Siemens SIMATIC S7-1500 System Manual page 169

Basics of program processing
14.1 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. S7-1500 CPUs
support the priority classes 1 (lowest) to 26 (highest). Events are executed with a defined
priority. The following items are essential to the execution of an event:
● the call of the assigned OB
● the update of the process image partition of the assigned OB
● the execution of the assigned OB.
The OBs are executed exclusively on a priority basis. This means that the OB with the
highest priority will be executed 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.
Events of the same priority are processed in order of occurrence.
Note
Communication
Communication always has priority 15. This makes it possible to assign a priority greater
than 15 to OBs so that these OBs are not interrupted by the communication.
Reference
Further information on organization blocks is available in the STEP 7 online help.
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S7-1500 Automation System
System Manual, 02/2014, A5E03461182-AB