User Manual

Chapter 1 Introduction 3
1.2.1 Overview
The Modbus/RTU and Modbus/ASCII protocols define how a “master”
device polls one or more “slave” devices to read and write real-time data
over RS-232, RS-422, or RS-485 serial data communication. The simplicity
of Modbus/RTU not only allows rapid implementation, but can also re-
main flexible enough to be applied in virtually all industrial situations.
During Modbus network communication, the protocol determines how
each controller will know its device address, recognize a message ad-
dressed to it, determine the kind of action to be taken, and extract any
data or other information contained in the message. If a reply is required,
the controller will construct the reply message and send it back using
Modbus protocol.
The way controllers communicate with each other is by using a master-
slave technique, in which only one device (the master) can initiate que-
ries. The other devices (the slaves) respond by supplying the requested
data to the master, or by taking the action requested in the query. Typical
master devices include host processors and programming panels. Typi-
cal slaves include programmable controllers.
It is the master that can address individual slaves and initiate a broadcast
message to all slaves. On the other hand, slaves return a response to
queries that are addressed to them individually. Responses are not re-
turned to broadcast queries from the master.
The Modbus protocol has a definite format for the master’s query, which
incorporates the device (or broadcast) address, a function code defining
the requested action, any data to be sent, and an error-checking field.
The slave’s response message, which is also constructed using Modbus
protocol, contains fields confirming the action taken, any data to be
returned, and an error-checking field. If an error occurred in receipt of the
message, or if the slave is unable to perform the requested action, the
slave will construct an error message and send it as its response.
The basic system architecture is as follows: