Siemens SIMATIC S7-1500 System Manual page 162

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.
With a synchronous instruction, execution is completed upon each call.
①
②
③
④
Figure 8-1
SIMATIC Drive Controller
System Manual, 11/2019, A5E46600094-AA
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
The synchronous instruction is completely executed at each call
Duration of complete execution
Difference between asynchronous and synchronous instructions
Basics of program execution
8.3 Asynchronous instructions
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