Hi,

here is an example how it works:

Failure of a central module or a power supply.
Initial situation: The S7-400H is in system state Redundant.
1. let the CPU0 fail by switching off the power supply.
Result: At the CPU1 the LEDs REDF, IFM1F and IFM2F are on.

The CPU1 goes into solo operation and the user program continues to run.

2. Switch on the power supply again.
Result:
- The CPU0 performs automatic CONNECT and UPDATE.
- The CPU0 changes to RUN and now works as reserve CPU.
- The S7-400H is now in system state Redundant

Detailed informations and more examples you will find in the manual

SIMATIC Fault-tolerant systems S7-400H (V6.0)

Regards,

Towome