1
Modicon
Modbus Protocol
Reference Guide
PI–MBUS–300 Rev. J
DOK-
3
Modicon
Modbus Protocol
Reference Guide
PI–MBUS–300 Rev. J
June 1996
MODICON, Inc., Industrial Automation Systems
One High Street
North Andover, Massachusetts 01845
PI-MBUS–300
Preface iii
Preface
This guide is written for the person who will use Modicon Modbus protocols and
messages for communication in Modicon programmable controller 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 not binding. 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 for improvements, 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 984 P190 SM85
ModConnect BM85 RR85 SQ85
Modcom BP85 SA85
DEC
is a registered 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
Related Publicationsiv
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.
GM–BM85–001 Modbus Plus Bridge/Multiplexer User’s Guide.
Modicon, Inc.
Refer to the following publication for details about the Modcom III Communications
Software Library for host computer applications:
GM–MC3A–001 Modcom III Communications Software Library User’s Guide.
Modicon, Inc.
PI–MBUS–300
Contents vii
Contents
Chapter 1 Modbus Protocol 1. . . . . . . . . . . . . . . . . . . . . . .
Introducing Modbus Protocol 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Transactions on Modbus Networks 4. . . . . . . . . . . . . . . . . . . . . . . . .
Transactions on Other Kinds of Networks 4. . . . . . . . . . . . . . . . . . .
The Query–Response Cycle 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The Two Serial Transmission Modes 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ASCII Mode 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
RTU Mode 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Modbus Message Framing 8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ASCII Framing 8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
RTU Framing 9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
How the Address Field is Handled 10. . . . . . . . . . . . . . . . . . . . . . . . . .
How the Function Field is Handled 10. . . . . . . . . . . . . . . . . . . . . . . . .
Contents of the Data Field 11. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Contents of the Error Checking Field 12. . . . . . . . . . . . . . . . . . . . . . .
How Characters are Transmitted Serially 13. . . . . . . . . . . . . . . . . . . .
Error Checking Methods 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Parity Checking 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
LRC Checking 15. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CRC Checking 16. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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