Siemens SIMATIC S7-1500/ET 200MP System Manual page 212

Difference between synchronous/asynchronous instructions
The figure below shows the difference between the processing of an asynchronous
instruction and a synchronous instruction. In this figure the CPU calls the asynchronous
instruction five times before its execution is complete, e.g. a data record has been completely
transferred.
In the case of a synchronous instruction, this is completely executed with each call.
①
②
③
④
Figure 9-1
Automation system
System Manual, 01/2023, A5E03461182-AK
First call of the asynchronous instruction, start of execution
Intermediate call of the asynchronous instruction, execution continues
Last call of the asynchronous instruction, completion of execution
A job is fully processed by a synchronous instruction with each call.
Duration of a fully processed job
Difference between asynchronous and synchronous instructions
Basics of program execution
9.2 Asynchronous instructions
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