Siemens SIMATIC S7-400H System Manual page 22

Fault-tolerant automation systems
2.1 Redundant automation systems in the SIMATIC series
Software redundancy
For many applications, the requirements for redundancy quality or the extent of plant sections
that may require redundant automation systems do not necessarily justify the implementation
of a special fault-tolerant system. Usually, simple software mechanisms are adequate to allow
a failed control task to be continued on a substitute system if a problem occurs.
The optional "SIMATIC S7 Software Redundancy" software package can be implemented on
S7-300 and S7-400 standard systems to control processes that tolerate failover delays to a
substitute system in the seconds range, such as water works, water treatment systems or
traffic flows.
Redundant I/O
Input/output modules are termed redundant when they exist twice and they are configured
and operated as redundant pairs. The use of a redundant I/O means maximum availability,
because such systems will tolerate failure of a CPU and a signal module; see section
Connecting redundant I/Os (Page 127).
If you require a redundant I/O, you use the blocks of the "Functional I/O Redundancy" block
library.
These blocks are available in the "Redundant IO(V1)" library (module-oriented) or
"Redundant IO CGP" library (channel-oriented) under STEP 7\S7_LIBS\RED_IO.
The functions and use of the blocks are described in the corresponding online help.
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System Manual, 09/2007, A5E00267695-03
S7-400H