Modbus over serial line
MODBUS.ORG
MODBUS over Serial Line Specification & Implementation guide V1.0
Modbus.org 12/02/02
http://www.modbus.org/
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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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