Figure 2-3 Example Of Redundancy In A Network Without An Error Or Fault; Figure 2-4 Example Of Redundancy In A 1-Of-2 System With Error/Fault; Figure 2-5 Example Of Redundancy In A 1-Out-Of-2 System With Total Failure - Siemens SIMATIC S7-400H System Manual

Fault-tolerant automation systems
2.2 Increasing system availability
Redundancy nodes
Redundant nodes represent the reliability of systems with redundant components in case of
failure. 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 entire system.
The availability of the entire system can be illustrated simply based on 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 2-3
With error/fault
The following figure shows how a component may fail without impairing the functionality of
the overall system.
Figure 2-4
Failure of a redundancy node (total failure)
The following figure shows that the system is no longer operable, because both subunits
have failed in a 1-of-2 redundancy node (total failure).
Figure 2-5
24
Example of redundancy in a network without an error or fault
Example of redundancy in a 1-of-2 system with error/fault
Example of redundancy in a 1-out-of-2 system with total failure
System Manual, 09/2007, A5E00267695-03
S7-400H