Siemens SIMATIC S7-1500 System Manual page 108

Basics of program processing
7.1 Events and OBs
Assignment between event source and OBs
The type of OB determines where you make the assignment between OB and event source:
● With hardware interrupts and isochronous mode interrupts, the assignment is made
during the configuration of the hardware or when the OB is created.
● STEP 7 automatically assigns OBs 91/92 to the MC-Servo interrupt and MC-Interpolator
interrupt as soon as a technology object is added.
● For all other types of OB, the assignment is made when the OB is created, where
applicable after you have configured the event source.
For hardware interrupts, you can change an assignment which has already been made
during runtime with the instructions ATTACH and DETACH. In this case, only the actually
effective assignment changes, and not the configured assignment. The configured
assignment takes effect after loading, and upon each startup.
The CPU ignores hardware interrupts to which you did not assign an OB in your
configuration or which occur after the DETACH instruction. The check as to whether an OB
is assigned to an event does not take place when the associated event occurs, but only
when the hardware interrupt actually has to be executed.
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:
● The call of the assigned OB
● The update of the process image partition of the assigned OB
● The execution 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
Further information on organization blocks is available in the STEP 7 online help.
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Automation system
System Manual, 12/2014, A5E03461182-AC