Modbus Message Structure - ICPDAS ET-2268 CR User Manual

ET-2200 Series Ethernet I/O Modules
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, 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
6-byte header
 The Leading 6 bytes of a Modbus/TCP Protocol Query
Byte 00 Byte 01
Transaction Identifier

Transaction identifier = Assigned by the Modbus/TCP master (client)

Protocol identifier = 0

Length field (upper byte) = 0 (since all messages are smaller than 256)

Length field (lower byte) = The number of following RTU data bytes
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RTU Data
Byte 02 Byte 03
Protocol Identifier
Bytes 06 - 11
Byte 04 Byte 05
Length Field Length Field
(upper byte ) (lower byte)
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