Siemens SIMATIC ET 200SP System Manual page 122

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:
● The call of the assigned OB
● The update of the process image partition of the assigned OB
● The processing 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 always has priority 15; always assign a priority >15 so that the OBs are not
interrupted by the communication.
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, 12/2015, A5E03576849-AF