Manual modbus

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Modicon Modbus Protocol Reference Guide
PI–MBUS–300 Rev. J

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Modicon Modbus Protocol Reference Guide
PI–MBUS–300 Rev. J

June 1996 MODICON, Inc., Industrial Automation Systems One High Street North Andover, Massachusetts 01845

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Preface
This guide is written for the person who will use Modicon Modbus protocols and messages for communication in Modicon programmablecontroller applications. It describes how messages are constructed, and how transactions take place using Modbus protocol. This guide should be used in conjunction with Modicon user guides for the types of networks and programmable controllers present in the application. Familiarity with your network layout, and with your control application, is assumed. The data and illustrations in this book are notbinding. We reserve the right to modify our products in line with our policy of continuous product improvement. Information in this document is subject to change without notice and should not be construed as a commitment by Modicon, Inc., Industrial Automation Systems. Modicon, Inc. assumes no responsibility for any errors that may appear in this document. If you have any suggestions forimprovements, or have found any errors in this publication, please notify us. No part of this document may be reproduced in any form or by any means, electronic or mechanical, without the express written permission of Modicon, Inc., Industrial Automation Systems. All rights reserved. The following are trademarks of Modicon, Inc.: Modbus ModConnect Modcom 984 BM85 BP85 P190 RR85 SA85 SM85 SQ85

DEC® is aregistered trademark of Digital Equipment Corporation. VAX™ and DECNET™ are trademarks of Digital Equipment Corporation. IBM® is a registered trademark of International Business Machines Corporation. IBM AT™, IBM XT™, Micro Channel™, and Personal System/2™ are trademarks of International Business Machines Corporation. Microsoft® and MS–DOS® are registered trademarks of Microsoft Corporation.Western Digital® is a registered trademark of Western Digital Corporation. Ethernet™ is a trademark of Xerox Corporation. Copyright © 1996, Modicon, Inc. Printed in U. S. A.

PI-MBUS–300

Preface

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Related Publications
Refer to the following publication for details about the application of Modicon 984 Programmable Controller systems: GM–0984–SYS 984 Programmable Controller Systems Manual.Modicon, Inc.

Refer to the following publications for details about the application and installation of the Modbus Plus network and related communications devices: GM–MBPL–001 Modbus Plus Network Planning and Installation Guide. Modicon, Inc. Modbus Plus Bridge/Multiplexer User’s Guide. Modicon, Inc.

GM–BM85–001

Refer to the following publication for details about the Modcom IIICommunications Software Library for host computer applications: GM–MC3A–001 Modcom III Communications Software Library User’s Guide. Modicon, Inc.

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Related Publications

PI-MBUS–300

Contents
Chapter 1 Modbus Protocol . . . . . . . . . . . . . . . . . . . . . . .
Introducing Modbus Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Transactions on ModbusNetworks . . . . . . . . . . . . . . . . . . . . . . . . . Transactions on Other Kinds of Networks . . . . . . . . . . . . . . . . . . . The Query–Response Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Two Serial Transmission Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ASCII Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . RTU Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Modbus Message Framing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ASCII Framing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . RTU Framing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ....
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