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Entry type: FAQ, Entry ID: 17991275, Entry date: 07/02/2009

What differences are there when configuring S7 connections?

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In this description of how to configure S7 connections it is assumed that all the stations are in one S7 project.

Types of S7 connections:
With S7 connections you differentiate between bilaterally configured and unilaterally configured S7 connections.

Bilaterally configured S7 connections
You recognize bilaterally configured connections by the fact that they receive a connection ID at both endpoints. The partner ID can be identical for both connection partners, but doesn't have to be. The remote endpoint of the S7 connection created is entered automatically in the connection table on the partner side.
You can use the following communication blocks in the S7-300 or S7-400 station for data communication via bilaterally configured S7 connections:

  • FB/SFB14 "GET" and FB/SFB15 "PUT"
  • FB/SFB12 "BSEND" and FB/SFB13 "BRCV" or
  • FB/SFB8 "USEND" and FB/SFB9 "URCV"

Unilaterally configured S7 connections
Unilaterally configured S7 connections are configured on the module that actively sets up the S7 connection.
You can use the communication blocks FB/SFB14 "GET" and FB/SFB15 "PUT" in the S7-300 or S7-400 station for data communication via unilaterally configured S7 connections. You can use these for data communication via unilaterally and bilaterally configured S7 connections. You can only use the communication blocks FB/SFB12 "BSEND" and FB/SFB13 "BRCV" and FB/SFB8 "USEND" and FB/SFB9 "URCV" for data communication via bilaterally configured S7 connections.

More information on the properties and special features of the S7 protocol and on the properties of the different S7 protocol services is available in Entry ID: 26483647.

In NetPro there is a connection table with all the configured communication connections. Unilaterally configured S7 connections have no entry in the "Partner ID" column of the connection table. This field remains empty in the connection table. No S7 connection is configured on the server and no communication blocks are called. The server is managed independently by the CPU's operating system and has resource 0x03. No system data is created for this in the connection partner.

Fig. 1: Bilaterally and unilaterally configured S7 connection


  • All S7 connections that have a connection ID at both connection endpoints are bilaterally configured S7 connections.

  • S7 connections that have no partner ID are unilaterally configured S7 connections or have been created via "unspecified" S7 connections with resource 0x03.


  • With bilaterally configured S7 connections you must load the configuration in both connection partners after creating the S7 connection.
  • With unilaterally configured S7 connections you must load only the one station in whose connection table the S7 connection is shown.

In the sample S7 connection configuration given above all the stations are in one S7 project. You can also set up an S7 connection to an unspecified partner and the endpoints of these connections can be in different projects. More information on this is available in the Online Help of STEP 7:

  • Configuring cross-project connections.
  • Connection types for partners in other projects.
  • Creating cross-project subnetworks.

Connection configuration, S7-300, S7-400, SIMATIC, Connections, Creating a connection, Client, Server, Connection setup, NetPro

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