Siemens SIMATIC S7-400H - Connecting Redundant I;O

Using I/O on the S7-400H
7-10
Automation System S7-400H Fault-tolerant Systems
A5E00068197-07
7.4 Connecting Redundant I/O
Was is redundant I/O?
I/O modules are considered redundant when there are two of each and are
configured and operated as redundant pairs. The use of redundant I/O provides
the highest degree of availability since it means that failure of a CPU failure and
failure of a signal module are both tolerated.
Configurations
The following configurations with redundant I/O are possible:
1. Redundant signal modules in the central and expansion devices
The signal modules are installed in pairs in the subsystems of CPU 0 and
CPU 1.
Redundant module pair
Redundant module pair
expansion unit
expansion unit
Central controller Central controller
Figure 7-3 Redundant I/O in central- and expansion devices