Applies To:
  • CitectSCADA 5.xx, 6.xx
  • CitectHMI 5.xx, 6.xx

What are the fundamentals of configuring CitectSCADA communications if the project is to utilise redundant servers as well as redundant Ethernet IO devices? 

Article Q2156 outlines how to configure redundant IO Servers to the same serial IO Device, however many control systems also utilise redundant IO devices and connect to them via Ethernet. For redundant servers connecting to redundant Modnet devices on a single network an illustrative configuration follows, extending on that suggested in article Q2156 and referring to the Driver Help for Modnet devices. This assumes you have two CitectSCADA machines - ‘CitectP’ which runs ‘IOserverP’ (primary), and ‘CitectS’ which runs ‘IOserverS’ (standby) - and real IO device ‘Device1’ (primary) at and real IO device ‘Device2’ (standby) at



IO Servers:        
Name: IOserverP   IOserverS  
Server Name: IOserverP   IOserverS  
Board Name: IOserverP_Board1   IOserverS_Board1  
Board Type: TCPIP   TCPIP  
Address: 0   0  
Server Name: IOserverP IOserverP IOserverS IOserverS
Port Name: Board1_Port1P Board1_Port2P Board1_Port1S Board1_Port2S
Board Name: IOserverP_Board1 IOserverP_Board1 IOserverS_Board1 IOserverS_Board1
Port Number: 1 2 1 2
Special Options: -I192.168.0.11 -I192.168.0.12 -I192.168.0.11 -I192.168.0.12
IO Devices:        
Server Name: IOserverP IOserverP IOserverS IOserverS
Name: Device1P Device2P Device1S Device2S
Number: 1 1 1 1
Port Name: Board1_Port1P Board1_Port2P Board1_Port1S Board1_Port2S
Startup Mode: Primary Standby Standby Standby

As you can see, you will need to configure in your project two IO Servers, each with one Board, which is in turn referred to by two Ports and their associated IO Devices. Thus, you have four Ports and four IO Devices configured. Note that the IO Device Address field is left blank for Modnet devices. Note also that only one of these IO Devices has Startup Mode ‘Primary’ and the other three have Startup Mode 'Standby’, however the IO Device Number is the same for all.

The IO Device Number is crucial since in the variable tag configuration you specify the device that is associated with that tag by its IO Device Name – always select the IO Device which has a Startup Mode of 'Primary', in this case ‘Device1P’. CitectSCADA knows that this tag is also associated with the standby devices (‘Device2P’, ‘Device1S’ and ‘Device2S’) because these devices share a common IO Device Number with ‘Device1P’. This means that even though the IO Device is actually a pair of real devices, which are in turn configured as four IO Devices in CitectSCADA, you only need to define each variable tag once in your project.

The order in which these devices appear in the units.dbf file will determine the order in which the standby devices are used in the event of a failure. It is recommended that the units.dbf file in your project folder is opened with a third party application such as Excel and you ensure the IO Devices appear in this order:


(As always, do not modify the column widths of any dbf file opened in Excel, and always close the file using the ‘Save/Close DBF’ option in the right-click menu enabled by running the SAVE_DBF.xls macro found in the Bin directory.)

After completing this configuration and restoring the project to the two machines, ensure you run the Computer Setup Wizard on each machine and select the correct IO Server in each case.


redundancy primary standby