Asynchronous Instructions - Siemens Simatic S7-1500R System Manual

7.5

Asynchronous instructions

Introduction
During program execution a distinction is made 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 instructions for transferring data (for example data
records for modules, communication data, diagnostics data).
S7-1500R/H redundant system
System Manual, 10/2018, A5E41814787-AA
Basics of program execution
7.5 Asynchronous instructions
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