Siemens SIMATIC S7-1500 System Manual page 325

Basics of program execution
10.3 Events and OBs
Response to start events
The occurrence of a trigger results in the following response:
• If the event comes from an event source to which you have assigned an OB, this event
triggers the execution of the assigned OB. The event enters the queue according to its
priority (exception: hardware interrupts).
• If the event comes from an event source to which you have not assigned an OB, the CPU
executes the default system reaction.
Note
Some event sources, such as startup, pull/plug, exist even if you do not configure them.
Response of OB 72 and OB 86 to system state transitions
If an IO device has failed, the OB 86 reports a "rack failure" if programmed. OB 72 "CPU
redundancy error" reports a loss of redundancy in the redundant system or failure of the
redundant synchronization.
The figure below shows the behavior of the two OBs during system state transitions from
RUN-Solo to RUN-Redundant and vice versa.
Figure 10-1 OB 72 and OB 86 during system state transitions
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S7-1500R/H redundant system
System Manual, 01/2024, A5E41814787-AF