Siemens Simatic S7-1500R System Manual page 163

Basics of program execution
7.5 Asynchronous instructions
Difference between synchronous/asynchronous instructions
The figure below shows the difference between the processing of an asynchronous
instruction and a synchronous instruction. In the figure below, an asynchronous instruction is
called in five cycles before execution is completed, for example, a data record is transferred
in full.
①
②
③
④
Figure 7-5
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.
Recommendation: Always access asynchronous instructions in the cyclic user program of
the S7-1500R/H redundant system, for example in OB 1.
162
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
Difference between asynchronous and synchronous instructions
S7-1500R/H redundant system
System Manual, 10/2018, A5E41814787-AA