Redundant Nodes; Example Of Redundancy In A Network Without Malfunction; Failure Of A Redundant Node (Total Failure); Example Of Redundancy In A 2-Out-Of-2 System With Total Failure - Siemens SIMATIC S7-400H Manual

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).
Figure 1-3
With malfunction
In Figure 1-4, one component may fail per redundant node without the functionality
of the overall system being impaired.
Figure 1-4

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).
Figure 1-5
Automation System S7-400H Fault-tolerant Systems
A5E00068197-07
PS CPU
PS
CPU
Redundant nodes with 2-out-of-2 redundancy

Example of redundancy in a network without malfunction

CPU CP
CP
CPU
Example of redundancy in a 2-out-of-2 system with malfunction
PS CPU
PS CPU
Redundant nodes with 2-out-of-2 redundancy

Example of redundancy in a 2-out-of-2 system with total failure

Fault-Tolerant Programmable Logic Controllers
Bus
IM 153-2
Bus
IM 153-2
Bus CP
CPU
CP CPU
Bus
Bus
IM 153-2
Bus
IM 153-2
SM
SM
1-5