Modbus over serial line

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MODBUS over serial line specification and implementation guide V1.0

MODBUS.ORG

MODBUS over Serial Line Specification & Implementation guide V1.0

Modbus.org 12/02/02

http://www.modbus.org/

1/44

MODBUS over serial line specification and implementation guide V1.0

MODBUS.ORG

Contents

1

Introduction..............................................................................4
1.1 1.2 1.3 1.4 1.5 Scope of this document ............................................................. 4 Protocol overview....................................................................... 5 Conventions ............................................................................... 5 Compliance ................................................................................ 6Glossary..................................................................................... 6 MODBUS Master / Slaves protocol principle.............................. 7 MODBUS Addressing rules........................................................ 8 MODBUS frame description....................................................... 8 Master / Slaves StateDiagrams................................................. 9 The two serial Transmission Modes......................................... 12 Error Checking Methods .......................................................... 19 Preamble.................................................................................. 20 Data Signaling Rates ............................................................... 20 ElectricalInterfaces.................................................................. 21 Multipoint System requirements............................................... 27 Mechanical Interfaces .............................................................. 29 Cables...................................................................................... 32 Visual Diagnosis ......................................................................32 Installation................................................................................ 33 User Guide............................................................................... 33

2

MODBUS Data Link Layer .......................................................7
2.1 2.2 2.3 2.4 2.5 2.6

3

PhysicalLayer........................................................................20
3.1 3.2 3.3 3.4 3.5 3.6 3.7

4

Installation and Documentation .............................................33
4.1 4.2

5 6

Implementation Classes ........................................................34 Appendix ................................................................................35
6.1 6.2 6.3 Appendix A - Management of Serial Line Diagnostic Counters 35 Appendix B -LRC/CRC Generation ......................................... 38 Appendix E - References ......................................................... 44

Modbus.org 12/02/02

http://www.modbus.org/

2/44

MODBUS over serial line specification and implementation guide V1.0

MODBUS.ORG

1.0

Document modifications Month-Year Modifications Nov 02 Creation. This document comprises adescription of Master / slave protocol and of the two different transmission modes ( RTU, ASCII). The main features of the physical layer ( RS485, RS232) and some recommendations are provided. Implementation classes are proposed to guide the implementation.

Modbus.org 12/02/02

http://www.modbus.org/

3/44

MODBUS over serial line specification and implementation guide V1.0

MODBUS.ORG

11.1

Introduction
Scope of this document

The MODBUS standard defines an application layer messaging protocol, positioned at level 7 of the OSI model that provides "client/server" communications between devices connected on different types of buses or networks. It standardizes also a specific protocol on serial line to exchange MODBUS request between a master and one or several slaves. The...
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