Siemens SIMATIC PCS 7 CPU 410-5H System Manual page 259

If the active subconnection fails, the already established second subconnection automatically
takes over communication.
Resource requirements of fault-tolerant S7 connections
The fault-tolerant CPU supports operation of 62 fault-tolerant S7 connections (see technical
specifications). Each connection needs a connection resource on the CPU; subconnections
do not need any additional connection resources. On the CP, on the other hand, each
subconnection needs a connection resource.
Note
If you have configured several fault-tolerant S7 connections for a fault-tolerant station,
establishing them may take a considerable time. If the configured maximum communication
delay was set too short, link-up and updating is canceled and the redundant system state is
no longer achieved (see Chapter Time monitoring (Page 123)).
CPU 410-5H Process Automation
System Manual, 09/2014, A5E31622160-AB
15.7 Basics and terminology of fault-tolerant communication
Supplementary information
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