These operating instructions assume that the installed controller supports the interfaces in this document and that all the requirements stipulated in the controller,

as well as the frequency converter, are strictly observed, along with all limitations therein.

10.6.2 What the User Should Already Know

The Modbus RTU (Remote Terminal Unit) is designed to communicate with any controller that supports the interfaces defined in this document. It is assumed that

the user has full knowledge of the capabilities and limitations of the controller.

10.6.3 Modbus RTU Overview

Regardless of the type of physical communication networks, the Modbus RTU Overview describes the process a controller uses to request access to another

device. This includes how it will respond to requests from another device, and how errors will be detected and reported. It also establishes a common format for

the layout and contents of message fields.

During communications over a Modbus RTU network, the protocol determines how each controller will learn its device address, recognise a message addressed

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.

Controllers communicate using a master-slave technique in which only one device (the master) can initiate transactions (called queries). The other devices (slaves)

respond by supplying the requested data to the master, or by taking the action requested in the query.

The master can address individual slaves, or can initiate a broadcast message to all slaves. Slaves return a message (called a response) to queries that are

addressed to them individually. No responses are returned to broadcast queries from the master. The Modbus RTU protocol establishes the format for the master's

query by placing into it 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 is also constructed using Modbus protocol. It contains fields confirming the action taken, any data to be returned, and an error-checking

field. If an error occurs 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

in response, or a time-out will occur.

10.6.4 Frequency Converter with Modbus RTU

The frequency converter communicates in Modbus RTU format over the built-in RS-485 interface. Modbus RTU provides access to the Control Word and Bus

Reference of the frequency converter.

The Control Word allows the Modbus master to control several important functions of the frequency converter:

•

Start

•

Stop of the frequency converter in various ways:

Coast stop

Quick stop

DC Brake stop

Normal (accel/decel) stop

•

Reset after a fault trip

•

Run at a variety of preset speeds

•

Run in reverse

•

Change the active set-up

•

Control the frequency converter's built-in relay

The Bus Reference is commonly used for speed control. It is also possible to access the parameters, read their values, and where possible, write values to them.

This permits a range of control options, including controlling the setpoint of the frequency converter when its internal PI controller is used.

190

AF-650 GP Design Guide

Table of Contents