Siemens SIMATIC S7-400H System Manual page 130

Using I/Os in S7-400H
10.5 Connecting redundant I/Os
4. Redundant I/O connected to a fault-tolerant CPU in single mode
Figure 10-5
Module-oriented redundancy and channel-oriented redundancy
You can specify whether you operate redundant modules with module-oriented redundancy
or with channel-oriented redundancy. There are two "Functional I/O redundancy" module
libraries for this.
You can check which modules you can operate module-oriented and which channel-oriented
in the section "Signal modules for redundancy".
Principle of module-oriented redundancy
Redundancy always applies to the entire module, rather than to individual channels. When a
channel error occurs in the first redundant module, the entire module and its channels are
passivated. If an error occurs on another channel on the second module before the first error
has been eliminated and the first module has been depassivated, this second error cannot
be handled by the system.
Principle of channel-oriented redundancy
Channel errors, whether due to discrepancy or diagnostic interrupt (OB 82), do not lead to
the entire module being passivated. Instead, only the channel involved is passivated.
Depassivation depassivates the channel involved as well as the modules passivated due to
module errors. Channel-oriented passivation significantly increases availability in the
following situations:
● Relatively frequent encoder failures
● Repairs that take a long time
● Multiple channel errors on one module
130
Redundant I/O in single mode
System Manual, 09/2007, A5E00267695-03
S7-400H