Siemens SIMATIC S7-400H - Example of Redundancy in a 2-Out-Of-2 System with Total Failure; Example of Redundancy in a Network Without Malfunction; Failure of a Redundant Node (Total Failure); Redundant Nodes

Fault-Tolerant Programmable Logic Controllers
1-5
Automation System S7-400H Fault-tolerant Systems
A5E00068197-07
Redundant nodes
Redundant nodes represent the fault tolerance of systems with redundant
components. The independence of a redundant node is given 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 in a simple manner by
means of a block diagram. With a 2-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.
Without malfunction (Figure 1-3).
PS
PS CPU
IM 153-2
Bus
Bus
Redundant nodes with 2-out-of-2 redundancy
CPU
IM 153-2
SM
Figure 1-3 Example of redundancy in a network without malfunction
With malfunction
In Figure 1-4, one component may fail per redundant node without the functionality
of the overall system being impaired.
CPU
CP
CP
CPU
CPUCPU
CP
CP
Bus
Bus
Figure 1-4 Example of redundancy in a 2-out-of-2 system with malfunction
Failure of a redundant node (total failure)
In Figure 1-5, the entire system is no longer operable since both subcomponents
have failed in a 1-out-of-2 redundant node (total failure).
PS
PS CPU
IM 153-2
Bus
Bus
Redundant nodes with 2-out-of-2 redundancy
CPU
IM 153-2
SM
Figure 1-5 Example of redundancy in a 2-out-of-2 system with total failure