Siemens SIMATIC S7-1500 System Manual page 223

NOTE
Station re-integration with errors
When a station returns with errors in an R/H-CPU, no attempt is made - in contrast to a
standard CPU - to output exact error information in the diagnostic buffer.
OB 72
If the system then switches to the RUN-Redundant system state, OB 72 "CPU redundancy
error" is called. OB 72 is called again:
• If the redundant system exits redundant operation and changes to the RUN-Solo system
state.
• If the redundant system is in redundant operation and the synchronization of the two R/H-
CPUs is no longer possible redundantly or is possible redundantly again.
All cases can be distinguished by the start information of OB 72. You can find more
information on this topic in the STEP 7 online help.
NOTE
Delayed execution of OB 72
There may also be a delay in executing OB 72 because the corresponding diagnostic event is
processed asynchronously to the user program.
Behavior of OB 72 and OB 86 for standard IO devices at primary-backup switchover
If the primary CPU fails or goes to STOP, the standard IO devices are temporarily separated
from the redundant S7-1500R/H system. From the CPU perspective, the standard IO devices
fail. OB 72 "Redundancy error" is called, additional OB 86 "Module rack failure" for the failed
IO devices are not called, however. To detect the failed IO devices, call the DeviceStates
instruction in OB 72. To detect all failed IO devices, the OB 72 must have priority 26 (default).
With the "Switched S1 device" function, the new primary CPU establishes the ARs to the
standard IO devices again. OB 86 is called for each return of an IO device.
Behavior of OB 70 and OB 86 at failure of a R1/S2 device in system state RUN-Redundant
If an R1/S2 device fails, then OB 86 reports "Module failure" if programmed accordingly.
• If two ARs existed before the R1/S2 device failure, the operating system calls OB 70 "I/O
redundancy error" before OB 86.
• If only one AR existed before the R1/S2 device failure and it is lost, no OB 70 is called.
Behavior of the OB 70 and the OB 86 at return of a R1/S2 device in system state RUN-Redundant
If an R1/S2 device returns, then OB 86 reports "Module return" if programmed accordingly.
S7-1500R/H redundant system
System Manual, 11/2022, A5E41814787-AD
Basics of program execution
9.3 Events and OBs
223