Siemens SIMATIC S7-400H Manual page 13

Why do we have fault-tolerant programmable logic controllers?
The objective of using high-availabilty programmable logic controllers is a reduction
of losses of production. It does not matter whether the losses are caused by an
error or as a result of maintenance work.
The higher the costs of a stoppage, the more worthwhile it is to use a fault-tolerant
system. The generally higher investment costs of fault-tolerant systems are quickly
compensated by the avoidance of losses of production.
Software redundancy
In a large number of applications, requirements in respect of the quality of
redundancy or the number of system sections that necessitate redundant PLCs
are not high enough to warrant the use of a specific fault-tolerant system.
Frequently, simple software mechanisms are sufficient to allow continuation of a
failed control task on a substitute system in the event of an error.
The "SIMATIC S7 Software Redundancy" options software can run on S7-300 and
S7-400 standard systems to control processes that tolerate transfer times to a
substitute system within seconds, such as water works, water treatment systems
or traffic flows.
S7-400H Programmable Controller Fault-Tolerant Systems
A5E00068197-04
Fault-Tolerant Systems in Automation Engineering
1-3