Siemens SIMATIC S7-400H - Communication between Fault-Tolerant Systems

Communication
14.6 Communication via fault-tolerant S7 connections
S7-400H
System Manual, 03/2012, A5E00267695-11
247
14.6.1 Communication between fault-tolerant systems
Availability
The easiest way to enhance availability between linked systems is to implement a redundant
plant bus, using a duplex fiber-optic ring or a dual electrical bus system. The connected
nodes may consist of simple standard components.
Availability can best be enhanced using a duplex fiber-optic ring. If the one of the multimode
fiber-optic cables breaks, communication between the systems involved is maintained. The
systems then communicate as if they were connected to a bus system (line). A ring topology
basically contains two redundant components and automatically forms a 1-out-of-2
redundancy node. A fiber-optic network can be set up as a line or star topology. However,
the line topology does not permit cable redundancy.
If one electrical cable segment fails, communication between the participating systems is
also upheld (1-out-of-2 redundancy).
The examples below illustrate the differences between the two variants.
Note
The number of connection resources required on the CPs depends on the network used.
If you implement a duplex fiber-optic ring (see figure below), two connection resources are
required per CP. In contrast, only one connection resource is required per CP if a double
electrical network (see figure after next) is used.
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