How does PLC hot standby redundancy work?

Hot standby redundancy uses two PLCs running simultaneously - primary executes control, standby mirrors state. Synchronization methods: memory replication (continuous or periodic), common I/O (both CPUs access same I/O), or bumpless transfer (standby maintains output states). Failure detection through heartbeat monitoring, watchdog timers, and diagnostics. Switchover triggered by CPU fault, communication loss, or manual command. Switchover time typically 10-50 ms. Critical considerations: synchronized time, identical programs, I/O module redundancy, and network redundancy. Used for continuous processes where downtime is unacceptable.