Siemens SIMATIC S7-1500 System Manual page 327

Basics of program execution
10.3 Events and OBs
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 on failure of a R1/S2 device in RUN-Redundant system state
If an R1/S2 device fails, then OB 86 reports "Rack 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, OB 70 is not called.
Behavior of the OB 70 and the OB 86 on return of a R1/S2 device in RUN-Redundant system state
If an R1/S2 device returns, then OB 86 reports "Rack return" if programmed accordingly.
The call of OB 86 indicates the station return, independent of the redundancy status of the
R1/S2 device at the time of the call of OB 86.
• If both ARs can be established on return, OB 70 is called after OB 86. The R1/S2 device is
redundant.
• If only one AR can be set up on return, OB 70 is not called. The R1/S2 device is not
redundant.
Priority of OB 70 and OB 86
Note
Priority of OB 70 and OB 86
Select the same priority for OB 70 and OB 86 or keep the default setting (priority 6). This
prevents the OBs from interrupting each other. The user program processes events of the
same priority in order of occurrence.
Behavior of the OB 70 and the OB 86 after SYNCUP
If an R1/S2 device becomes redundant after the system state RUN-Redundant has been
accessed, OB 70 (outgoing) is called.
If an IO device is only accessible by the backup CPU and was not reachable at all before the
SYNCUP, then in the system state RUN-Redundant OB 86 (outgoing) is called, but not OB 70.
This indicates the return of the IO device without redundancy.
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S7-1500R/H redundant system
System Manual, 01/2024, A5E41814787-AF