Siemens SIMATIC ET 200pro Operating Instructions Manual page 51

Basics of program execution
6.1 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 priorities 1 (lowest priority) to 26 (highest priority). The following items are
essential to the execution of an event:
● Call and execution 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
The communication (e.g. test functions with the PG) always works with priority 15. To avoid
unnecessarily prolonging the program runtime in the case of time-critical applications, make
sure that these OBs are not delayed or interrupted by communication. Assign a priority > 15
for these OBs.
Reference
You can find additional information on organization blocks in the STEP 7 online help.
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CPU 1516pro-2 PN (6ES7516-2PN00-0AB0)
Operating Instructions, 11/2019, A5E35873416-AC