Modbus; Modbus Tcp Segment; Modbus/Tcp Header - WAGO 750 Series Manual

144 • ETHERNET
Network Communication

5.3.4.1 MODBUS

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
controller 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.
For the MODBUS protocol five 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 five stations simultaneously.
For this purpose a set of MODBUS functions from the OPEN MODBUS /TCP
SPECIFICATION is realized.
More information
These functions and their application are described in detail in Chapter 6
"MODBUS functions".

MODBUS TCP segment

The general MODBUS/TCP header is as follows:
Byte:
Fig. 5-12: MODBUS/TCP Header
More information
The structure of a datagram is specific for the individual function. Refer to
Chapter "MODBUS functions".
0 1 2
Identifier Protocol-
(entered by identifier
receiver) (is always 0)
3 4 5
Length field
(High byte, Low identifier
byte)
WAGO-I/O-SYSTEM 750
6 7 8 - n
Unit MODB Data
US
(Slave function
address) code
ETHERNET TCP/IP