Siemens SIMATIC S7-1500 System Manual page 119

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). 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.
Not e
Communication
The communication (e.g. test functions with the PG) always works with priority 15. To
prevent extending the program runtime unnecessarily in time-critical applications, these OBs
should not be interrupted by communication. Assign a priority > 15 for these OBs.
Reference
Additional information on organization blocks is available in the STEP 7 online help.
Automation system
System Manual, 09/2016, A5E03461182-AD
Basics of program execution
7.1 Events and OBs
119