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ET-7000/PET-7000 AIO Series User Manual, version 1.0.1 beta3 Page: 90
Copyright © 2009 ICP DAS Co., Ltd. All Rights Reserved. E-mail: [email protected]
4.1. What is Modbus TCP/IP?
Modbus is a communication protocol developed by Modicon in 1979.
Different versions of Modbus used today include Modbus RTU (based on serial
communication like RS485 and RS232), Modbus ASCII and Modbus TCP, which is 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 reliable. The
master query’s the slave and the slave responds with the reply. The protocol is open and
hence highly scalable.
4.2. 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 supplying the requested data to the master, or by
taking the action requested in the query.
A master’s query 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 slave’s
response consists of fields confirming the action taken, any data to be returned, and an
error checking field.
Address
(1 byte)
Function
Code
(1 byte)
Data
(0 to 252 bytes)
Checksum
(2 bytes)
Address
specifies the address of the receiver.
Function Code
specifies the message type.
Data
is the data block.
Checksum
specifies the numerical check value for testing the validity of the protocol.