Siemens Simantic ET200SP System Manual page 152

Basics of program execution
7.1 Events and OBs
OB priority and runtime behavior
If you have assigned an OB to the event, the OB has the priority of the event. The CPU
supports the priority classes 1 (lowest priority) to 26 (highest priority). The following items are
essential to the processing of an event:
● Calling and processing of the assigned OB
● The update of the process image partition of the assigned OB
The user program processes the OBs exclusively on a priority basis. This means the
program processes the OB with the highest priority first when multiple OB requests occur at
the same time. If an event occurs that has a higher priority than the currently active OB, this
OB is interrupted. The user program processes events of the same priority in order of
occurrence.
Note
Communication
Communication (e.g. test functions with the PG) always works permanently with the priority
15. So as not to unnecessarily prolong program runtime in the case of time-critical
applications, these OBs should not be interrupted by communication. Assign a priority >15
for these OBs.
Reference
You can find more information on organization blocks in the STEP 7 online help.
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Distributed I/O system
System Manual, 12/2016, A5E03576849-AG