Using CitectSCADA > Building Redundancy Into Your System > Data Path Redundancy > Multiple Device Redundancy (Standby Data Paths)

Multiple Device Redundancy (Standby Data Paths)

If your I/O Devices support peer-to-peer communication, you can add another level of redundancy to your system by duplicating the I/O Devices.

Note: Although I/O Servers are not assigned the Primary or Standby role based on the I/O devices to which they are connected, it is common practice in redundant I/O systems to connect the Primary I/O Devices to the Primary I/O Server and the Standby I/O Devices to the Standby I/O Server. One I/O Server can connect to a mixture of Primary and Standby I/O Devices. The I/O Server can support any number of Standby Data Paths.

The following diagram demonstrates multiple device redundancy and Standby Data Paths:

In this scenario, we have three I/O Servers connected to three I/O Devices in the following manner:

I/O Server

I/O Devices Connected


I/O Device1 (Primary)
I/O Device2 (Standby)
I/O Device3 (Primary)


I/O Device1 (Standby)
I/O Device2 (Primary)


I/O Device1 (Standby)
I/O Device2 (Standby)
I/O Device3 (Standby)

The following is known:

Redundancy is provided as follows:

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.

Since we can place primary and standby I/O Devices on various I/O Servers, share the primary I/O Devices between your I/O Servers to balance the loading across the I/O Servers. However, this might not apply for every protocol because the loading could be dependent on the PLC network and not the I/O Server CPU. In this case, more than one active I/O Server on the same PLC Network can degrade the PLC network and therefore, slow the total response.

See Also