Communications Between Fault-Tolerant Systems - Siemens SIMATIC S7-400H Manual

Communication Functions
8.4.1

Communications between Fault-Tolerant Systems

Availability
The simplest method of enhancing the availability of interconnected systems is to
use a redundant system bus configured with an optical two-fiber ring or a
duplicated electrical bus system. In this case the connected nodes may consist of
simple standard components.
An increase in availability can best be obtained with an optical two-fiber ring.
Should the two-fiber optic cable rupture, communications continue to exist between
the systems involved. The systems then communicate as if they were connected
to a bus system (line). A ring system contains two redundant components as a
matter of principle and therefore automatically forms a 2-out-of-2 redundant node.
An optical network can also be configured in line-type or star topology. There is no
cable redundancy with line-type topology, however.
Should one electrical cable segment fail, communications between the systems
involved similarly continue to exist (2-out-of-2 redundancy).
The following examples illustrate the differences between the two versions.
Note
The number of connection resources required on the CPs depends on the network
you are using.
If you are using an optical two-fiber ring (refer to Figure 8-3), two connection
resources are required per CP. In contrast to this, only one connection resource is
required per CP if a duplicated electrical network (refer to Figure 8-4) is being
used.
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Automation System S7-400H Fault-tolerant Systems
A5E00068197-07