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. This is done by allowing the scan rate to be specified per I/O device, and then configuring multiple I/O devices to the same PLC.

The inactive unit under redundancy will receive a STATUS_UNIT command from Plant SCADA every configured WatchTime. The reply sent back to Plant SCADA will be based on the current state of the status tag condition. If the condition isn't met the standby unit will be set to UNIT_OFFLINE until the status tag condition passes. The status group polling rate remains unchanged on the inactive unit. See Stop_Unit Handling.

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.

Both the primary and standby devices will be offline if both have the same status condition and the condition is not passing. All points in the system for that unit would therefore be #COM.

The status tag can be defined either in the main [ABCLX] section to apply to all devices or a status can be defined specifically for a particular device within the [<IOServerName>.<IOPortName>] section.

The inactive unit under redundancy will receive a STATUS_UNIT command from Plant SCADA every configured WatchTime. The reply sent back to Plant SCADA will be based on the current state of the status tag condition. If the condition isn't met, the standby unit will be set to UNIT_OFFLINE until the status tag condition passes. The status group polling rate remains unchanged on the inactive unit.