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10 Modbus TCP
10.1 General Modbus Description
Modbus is an application layer messaging protocol, positioned at level 7 of the OSI model, that pro-
vides client/server communication between devices connected on different types of buses or net-
works.
The industry’s serial de facto standard since 1979, Modbus continues to enable millions of automa-
tion devices to communicate. Today, support for the simple and elegant structure of Modbus con-
tinues 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 func-
tion codes are elements of Modbus request/reply PDUs (Protocol Data Unit).
Currently, the following sub protocols are implemented:
TCP/IP over Ethernet. (used for the TBEN-L modules and described in the following)
Asynchronous serial transmission over a variety of media (wire: RS232, RS422, RS485, optical:
fiber, radio, etc.)
Modbus PLUS, a high speed token passing network.
Schematic representation of the Modbus Communication Stack (according to Modbus Application
Protocol Specification V1.1 of Modbus-IDA):
NOTE
The following description of the Modbus protocol is taken from the Modbus Application
Protocol Specification V1.1 of Modbus-IDA.
Modbus Application Layer
Modbus TCP
TCP
IP
andere
Master/Slave
Client/Server
Physical layer
Modbus plus
Physical layer
Rsxxx
Physical layer
Ethernet
Physical layer