Table of Contents
Advertisement
A.1. 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.
Modbus/TCP Message Structure
Bytes 00 - 05
6-byte header
The Leading 6 bytes of a Modbus/TCP Protocol Query
Bytes 00-01
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
Modbus/RTU Message Structure
Bytes 06
Net ID
(Station Number)
Net ID = specifies the address of the receiver (Modbus/TCP slave).
Function Code = specifies the message type.
Data Field = is the data block.
PET-AR400 User Manual
Bytes 02-03
Protocol identifier
Bytes 07
Function Code
V1.0.0
Bytes 06 - 11
RTU Data
Bytes 04
Length Field
(upper byte )
Bytes 08-09
Data Field
Reference Number
(Address Mapping)
Top
Bytes 05
Length Field
(lower byte)
Bytes 10-11
Number of Points
P.54
Advertisement
Table of Contents
