Using CitectSCADA > Building Redundancy Into Your System > I/O Server Redundancy > I/O Device promotion

I/O Device promotion

I/O Device promotion refers to when a system event forces a standby I/O Device to assume a primary role during Runtime.

If there is more than one standby device available within a cluster, the order they are promoted in will depend on how your project has been configured.

If necessary, you can manually set the order your standby devices are promoted in via the Priority field on the extended I/O Devices Properties form. If priorities are not set, the compiler automatically allocates a priority value to each standby device based on the following rules:

These rules apply regardless of the connections between the I/O Devices and I/O Servers configured in a cluster.

The standby device priorities are confirmed and/or allocated during project compilation. Notification is provided for each allocation issued by the compiler. Compile errors will occur under the following circumstances:


The following diagram shows four I/O Devices connected to two I/O Servers, with just one primary device configured.

If no priorities were set for the standby devices, the compiler would allocate the following:

This presumes the devices were configured in numerical order.

If I/O Device3 has been manually set to priority 2, the compiler would allocate the following:

In this case, the setting for I/O Device3 has taken precedence. The remaining standby devices are set for promotion based on the order they were configured.

Note: The automated priorities work on an n+1 equation; if I/O Device3 has been set to Priority 5, the remaining devices would have been allocated priority 6 and 7.

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