Siemens SIMATIC PCS 7 System Manual page 57

You yourself decide on any other components you want to duplicate to increase availability
depending on the specific process you are automating.
Redundancy nodes
Redundant nodes represent the fail safety of systems with redundant components. A
redundant node can be considered as independent when the failure of a component within
the node does not result in reliability constraints in other nodes or in the overall system.
The availability of the overall system can be illustrated simply in a block diagram. With a 1-
out-of-2 system, one component of the redundant node may fail without impairing the
operability of the overall system. The weakest link in the chain of redundant nodes
determines the availability of the overall system
No error/fault
Figure 4-6
With error/fault
The following figure shows how a component may fail without impairing the functionality of
the overall system.
CPU 410-5H Process Automation/CPU 410 SMART
System Manual, 09/2014, A5E32631667-AB
4.3 Fault-tolerant automation systems (redundancy operation)
Example of redundancy in a network without error
I/O configuration variants
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