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Entry type: FAQ, Entry ID: 22276762, Entry date: 11/22/2013

How does connection multiplexing work and how do you configure it for SIMATIC S7-300 Ethernet CPs?

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Connection multiplexing means that all S7, PG and OP connections, which run through the CP, are switched to a multiplex connection between the CP and the CPU. This means that the CP always reserves one connection resource in the CPU. The reservation of an S7 connection resource in the CPU for multiplex operation is independent of whether an S7 connection is configured. No other connection resource in the CPU is reserved by any of the other bilateral S7 connections that you configure. Another characteristic is that the communication speed diminishes with every connection configured. The advantage is that with multiplexing you can configure more connections than you can without.

Fig. 01 shows that a total of seven S7 connections have been configured, but only two connection resources are reserved in the CPU.

Fig. 01

The following Ethernet CPs support connection multiplexing.

Name Order number
CP343-1 6GK7343-1EX11-0XE0
CP343-1 6GK7343-1EX20-0XE0
CP343-1 6GK7343-1EX21-0XE0
CP343-1 6GK7343-1EX30-0XE0
CP343-1 IT 6GK7343-1GX11-0XE0
CP343-1 IT 6GK7343-1GX20-0XE0
CP343-1 Advanced 6GK7343-1GX21-0XE0
CP343-1 Advanced 6GK7343-1GX30-0XE0
CP343-1 Advanced 6GK7343-1GX31-0XE0
Table 01

Proceed on the basis of the following instructions to activate connection multiplexing in your CP.

No. Procedure
1 Open the CP's object properties in the hardware configuration.
2 Open the "Options" tab.
3 Enable the "Reserve CPU connection resources" option.

Fig. 02

4 Save and compile the hardware configuration.
5 Load the configuration into the CPU.
Table 02


  • Connection multiplexing only works for fully specified bilateral S7 connections.
  • As soon as connection multiplexing is enabled, a CPU connection resource is reserved, even if a connection has not yet been configured.
  • A separate connection resource is reserved in the CPU for each unilateral S7 connection.
  • The multiplex connections and the unspecified S7 connections do not appear in the CPU's Properties dialog ("Communication" tab), although they still count. When communication services log on (which means that the connection is set up), the CPU connection resources are reserved in chronological order.

You have a CPU that can handle a maximum of 32 connections. Taking away the connections for PG/OP communication, that leaves 30 connections for the S7 communication (value for S7 basic communication has been set to 0).

Fig. 03

You configure one or more bilateral S7 connections to other devices.

Fig. 04

These connections run through the multiplexing, which means that only one connection resource is reserved in the CPU. However, this does not appear in the Properties dialog.

Fig. 05

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