Siemens SIMATIC PCS 7 CPU 410-5H System Manual page 274

Supplementary information
15.10 Communication via fault-tolerant S7 connections
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 within a fault-tolerant S7 connection.
Response to failure
With a duplex optic-fiber ring, 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 Figure 11-14).
If a double error, e.g., CPUa1 and CPb2, occurs in the first case of a redundant electrical
bus system (see Figure 11-15), 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 Figure 11-16),
only a double error within a fault-tolerant system, e.g., CPUa1 and CPa2, 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.
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CPU 410-5H Process Automation
System Manual, 09/2014, A5E31622160-AB