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11.2

Modbus Functions

11.2.1

General

Modbus is a manufacturer-independent, open fieldbus standard for diverse

applications in manufacturing and process automation.

The Modbus protocol is implemented according to the current Internet Draft of

the IETF (Internet Engineering Task Force) and performs the following functions:

•

Transmission of the process image

•

Transmission of the fieldbus variables

•

Transmission of different settings and information on the coupler/controller

The data transmission in the fieldside takes place via TCP and via UDP.

The Modbus TCP protocol is a variation of the Modbus protocol, which was

optimized for communication via TCP/IP connections.

This protocol was designed for data exchange in the field level (i.e. for the ex-

change of I/O data in the process image).

All data packets are sent via a TCP connection with the port number 502.

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Modbus TCP segment

The general Modbus TCP header is as follows:

Table 67: Modbus TCP Header

Byte

Identifier

(entered by

receiver)

Additional Information

The structure of a datagram is specific for the individual function. Refer to the

descriptions of the Modbus Function codes.

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For the MODBUS protocol 15 connections are made available over TCP. Thus it

allows digital and analog output data to be directly read out at a fieldbus node

and special functions to be executed by way of simple MODBUS function codes

from 15 stations simultaneously.

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For this purpose a set of MODBUS functions from the Open MODBUS/TCP

specification is realized.

Manual

Version 1.3.0

Protocol-identifier

(is always 0)

(High byte, low

WAGO-I/O-SYSTEM 750 XTR

750-352/040-000 FC ETHERNET G3 XTR

Length field

Unit identifier

(Slave