Siemens SIMATIC ET 200SP System Manual page 125

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
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.
● Sufficient resources are available in the CPU.
The figure below shows the parallel processing of two jobs of the instruction WRREC. The
two instructions are processed in parallel for a specific period here.
Figure 7-2
Distributed I/O system
System Manual, 12/2015, A5E03576849-AF
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
Parallel processing of the asynchronous instruction WRREC
Basics of program execution
7.3 Asynchronous instructions
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