Siemens SIMATIC S7-1500/ET 200MP System Manual page 210

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.
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 interrupted by communication. Assign a priority > 15 for these
OBs.
Programming style guide
The programming guidelines described in the programming style guide help you to create a
uniform program code. You can better maintain and reuse the uniform program code. This
allows you to detect or avoid errors early on, for example, through compilers.
The programming style guide is available on the Internet
(https://support.industry.siemens.com/cs/de/en/view/109478084).
Reference
More information on organization blocks is available in the STEP 7 online help.
Automation system
System Manual, 01/2023, A5E03461182-AK
Basics of program execution
9.1 Events and OBs
209