Configuring Redundancy

Redundancy is handled in Plant SCADA by the Plant SCADA clients. They determine which I/O server to request I/O device data from by allowing the scan rate to be specified per I/O device, and then configuring multiple I/O devices to the same PLC.

A status tag can be defined in the Citect.ini file to determine whether or not to set a unit offline or online depending on the condition of the status tag. If the condition is true the unit is allowed to come online, otherwise the unit is set to offline.

The status tag is polled at the same rate configured for the groups of the corresponding unit. The driver supports STATUS_UNIT commands on the inactive server, so even though a particular unit is not currently servicing Plant SCADA requests, the state of the status condition will be checked periodically allowing the unit to be taken either offline or online (assuming that [IOServer]WatchDog has not changed from its default of 1).

Both the primary and standby devices will be offline if both have the same status condition and the condition is an error condition. All points in the system for that unit would therefore be #COM.

Status tags can be defined on a per-device or global basis. For more information, see OPC Status Tag Parameters and OPC Device-specific Parameters.