Siemens SIMATIC PCS 7 Operating Manual page 31

Note
If you are using redundant standard signal modules, the system will generate two data
blocks in the AS: DB 1, with the number specified, and DB 2, with the number following this.
Make sure that these blocks are not used twice (e.g. in a Modbus connection). If necessary,
adjust the DB numbers in the H parameters.
Compendium Part B - Process Safety (V8.2)
Operating Manual, 07/2016, A5E35031794-AB
Configuring S7F/FH hardware
6.1 Adapting CPU parameters (single F-system)
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