Siemens SIMATIC S7-400H System Manual page 234

Communication
14.2 Basics and terminology of fault-tolerant communication
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 62/46 (see the technical specifications) fault-
tolerant S7 connections. 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.
NOTICE
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 is set too short, link-up and updating is canceled and the redundant system mode is
no longer reached (see section Time monitoring (Page 155)).
Each CPU provides the connection resources according to its configuration, which means
that a CPU 417-5H that was configured as CPU 412-5H only provides the connection
resources of a CPU 412-5H.
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