Siemens SIMATIC S7-400H System Manual page 139

Using redundant digital input modules with non-redundant encoders
With non-redundant encoders, you use digital input modules in a 1-out-of-2 configuration:
Figure 10-6
The use of redundant digital input modules increases their availability.
Discrepancy analysis detects "Continuous 1" and "Continuous 0" errors of the digital input
modules. A "Continuous 1" error means the value 1 is applied permanently at the input, a
"Continuous 0" error means that the input is not energized. This can be caused, for example,
by a short-circuit to L+ or M.
The current flow over the chassis ground connection between the modules and the encoder
should be the minimum possible.
When connecting an encoder to several digital input modules, the redundant modules must
operate at the same reference potential.
If you want to replace a module during operation and are not using redundant encoders, you
will need to use decoupling diodes.
You will find connection examples in Appendix F.
Note
Remember that the current output by proximity switches (Beros) must be twice the current
specified in the technical specifications of the individual modules.
S7-400H
System Manual, 09/2007, A5E00267695-03
Fault-tolerant digital input module in 1-out-of-2 configuration with one encoder
Using I/Os in S7-400H
10.5 Connecting redundant I/Os
139