Siemens SIMATIC S7-1500 System Manual page 122

Basics of program execution
7.3 Asynchronous instructions
Processing of asynchronous instructions
The figure below shows the difference between the processing of an asynchronous
instruction and a synchronous instruction. In this figure the asynchronous instruction is called
five times before its execution is complete, e.g. a data record has been completely
transferred.
①
②
③
④
Figure 7-1
Parallel processing of asynchronous instruction jobs
A CPU can process several asynchronous instruction jobs in parallel. The CPU processes
the jobs in parallel under the following conditions:
● Several asynchronous instruction jobs are called at the same time.
● The maximum number of simultaneously running jobs for the instruction is not exceeded.
The figure below shows the parallel processing of two jobs of the WRREC instruction. The
two instructions are executed simultaneously for a certain duration.
Figure 7-2
122
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
Parallel processing of the asynchronous instruction WRREC
Automation system
System Manual, 09/2016, A5E03461182-AD