Siemens Simatic S7-1500 Function Manual page 54

Time error
As with non-redundant CPUs, you can specify the response to a time error for the CPUs of
the S7-1500R/H redundant system. In RUN-Solo system state, the redundant CPUs behave
like non-redundant CPUs when the maximum cycle time is exceeded (see section Cycle time
(Page 24)).
In the SYNCUP and RUN-Redundant system states the redundant CPUs behave as follows:
Behavior without OB 80
Without loaded OB 80 the backup CPU switches to STOP operating state after a maximum
cycle time has been exceeded once in the same cycle.
Note
System state change after STOP without OB 80
The primary CPU also switches to STOP after the maximum cycle time has been exceeded
twice in the same cycle.
Behavior with OB 80
When OB 80 is loaded, the backup CPU switches to STOP operating state after the
maximum cycle time has been exceeded twice in the same cycle.
Note
System state change after STOP with OB 80
The primary CPU also switches to STOP after the maximum cycle time has been exceeded
three times in the same cycle.
Ensure that the actual maximum cycle time is < 60% of the parameterized maximum cycle
time.
Switchover of the backup CPU to STOP operating state when the maximum cycle time is
exceeded
A switchover of the backup CPU to STOP operating state reduces the synchronization load
and stabilizes the primary CPU.
Cycle and response times
Function Manual, 10/2018, A5E03461504-AD
Cycle and response times of the S7-1500R/H redundant system
5.3 Influences on the cycle time of the S7-1500R/H redundant system
53