the communication module  30  checks whether the ‘a’ value of the parameter ‘ 01 - a ’ of the information received from the safety PLC  12  corresponds to the ‘A’ value of the parameter ‘ 02 -A’ related to data received from the safety PLC of the information stored in the memory  34 . 
     When determining that the received information and the information stored in the memory  34  correspond to each other in Step S 9 , the communication module  30  of each of the safety PLCs  10  and  11  determines that the normal state in which data is correctly transmitted between the safety PLC  12  and the corresponding safety PLC has been confirmed (Step S 10 ). On the other hand, when determining that the received information and the information stored in the memory  34  does not correspond to each other, the communication module  30  of each of the safety PLCs  10  and  11  determines that the abnormal state in which data cannot be correctly transmitted between the safety PLC  12  and the corresponding safety PLC has been confirmed (Step S 10 ). 
     After both the safety PLCs  10  and  11  confirm the normal state in Step S 10 , that is, when all of the safety PLCs  10 ,  11 , and  12  confirm the normal state, the communication module  30  of the safety PLC  10  instructs the CPU module  20  of the safety PLC  10  to start normal communication (Step S 11 ). Meanwhile, when any one of the safety PLCs  10 ,  11 , and  12  confirms the abnormal state, the communication module  30  of the safety PLC  10  instructs the CPU module  20  of the safety PLC  10  to prohibit the normal communication (Step S 12 ). 
     According to the above-mentioned first embodiment, when the system starts or restarts, each of the safety PLC  10 ,  11 , and  12  determines whether data is normally transmitted between the corresponding safety PLC and the other safety PLCs on the basis of the collation of its own the common-data specifying information with the common-data specifying information received from other PLCs. In other words, in the first embodiment, it is possible to automatically determine whether communication among the safety PLCs  10 ,  11 , and  12  is correctly performed without the operator. Therefore, it is possible to ensure the safety of the communication among the safety PLCs  10 ,  11 , and  12  and to improve the maintenance property of the overall system. In particular, since the safety of the communication among safety PLCs  10 ,  11 , and  12  is secured, it is possible to ensure the safety of the output devices  4  connected to the safety PLCs  10 ,  11 , and  12  with high reliability. 
     Further, according to the first embodiment, the parameters ‘d-a’ and ‘D-A’, which are the common-data specifying information, include the bit addresses ‘a’ and ‘A’ to which common data is assigned, in the frame used for the communication of the distributed control system. For this reason, even though a plurality of common data are assigned to a plurality of addresses of one frame and are simultaneously transmitted thereto, it is possible to secure the safety of the transmission on the basis of the above-mentioned theory. Therefore, it is possible to improve the maintenance property and to reduce the cost by reducing the number of signal lines while securing the safety of the communication among the safety PLCs  10 ,  11 , and  12 . 
     Furthermore, according to the first embodiment, the parameters ‘d-a’ and ‘D-A’, which are the common-data specifying information, include the exchange numbers ‘d’ and ‘D’ for identifying each of the safety PLCs  10 ,  11 , and  12 . The exchange numbers make it possible for the safety PLCs  10 ,  11 , and  12  to determine which of the safety PLCs  10 ,  11 , and  12  the received common-data specifying information is received from. Therefore, the safety PLCs  10 ,  11 , and  12  can accurately perform the above-mentioned collation for common-data specifying information received from other PLCs. 
     In addition, according to the first embodiment, the sequence program to be executed by the safety PLCs  10 ,  11 , and  112  and the common-data specifying information can be input to the safety PLCs  10 ,  11 , and  12  through the I/O devices  50 ,  51 , and  52 . Therefore, the sequence program and the common-data specifying information can be freely changed in response to the requirement of the system specification, thereby expanding the versatility of the distributed control system  1 . 
     Furthermore, according to the first embodiment, the I/O devices  50 ,  51 , and  52  can output the common-data specifying information on the ladder diagram representing the content of the sequence program to be executed by the safety PLCs  10 ,  11 , and  12 , with the common-data specifying information associated with each other. Therefore, the operator can check from the output results of the I/O devices  50 ,  51 , and  52  with eyes whether the common-data specifying information correctly specifies the common data to be transmitted among the safety PLCs  10 ,  11 , and  12 , as shown in the ladder diagram. As a result, it is expected to further improve the maintenance property. 
     The above-mentioned safety PLCs  10 ,  11 , and  12  correspond to ‘a PLC for distributed control’ and ‘other PLCs’ described in the appended claims. Further, the memories  22  and  34  of each of the safety PLCs  10 ,  11 , and  12  correspond to ‘storage units’ described in the appended claims. The communication module  30  of each of the safety PLCs  10 ,  11 , and  12  correspond to ‘a receiving unit’, ‘a collating unit’, and a ‘transmitting unit’ described in the appended claims. 
     Second Embodiment 
     A second embodiment of the invention is a modification of the first embodiment. In the second embodiment, the same parts as those in the first embodiment are denoted by the same numerals, and thus a description thereof will be omitted. 
