Siemens SIMATIC S7-400H Manual page 78

Using I/O on the S7-400H
Figure 6-2
Rule
When you use a single-channel, switched I/O, the configuration must always be
symmetrical, in other words:
the CPU 417-4 H and other DP masters must be located in both subsystems in
identical slots – for example, in slot 4 on both subsystems – or
the DP masters must be connected on both subsystems to the same integrated
interface – for example, to the PROFIBUS-DP interfaces of the two CPUs
417-4 H.
6-6
Single-Channel, Switched ET 200M Distributed I/O
S7-400H Programmable Controller Fault-Tolerant Systems
Switched ET 200M
distributed I/O
DP/PA coupler IM 157
A5E00068197-04