Siemens SIMATIC S7-1500 System Manual page 230

Basics of program execution
9.5 Asynchronous instructions
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.
With a synchronous instruction, the instruction is fully executed in each call.
①
②
③
④
Figure 9-5 Difference between asynchronous and synchronous instructions
NOTE
Processing of an asynchronous instruction during the SYNCUP system state
If the S7-1500R/H redundant system executes SYNCUP, this extends the processing time for
an asynchronous instruction.
The dynamic behavior of the system state SYNCUP requires that you call asynchronous
instructions cyclically (e.g. in OB 1).
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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
A job is fully processed by a synchronous instruction with each call.
Duration of a fully executed job
S7-1500R/H redundant system
System Manual, 11/2022, A5E41814787-AD