A communication/PLC failure occurs where the enabler can no longer communicate over any network path to a running healthy PLC; for example network down, PLC powered down, PLC stopped or Health bit clear.
Master PLC - the enabler updates the diagnostic points then performs a failover to the next highest ranking running Healthy PLC with the Health and master mode bits set.
Standby PLC - the enabler updates the diagnostic points for the PLC.
The enabler then retries communications to the failed PLC at the configured heartbeat rate.
Series 90 TCP/IP Redundancy failure conditions and actions.