Basics of program execution
10.5 Asynchronous instructions
S7-1500R/H redundant system
System Manual, 01/2024, A5E41814787-AF
333
10.5 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, or diagnostics data.
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.
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