Siemens SIMATIC PCS 7 Operating Manual page 24

Configuring S7F/FH hardware
6.1 Adapting CPU parameters (single F-system)
Note
No reduction ratio or phase offset may be configured for cyclic interrupt OBs with F-program.
The flow diagram below demonstrates one method you can use for structuring your program
according to process requirements, using the cyclic interrupt OBs.
24
Compendium Part B - Process Safety (V8.2)
Operating Manual, 07/2016, A5E35031794-AB