Siemens SIMATIC S7-400H System Manual page 185

Fault-tolerant systems

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The valid values that can be processed by the user program are always located at the lower

address of both redundant modules. This means that only the lower address can be used for

the application; the values of the higher address are not relevant for the application.

Use of FB 450 "RED_IN" and 451 "RED_OUT" when using process image partitions

For each priority class used (OB 1, OB 30 ... OB 38), you must use a separate process

image partition.

System Manual, 03/2012, A5E00267695-11

13.4 Connecting redundant I/O to the PROFIBUS DP interface

Using I/Os in S7–400H