Siemens SIMATIC S7-1500 System Manual page 222

Basics of program execution
9.3 Events and OBs
The figure below shows the behavior of the two OBs during system state transitions from
RUN-Solo to RUN-Redundant and vice versa.
Figure 9-1 OB 72 and OB 86 during system state transitions
OB 86
There are three IO devices in the example. Each failure of one of the three IO devices is
followed by recovery of the IO device. Each IO device failure/IO device recovery is signaled.
Cyclic program execution is interrupted with an OB 86 call.
In the "Copy main memory" phase of the SYNCUP system state those OBs that interrupt the
cyclic program processing are processed. New diagnostic events are signaled but the OBs are
not yet processed. In the example, the failure of IO device 2 and the recovery of IO device 3
are signaled. However, the OB 86 are not processed until the following phase, "Making up
backup CPU lag".
NOTE
Order of execution of OB 86
Please note that the order of processing of the OB 86 may differ from the order of processing
of the associated diagnostic events.
222
S7-1500R/H redundant system
System Manual, 11/2022, A5E41814787-AD