Siemens SIMATIC PCS 7 CPU 410-5H System Manual page 257

Connection (S7 connection)
A connection represents the logical assignment of two communication peers for executing a
communication service. Every connection has two end points containing the information
required for addressing the communication peer as well as other attributes for establishing
the connection.
An S7 connection is the communication link between two standard CPUs or from a standard
CPU to a CPU in a fault-tolerant system.
In contrast to a fault-tolerant S7 connection, which contains at least two partial connections,
an S7 connection actually consists of just one connection. If that connection fails,
communication is terminated.
Figure 15-6
Note
Generally speaking, "connection" in this manual means a "configured S7 connection". For
other types of connection, refer to Manuals
SIMATIC 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 underlying
subconnections. From the perspective of the user program, configuration, and connection
diagnostics, the fault-tolerant S7 connection with its underlying subconnections is
represented by exactly one ID (just like a standard 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.
CPU 410-5H Process Automation
System Manual, 09/2014, A5E31622160-AB
15.7 Basics and terminology of fault-tolerant communication
Example of an S7 connection
Supplementary information
SIMATIC NET NCM S7 for PROFIBUS
.
and
257