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EtherCAT to Modbus RTU Gateway
5. Modbus Information
Modbus is a communication protocol developed by Modicon in 1979. You can also visit
http://www.modbus.org
to find more valuable information.
The Different versions of Modbus used today include Modbus RTU (based on serial communication
interfaces such as RS-485 and RS-232), Modbus ASCII and Modbus TCP, which is the Modbus RTU
protocol embedded into TCP packets.
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 query from a master will consist of a slave 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.
RTU Data Structure
Byte 00
Net ID
(Station number)
Net ID: specifies the address of the receiver (Modbus RTU slave).
Function Code: specifies the message type.
Data Field: is the data block.
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Byte 01
Function Code
Reference Number
(Address Mapping)
Byte 02-03
Data Field
Number of Points
Byte 04-05
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