5 Implementation of Modbus TCP
5.1 Common Modbus description
XI/ON: XNE-GWBR-2ETH-MB
10/2011 MN05002008Z-EN
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5
Implementation of Modbus TCP
5.1
Common Modbus description
Modbus is an application layer messaging protocol, positioned at level 7 of the OSI model, that
provides client/server communication between devices connected on different types of buses
or networks.
The industry’s serial de facto standard since 1979, Modbus continues to enable millions of
automation devices to communicate. Today, support for the simple and elegant structure of
Modbus continues to grow. The Internet community can access Modbus at a reserved system
port 502 on the TCP/IP stack.
Modbus is a request/reply protocol and offers services specified by function codes. Modbus
function codes are elements of Modbus request/reply PDUs.
It is currently implemented using:
• TCP/IP over Ethernet. (that is used for the XI/ON Modbus-TCP gateways and described in
the following)
• Asynchronous serial transmission over a variety of media (wire: RS232, RS422, RS485,
fiber, radio, etc.)
• Modbus PLUS, a high speed token passing network.
Schematic representation of the Modbus Communication Stack (according to Modbus Appli-
cation Protocol Specification V1.1 of Modbus-IDA):
Note
The following description of the Modbus protocol is taken from the Modbus Appli-
cation Protocol Specification V1.1 of Modbus-IDA.
Figure 32:
Schematic
representation
of the Modbus
Communication
Stack
Modbus Application Layer
Modbus TCP
TCP
IP
others
Master/Slave
Client/Server
Physical layer
Modbus plus
Physical layer
Rsxxx
Physical layer
Ethernet
Physical layer
