Siemens SIMATIC S7-1500 System Manual page 216

Basics of program execution
9.1 Programming the S7-1500R/H
OB 72 (CPU redundancy error)
In addition to the OBs of the S7‑1500 CPU, you can also use OB 72 (CPU redundancy error).
The operating system of each CPU of an R/H-system calls up the CPU redundancy error OB
(OB72) when one of the following events occurs:
• Loss of redundancy due to a CPU failure
• Loss of redundancy due to a CPU transition to STOP mode (triggered by the user or the
system)
Up to firmware version V2.8:
• The R/H system has entered the RUN-Redundant system state.
As of firmware version V2.9:
• The R/H-system has entered RUN-Redundant system state and the synchronization of the
two R/H-CPUs is possible redundantly.
• The R/H-system has entered RUN-Redundant system state, but the synchronization of the
two R/H-CPUs is not possible redundantly.
• The R/H-system is still in RUN-Redundant system state and the synchronization of the two
R/H-CPUs is possible redundantly now or again.
• The R/H-system is still in RUN-Redundant system state, but the synchronization of the two
R/H-CPUs is no longer possible redundantly.
NOTE
An R/H-system with non-redundant synchronization may not tolerate any further failures. In
this case, you need to check and possibly repair the PROFINET ring (R-system) or the fiber-
optic cables (H-system) immediately.
OB 70 (IO redundancy error)
As of firmware version V3.0, OB 70 (I/O redundancy error) is available for the diagnostics of
R1 and S2 devices in S7-1500H systems.
The operating system of the CPUs calls OB 70 if a redundancy loss or a redundancy return
occurs in the system state RUN-Redundant at a redundant R1/S2 device. Loss of redundancy
means that one AR of two ARs of an R1/S2 device fails. Redundancy return means that the
second AR of an R1/S2 device returns.
For example, the loss of redundancy is caused by one of the following events:
• Failure of an S2 device due to voltage failure
• Failure of one of two interface modules of the R1 device
• Line interruption on one side of the R1 or S2 device
OB 70 is only called in the system state RUN-Redundant. When the H-system leaves the
system state RUN-Redundant, OB 72 (CPU redundancy error) is called.
STEP 7 online help
You can find additional information on instructions and blocks in the STEP 7 online help.
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S7-1500R/H redundant system
System Manual, 11/2022, A5E41814787-AD