     A distributed control system  1  according to the second embodiment is a master-slave network system that has a safety PLC  10  serving as a master and safety PLCs  11  and  12  serving as slaves. Therefore, in the second embodiment, frame data is received and transmitted between the safety PLCs  10  and  11  and between the safety PLCs  10  and  12 , while frame data is not received and transmitted between the safety PLCs  11  and  12 . 
     In I/O devices  50 ,  51 , and  52  of the second embodiment, common-data specifying information (parameters ‘d-a’ and ‘D-A’) input to a computer  53  is stored in a memory  57  of the computer  53 , is processed into a frame format by the computer  53 , and is stored in the memory  57 . 
     More specifically, the computer  53  connected to the safety PLC  10  generates a frame in which data corresponding to bit addresses represented by the values ‘a’ and ‘A’ of the parameters ‘ 01 - a ’ and ‘ 01 -A’ related to common data shared by the safety PLCs  10  and  11  are represented by ‘use (U)’ and data corresponding to the other bit addresses are represented by ‘non-use (N)’. Therefore, the computer  53  connected to the safety PLC  10  generates a frame shown in FIG.  4 A 1  in which bit data having bit addresses ‘L 00 ’, ‘L 42 ’, and ‘L 43 ’ corresponding to parameters ‘ 01 -L 00 ’, ‘ 01 -L 42 ’, and ‘ 01 -L 43 ’ shown in  FIG. 3A  are ‘U’. The computer  53  also generates a frame in which data corresponding to bit addresses represented by the values ‘a’ and ‘A’ of parameters ‘ 02 - a ’ and ‘ 02 -A’ related to the common data shared by the safety PLCs  10  and  12  are ‘U’, and data corresponding to the other addresses are ‘N’. Therefore, the computer  53  connected to the safety PLC  10  generates a frame shown in FIG.  4 A 2  in which bit data having, for example, bit addresses ‘L 04 ’, ‘L 44 ’, and ‘L 45 ’ corresponding to parameters ‘ 02 -L 04 ’, ‘ 02 -L 44 ’, and ‘ 02 -L 45 ’ shown in  FIG. 3A  are ‘U’. 
     The computer  53  connected to the safety PLC  11  generates a frame in which data corresponding to bit addresses represented by the values ‘A’ and ‘a’ of the parameters ‘ 00 -A’ and ‘ 00 - a ’ related to common data shared by the safety PLCs  10  and  11  are represented by ‘use (U)’ and data corresponding to the other bit addresses are represented by ‘non-use (N)’. Therefore, the computer  53  connected to the safety PLC  11  generates a frame shown in  FIG. 4B  in which bit data having bit addresses ‘L 00 ’, ‘L 42 ’, and ‘L 43 ’ corresponding to parameters ‘ 00 -L 00 ’, ‘ 00 -L 42 ’, and ‘ 00 -L 43 ’ shown in  FIG. 3B  are ‘U’. 
     The computer  53  connected to the safety PLC  12  generates a frame in which data corresponding to bit addresses represented by values ‘A’ and ‘a’ of parameters ‘ 00 -A’ and ‘ 00 - a ’ related to the common data shared by the safety PLCs  10  and  12  are ‘use (U)’ and data corresponding to the other addresses are ‘non-use (N)’. Therefore, the computer  53  connected to the safety PLC  12  generates a frame shown in  FIG. 4C  in which bit data having, for example, bit addresses ‘L 04 ’, ‘L 44 ’, and ‘L 45 ’ corresponding to parameters ‘ 00 -L 04 ’, ‘ 00 -L 44 ’, and ‘ 00 -L 45 ’ shown in  FIG. 3C  are ‘U’. 
     The common-data specifying information processed into the frame format as described above is input from the computer  53  to the CPU module  20  of the safety PLC connected to the computer  53  together with the exchange numbers and the sequence program, and is stored in the memory  22  of the corresponding safety PLC. In an initial operation to be described below, the common-data specifying information stored in the memory  22  in the frame format is transferred to the memory  34  of the communication module  30  through the bus  21  and is then transmitted from the communication module  30  to another safety PLC. 
     The initial operation according to the second embodiment is performed as shown in  FIG. 5 . That is, when the initial operation starts, each of the safety PLCs  10 ,  11 , and  12  transfers the common-data specifying information of the frame format from the memory  22  of the CPU module  20  to the memory  34  of the communication module  30  (Step S 101 ). 
     Subsequently, the communication module  30  of the safety PLC  10  reads, from the memory  34 , a part of the common-data specifying information of the frame format that relates to the common data shared by the safety PLCs  10  and  11  and is shown in, for example, FIG.  4 A 1 , and transmits the read information to the safety PLC  11  (Step  102 ). Then, the communication module  30  of the safety PLC  10  reads, from the memory  34 , a part of the common-data specifying information of the frame format that relates to the common data shared by the safety PLCs  10  and  12  and is shown in, for example, FIG.  4 A 2 , and transmits the read information to the safety PLC  12  (Step S 102 ). 
