Siemens SIMATIC S7-400H System Manual page 157

Response to time-outs
If one of the times monitored exceeds the configured maximum value, the following
procedure is started:
1. Cancel update
2. Fault-tolerant system remains in standalone mode, with the previous master CPU in RUN
3. Cause of cancelation is entered in diagnostic buffer
4. Call OB 72 (with corresponding start information)
The standby CPU then reevaluates its system data blocks.
Then, but after at least one minute, the CPU tries again to perform the link-up and update. If
still unsuccessful after a total of 10 retries, the CPU abandons the attempt. You yourself will
then need to start the link-up and update again.
A monitoring timeout can be caused by:
● High interrupt load (e.g. from I/O modules)
● High communication load causing prolonged execution times for active functions
● In the final update phase, the system needs to copy large amounts of data to the
S7-400H
System Manual, 03/2012, A5E00267695-11
standby CPU.
Link-up and update
12.4 Time monitoring
157