Asynchronous Instructions - Siemens Simantic ET200SP System Manual

7.3

Asynchronous instructions

Difference between synchronous/asynchronous instructions
In program processing, a differentiation 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 ended, the execution is also ended.
This is different in the case of asynchronous instructions: When the call of an asynchronous
instruction is ended, the execution of the asynchronous instruction is not necessarily ended
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 (data records for
modules, communication data, diagnostics data).
Processing of 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.
①
②
③
④
Figure 7-1
Distributed I/O system
System Manual, 12/2016, A5E03576849-AG
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
Basics of program execution
7.3 Asynchronous instructions
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