Siemens SIMATIC CPU 410 System Manual page 323

Note
"Connection" in this manual refers in general to a "configured S7 connection". For other
types of connection, refer to Manuals
NET NCM S7 for Industrial Ethernet
Fault-tolerant S7 connections
The requirement for higher availability with communication components (for example CPs
and buses) means that redundant communication connections are necessary between the
systems involved.
Unlike an S7 connection, a fault-tolerant S7 connection consists of at least two subordinate
partial connections. For the user program, configuration and connection diagnostics, a fault-
tolerant S7 connection and its subordinate partial connections are represented by precisely
one ID (like an S7 connection). Depending on the configuration, it can consist of a maximum
of four subconnections. To maintain communication in the event of an error, two of the four
subconnections are always connected (active) at any given time. The number of
subconnections depends on the possible alternative paths (see figure below) and is
determined automatically. Within an S7-H connection, only subconnections over CP or over
the integrated CPU interface are used in the configuration.
The following examples and the possible configurations in STEP 7 are based on a maximum
of two subnets and a maximum of 4 CPs in the redundant fault-tolerant system.
Configurations with a higher number of CPs or networks are not supported in STEP 7.
CPU 410 Process Automation/CPU 410 SMART
System Manual, 05/2017, A5E31622160-AC
18.8 Basics and terminology of fault-tolerant communication
SIMATIC NET NCM S7 for PROFIBUS
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Supplementary information
SIMATIC
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