Siemens SIMATIC S7-400H System Manual page 249

Configuration view = Physical view
You decide during configuration if the additional CPs are used to increase resources or
availability. This configuration is typically used to increase availability.
Note
Internal and external interface
Communication between fault-tolerant systems may only take place between internal
interfaces or external interfaces (CP). Communication between internal interface and CP is
not possible.
Response to failure
If a duplex fiber-optic ring is used, only a double error within a fault-tolerant system (e.g.
CPUa1 and CPa2 in one system) leads to total failure of communication between the
systems involved (see first figure).
If a double error (e.g. CPUa1 and CPb2) occurs in the first case of a redundant electrical bus
system (see second figure), this results in a total failure of communication between the
systems involved.
In the case of a redundant electrical bus system with CP redundancy (see third figure), only
a double error within a fault-tolerant system (e.g. CPUa1 and CPUa2) or a triple error (e.g.
CPUa1, CPa22, and bus2) will result in a total failure of communication between the systems
involved.
Fault-tolerant S7 connections
Any disruption of subconnections while communication jobs are active over fault-tolerant S7
connections leads to extended delay times.
S7-400H
System Manual, 03/2012, A5E00267695-11
14.6 Communication via fault-tolerant S7 connections
Communication
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