PET-AR400 User Manual
V1.0.0
P.51
5.
Modbus Application
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.
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.