     The communication module  30  of each of the safety PLCs  11  and  12  having received the common-data specifying information of the frame format from the safety PLC  10  collates the received common-data specifying information of the frame format with the common-data specifying information of the frame format stored in the memory  34  (Step S 103 ). More specifically, the communication module  30  of the safety PLC  11  collates bit data of the frame received from the safety PLC  10  as the common-data specifying information with bit data of the frame stored in the memory  34  as the common-data specifying information so as to determine whether the bit data coincide with each other for each same address. For example, when a frame in which addresses where bit data are ‘U’ are ‘L 00 ’, ‘L 42 ’, and ‘L 43 ’ as shown in FIG.  4 A 1  is received and a frame in which addresses where bit data are ‘U’ are ‘L 00 ’, ‘L 42 ’, and ‘L 43 ’ as shown in  FIG. 4B  is stored, the communication module  30  of the safety PLC  11  determines that the bit data coincide with each other. Further, the communication module  30  of the safety PLC  12  collates bit data of the frame received from the safety PLC  10  as the common-data specifying information with bit data of the frame stored in the memory  34  as the common-data specifying information so as to determine whether the bit data coincide with each other for each same address. For example, when a frame in which addresses where bit data are ‘U’ are ‘L 04 ’, ‘L 44 ’, and ‘L 45 ’ as shown in FIG.  4 A 2  is received and a frame in which addresses where bit data are ‘U’ are ‘L 04 ’, ‘L 44 ’, and ‘L 45 ’ as shown in  FIG. 4C  is stored, the communication module  30  of the safety PLC  12  determines that the bit data coincide with each other. 
     On the basis of the collation results in Step S 103 , the communication modules  30  of the safety PLCs  11  and  12  perform determination, similar to Step S 4  of the first embodiment (Step S 104 ). When both the communication modules  30  of the safety PLCs  11  and  12  confirm the normal state in Step  104 , the communication module  30  of the safety PLC  11  reads, from the memory  34 , a part of the common-data specifying information of the frame format that relates to the common data shared by the safety PLCs  10  and  11  and is shown in, for example,  FIG. 4B , and transmits the read information to the safety PLC  10  (Step S 105 ). 
     The communication module  30  of the safety PLC  10  having received the frame format common-data specifying information from the safety PLC  11  collates the received frame format common-data specifying information with the frame format common-data specifying information stored in the memory  34  according to Step S 103  (Step  106 ). On the basis of the collation result, the communication module  30  of the safety PLC  10  performs determination, similar to Step S 7  of the first embodiment (Step S 107 ). 
     When the communication module  30  of the safety PLC  10  confirms the normal state in Step S 107 , the communication module  30  of the safety PLC  12  reads, from the memory  34 , a part of the frame format common-data specifying information that relates to the common data shared by the safety PLCs  10  and  12  and is shown in, for example,  FIG. 4C , and transmits the read information to the safety PLC  10  (Step S 108 ). 
     The communication module  30  of the safety PLC  10  having received the common-data specifying information of the frame format from the safety PLC  12  collates the received common-data specifying information of the frame format with the common-data specifying information of the frame format stored in the memory  34  according to Step S 103  (Step S 109 ). On the basis of the collation result, the communication module  30  of the safety PLC  10  performs determination based on Step S 10  in the first embodiment (Step S 110 ). 
     When the communication module  30  of the safety PLC  10  confirms the normal state in Step S 110 , it instructs the CPU module  20  of the safety PLC  10  to start normal communication, similar to Step S 11  of the first embodiment (Step S 111 ). Meanwhile, when any one of the safety PLCs  10 ,  11 , and  12  confirms the abnormal state in Steps S 104 , S 107 , and S 110 , it instructs the CPU module  20  thereof to prohibit the normal communication based on Step S 12  in the first embodiment (Step S 112 ). 
     As described above, each of the safety PLCs  10 ,  11 , and  12  according to the second embodiment can also determine the normal state or the abnormal state between the corresponding safety PLC and another safety PLC by collating the common-data specifying information stored in the corresponding safety PLC with the common-data specifying information received from another safety PLC. Therefore, the second embodiment can obtain the same effects as the first embodiment. 
     The embodiments of the invention have been described above, but the invention is not limited thereto. 
     For example, in the first embodiment, each of the safety PLCs  10 ,  11 , and  12  may transmit the parameter ‘D-A’ related to the received data of the corresponding safety PLC to another safety PLC and may collate the parameter ‘D-A’ with the parameter ‘d-a’ stored in the corresponding safety PLC in relation to the transmitted data. 
     In the first and second embodiments, the invention is applied to the distributed control system  1  connected to a plurality of PLCs each securing the safety of a control object. However, the invention can be applied to a distributed control system connected to a plurality of PLCs each having a function other than the function of securing the safety of a control object. Further, a topology of a distributed control system to which the invention is applied may be a bus or star other than the ring described in the first and second embodiments, regardless of the type of PLC.