Asynchronous Instructions - Siemens SIMATIC S7-1500 System Manual

Basics of program execution

8.3 Asynchronous instructions

OB priority and runtime behavior
If you have assigned an OB to the event, the OB has the priority of the event. The CPUs of
the SIMATIC Drive Controller support priorities 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
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.
Additional information
Additional information on organization blocks is available in the STEP 7 online help.
8.3 Asynchronous instructions
Introduction
Program execution makes a distinction between synchronous and asynchronous
instructions.
The "synchronous" and "asynchronous" properties relate to the temporal relationship
between the call and execution of the instruction.
The following applies to synchronous instructions: When the call of a synchronous instruction
is complete, execution of the instruction is also complete.
This is different in the case of asynchronous instructions: When the call of an asynchronous
instruction is complete, execution of the asynchronous instruction is not necessarily
complete yet. This means the execution of an asynchronous instruction can extend over
multiple calls. The CPU processes asynchronous instructions in parallel with the cyclic user
program. Asynchronous instructions generate jobs in the CPU for their processing.
Asynchronous instructions are usually Motion Control instructions or instructions for
transferring data, e.g. data records for modules, communication data, or diagnostics data.
160
SIMATIC Drive Controller
System Manual, 11/2019, A5E46600094-AA