Siemens SIMATIC S7-400H System Manual page 159

Connection (S7 connection)
A connection represents the logical assignment of two communication partners executing a
communication service. Every connection has two end points containing the information
required for addressing the communication partner as well as other attributes for establishing
the connection.
An S7 connection is the communication link between two standard CPUs or from one 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 11-1
Note
Generally speaking, "connection" in this manual means a "configured S7 connection". For
other types of connection please refer to the
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 a user program, configuration and connection diagnostics perspective,
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
up to four subconnections, of which two are always established (active) to maintain
communication in the event of an error. The number of subconnections depends on the
possible alternative paths (see figure below) and is identified automatically.
S7-400H
System Manual, 09/2007, A5E00267695-03
Example of an S7 connection
11.2 Fundamentals and basic concepts
SIMATIC NET NCM S7 for PROFIBUS
manuals.
Communication
and
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