Technical data

214 • Fieldbus Communication
MODBUS Functions
WAGO-I/O-SYSTEM 750
BACnet/IP Controller
4.3 MODBUS Functions
4.3.1 General
MODBUS is a manufacturer-independent, open fieldbus standard for diverse
applications in manufacturing and process automation.
The MODBUS protocol is implemented for the transmission of the process
image, the fieldbus variables, different settings and information on the control-
ler according to the current Internet Draft of the IETF (Internet Engineering
Task Force).
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 .
MODBUS/TCP segment
The general MODBUS/TCP header is as follows:
Byte: 0 1 2 3 4 5 6 7 8 - n
Identifier
(entered by
receiver)
Protocol-
identifier
(is always 0)
Length field
(High byte,
Low byte)
Unit
identifier
(Slave address)
MODBUS
function code
Data
Fig. 4-15: MODBUS/TCP Header
Additional Information
The structure of a datagram is specific for the individual function. Refer to
the descriptions of the MODBUS Function Codes.
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 field-
bus node and special functions to be executed by way of simple MODBUS
function codes from 15 stations simultaneously.
For this purpose a set of MODBUS functions from the OPEN MODBUS /TCP
SPECIFICATION is realized.
Additional Information
More information on the OPEN MODBUS / TCP SPECIFICATION you can
find in the Internet: www.modbus.org.