Siemens SIMATIC ET 200SP System Manual page 147

Basics of program execution
8.2 Asynchronous instructions
Difference between synchronous/asynchronous instructions
The figure below shows the difference between processing an asynchronous instruction and
processing a synchronous instruction. In this figure the asynchronous instruction is called
five times before the execution is completed, for example, a data record is completely
transferred.
With a synchronous instruction, the instruction is fully executed in each call.
①
②
③
④
Figure 8-1
146
First call of the asynchronous instruction, start of processing
Intermediate call of asynchronous instruction, continuation of processing
Last call of the asynchronous instruction, termination of processing
The synchronous instruction is completely processed at each call
Duration of the complete processing
Difference between synchronous and asynchronous instructions
Distributed I/O system
System Manual, 09/2019, A5E03576849-AJ