Fieldbus
Communication
•
117
MODBUS
Functions
WAGO-I/O-SYSTEM 750
ETHERNET TCP/IP
4.2 MODBUS Functions
4.2.1 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 coupler
according to the current Internet Draft.
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
exchange 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. 4-12: MODBUS/TCP Header
More 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
fieldbus 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.
More information
More information on the
OPEN MODBUS / TCP SPECIFICATION
you can
find in the Internet:
Therefore the MODBUS protocol based essentially on the following basic
data types:
Datatype
Length
Description
Discrete Inputs
1 Bit
Digital Inputs
Coils
1 Bit
Digital Outputs
Input Register
16 Bit
Analog-Input data
Holding Register
16 Bit
Analog-Output data
For each basic data type one or more „FunctionCodes“ are defined.
