Ethernet I/O Modules
ICP DAS CO., LTD.
PETL/tET/tPET DIO Series User Manual, Ver. 2.2, Aug. 2017, Page: 84
6.1 What is Modbus TCP/IP?
Modbus is a communication protocol that was developed by Modicon Inc. in 1979, and was originally
designed for use with Modicon controllers. Detailed information regarding the Modbus protocol can
be found at:
http://www.modbus.org
.
The different versions of the Modbus protocol used today include Modbus RTU, which is based on
serial communication interfaces such as RS-485 and RS-232, Modbus ASCII and Modbus TCP, which
uses the Modbus RTU protocol embedded into TCP packets.
Modbus TCP is an internet protocol. The protocol embeds a Modbus frame into a TCP frame so that a
connection oriented approach is obtained, thereby making it more reliable. The master queries the
slave and the slave responds with a reply. The protocol is open and, hence, highly scalable.
6.2 Modbus Message Structure
Modbus devices communicate using a master-slave (client-server) technique in which only one device
(the master/client) can initiate transactions (called queries). The other devices (slaves/servers)
respond by either supplying the requested data to the master, or by taking the action requested in the
query.
A query from a master will consist of a slave address (or broadcast address), a function code defining
the requested action, any required data, and an error checking field. A response from a slave consists
of fields confirming the action taken, any data to be returned, and an error checking field.
The Modbus/TCP Message Structure
Bytes 00 - 05
Bytes 06 - 11
6-byte header
RTU Data
