Applies To:
  • CitectSCADA 4.xx, 5.xx
Summary:
How do I setup a Citect project with redundant I/O Servers and more then one Ethernet connection to a PLC? 

Solution:
In Citect you can have one Primary I/O Device and multiple Standby I/O Devices. They all must have the same I/O Device Number. The Standby I/O Devices will be activated strictly in the order in which they are first created in the project. This can be viewed by looking in the Units.dbf file in the project directory.

Example -

Two I/O Servers each with two NICs (Network Interface Cards) communicating to one I/O Device, that has two Ethernet modules. Protocol used is Modbus over TCP/IP.

I/O Server 1: NIC1 199.40.35.3 I/O Server 2: NIC1 199.40.35.4
NIC2 198.40.35.3 NIC2 198.40.35.4
PLC Addresses: IP address 199.40.35.1, TCP port: 1025, Modbus address: 1
IP address 198.40.35.1, TCP port: 1025, Modbus address: 1

In Citect the following setup is used (IOServer, Boards, Ports, IODevices)

IOServer1

        TCPIP, Address 0

            Port1_Board1,     Special Options:     -I199.40.35.1 -P1025

            Port1_Board2,     Special Options:     -I198.40.35.1 -P1025

                    IODev1:

                                    Protocol: Modbus

                                    Address: 1

                                    Startup mode: Primary

                                    Port: Port1_Board1

                                    IODevice No: 1

                    IODev1:

                                    Protocol: Modbus

                                    Address: 1

                                    Startup mode: Standby

                                    Port: Port1_Board2

                                    IODevice No: 1

IOServer2

        TCPIP, Address 0

            Port1_Board1,     Special Options:     -I199.40.35.1 -P1025 -U

            Port1_Board2,     Special Options:     -I198.40.35.1 -P1025 -U

                    IODev1:

                                    Protocol: Modbus

                                    Address: 1

                                    Startup mode: Standby

                                    Port: Port1_Board1

                                    IODevice No: 1

                    IODev1:

                                    Protocol: Modbus

                                    Address: 1

                                    Startup mode: Standby

                                    Port: Port1_Board2

                                    IODevice No: 1

Important to note is that in Citect only one TCPIP board is defined.  This defines the interface to the Windows TCP/IP interface.  Windows will send the TCPIP message to the correct NIC.

When the primary device goes offline, the first standby is activated (on IOServer1), when that one goes offline the second standby is activated (on IOServer2). When the second standby goes offline, the third standby is activated.

When the second standby is re-activated, no switch over takes place and the third standby keeps communicating.  When the first standby is re-activated the communication switches over to IOServer1 and the standby device starts communicating.  When the primary is activated, communications switches back to the primary device.

 
Keywords:
LAN LANs 

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