For PLC redundancy, the Series 90 TCP/IP Redundancy enabler operates in "Hot Standby" mode, such that CIMPLICITY software automatically communicates with the slave PLC in the redundant pair when the master PLC fails. CIMPLICITY software connects to both PLCs, but only reads data from and writes data to the active master. To the user, the two physical devices are viewed as a single logical device. Points are configured for the logical device thus eliminating the need for duplicate configuration.

You configure the logical device by entering two TCP/IP addresses, one for the configured primary PLC and one for the configured secondary PLC. When the enabler starts, the primary PLC is the master and the secondary PLC is the slave. Under normal operating conditions, the enabler reads data from and writes data to only the master PLC, and checking the "health" of the slave PLC.

The enabler requires the redundant PLC pair to communicate with each other and to indicate (via a mode flag) which programmable controller is acting as the current master. The PLC sets the "mode flag", at a configured address, whenever a failover takes place. CIMPLICITY software monitors the mode flag, at a configured interval, to determine which PLC is acting as the master.

 Important: Since the Mode Address is specified at the port level, all redundant PLCs associated with a port must use the same Mode Address. This means that all PLCs on the port must have the same model type (that is all 90-70s or all 90-30s)

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