SEND/RECEIVE Interface in the User Program

3.4Additional Information

3.4.1Programming Data Transfer on TCP Connections

Purpose of TCP Connections

TCP connections should be used above all for connections to systems of a different type if they do not support RFC1006.

For communication between devices of the SIMATIC family, you should use the more convenient ISO-on-TCP connections! The following sections explain certain special points to note.

Special Features

SBlock types (FC calls)

Use only the following functions (FCs) for data transmission:

With the S7-CPs for S7-400, you must use FCs AG_LSEND (FC 50) and AG_LRECV (FC 60) on TCP connections!

With the latest S7-CPs for S7-300, you also use FCs AG_SEND (FC 5) and AG_RECV (FC 6) for TCP connections.

SFrame length

On the TCP connections, the protocol contains no information about the end of a message and start of a new message.

The receiving station must therefore know how many bytes belong to a message and transfer an ANY pointer corresponding exactly to this length when FC AG_LRECV is called. (Example: if 100 bytes of data are always transferred, the ANY pointer could appear as: P#DB100.DBX 0.0 Byte 100)

If you want to receive data with a variable length, follow the steps below:

Include information on the length of the user data in the frame before the actual user data. Initially, you should only evaluate the length information on the receiving station. Fetch the appropriate length of user data in another receive job by specifying an ANY pointer of the required length on the FC interface for fetching the actual user data.

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SIMATIC NET NCM S7 for Industrial Ethernet

Release 6/2003

C79000–G8976–C129–07

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Siemens NCM S7 appendix Additional Information, Programming Data Transfer on TCP Connections, Purpose of TCP Connections

NCM S7 specifications

Siemens NCM S7, which stands for Siemens Network Control Module for the S7 series, represents a significant advancement in industrial automation technology. This robust control system is designed to enhance productivity and efficiency in various manufacturing processes. Its primary purpose is to facilitate seamless communication between field devices and higher-level control systems, ensuring that processes run smoothly and effectively.

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