Siemens SIMATIC S7-1500 System Manual page 335

Basics of program execution
10.5 Asynchronous instructions
With a synchronous instruction, the instruction is fully executed in each call.
①
②
③
④
Figure 10-5
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.
The dynamic behavior of the system state SYNCUP requires that you call asynchronous
instructions cyclically (e.g. in OB 1).
334
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, 01/2024, A5E41814787-AF