MODBUS over serial line specification and implementation guide V1.02
Modbus.org
Modbus.org http://www.modbus.org/ 1/44
Dec 20, 2006
MODBUS over Serial Line
Specification and Implementation Guide
V1.02
MODBUS over serial line specification and implementation guide V1.02
Modbus.org
Modbus.org http://www.modbus.org/ 2/44
Dec 20, 2006
Contents
1 Introduction ..............................................................................4
1.1 Scope of this document ............................................................. 4
1.2 Protocol overview....................................................................... 5
1.3 Conventions............................................................................... 5
1.4 Compliance................................................................................ 6
1.5 Glossary..................................................................................... 6
2 MODBUS Data Link Layer.......................................................7
2.1 MODBUS Master / Slaves protocol principle.............................. 7
2.2 MODBUS Addressing rules........................................................ 8
2.3 MODBUS frame description....................................................... 8
2.4 Master / Slaves State Diagrams................................................. 9
2.5 The two serial Transmission Modes......................................... 12
2.6 Error Checking Methods .......................................................... 19
3 Physical Layer........................................................................20
3.1 Preamble.................................................................................. 20
3.2 Data Signaling Rates ............................................................... 20
3.3 Electrical Interfaces.................................................................. 21
3.4 Multipoint System requirements............................................... 27
3.5 Mechanical Interfaces.............................................................. 29
3.6 Cables...................................................................................... 32
3.7 Visual Diagnosis ...................................................................... 32
4 Installation and Documentation .............................................33
4.1 Installation................................................................................ 33
4.2 User Guide............................................................................... 33
5 Implementation Classes ........................................................34
6 Appendix................................................................................35
6.1 Appendix A - Management of Serial Line Diagnostic Counters35
6.2 Appendix B - LRC/CRC Generation......................................... 38
6.3 Appendix E - References ......................................................... 44
MODBUS over serial line specification and implementation guide V1.02
Modbus.org
Modbus.org http://www.modbus.org/ 3/44
Dec 20, 2006
Document modifications
Month-Year Modifications
1.0 Nov 02 Creation.
This document comprises a description 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.
1.01 Aug 30, 2006 Minor clarifications and correction of typos.
1.02 Dec 20, 2006 Minor clarifications and correction of typos.
MODBUS over serial line specification and implementation guide V1.02
Modbus.org
Modbus.org http://www.modbus.org/ 4/44
Dec 20, 2006
1 Introduction
1.1 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 objective of this document is to present the MODBUS protocol over serial line, in order to be used by all system designers when
they want to implement MODBUS protocol on their serial line products. Thus, this document will facilitate interoperability between
devices using the MODBUS protocol.
This document comes in complement to the document called "MODBUS Application Protocol Specification".
In chapter 5 different implementation classes are defined for "MODBUS Serial Line". Specification of a class is the sum of
requirements that a device must
respect in order to belong to that class.
Figure 1: General overview of MODBUS documents
MODBUS
Application
Protocol
Specification
The MODBUS
application protocol
(
OSI Level 7)
MODBUS over
Serial Line
Specification &
Implementation
Guide
Serial Line specification
(OSI Levels 1 & 2)
This
document
MODBUS over serial line specification and implementation guide V1.02
Modbus.org
Modbus.org http://www.modbus.org/ 5/44
Dec 20, 2006
1.2 Protocol overview
This document describes the MODBUS over Serial Line protocol. MODBUS Serial Line protocol is a Master-Slave protocol. This
protocol takes place at level 2 of the OSI model.
A master-slave type system has one node (the master node) that issues explicit commands to one of the "slave" nodes and processes
responses. Slave nodes will not typically transmit data without a request from the master node, and do not communicate with other
slaves.
At the physical level, MODBUS over Serial Line systems may use different physical interfaces (RS485, RS232). TIA/EIA-485 (RS485)
Two-Wire interface is the most common. As an add-on option, RS485 Four-Wire interface may also be implemented. A TIA/EIA-232-
E (RS232) serial interface may also be used as an interface, when only short point to point communication is required. (see chapter
"Physical Layer")
The following figure gives a general representation of MODBUS serial communication stack compared to the 7 layers of the OSI
model.
Layer ISO/OSI Model
7 Application MODBUS Application Protocol
6 Presentation Empty
5 Session Empty
4 Transport Empty
3 Network Empty
2 Data Link MODBUS Serial Line Protocol
1 Physical EIA/TIA-485 (or EIA/TIA-232)
Figure 2: MODBUS Protocols and ISO/OSI Model
MODBUS application layer messaging protocol, positioned at level 7 of the OSI model, provides client/server communication between
devices connected on buses or networks. On MODBUS serial line the client
role is provided by the Master of the serial bus and the
Slaves nodes act as servers
.
1.3 Conventions
In this document, the following words are used to define the significance of each particular requirement.
"MUST" / "REQUIRED"
All requirements containing the word "MUST" are mandatory
. The word MUST, or the adjective "REQUIRED", means that the item is
an absolute requirement of the implementation. These words are underlined.
"SHOULD" / "RECOMMENDED"
All recommendations containing the word "SHOULD", or the adjective “RECOMMENDED”, are considered desired behavior. These
recommendations should
be used as a guideline when choosing between different options to implement functionality. There may be
valid reasons in particular circumstances to ignore this item, but the full implications should be understood and the case carefully
weighed before choosing a different course. These words are underlined
.
"MAY" / "OPTIONAL"
The word “MAY”, or the adjective "OPTIONAL", means that this item is truly optional. One designer may choose to include the item
because a particular marketplace requires it or because it enhances the product, for example; another designer may omit the same
item.
EIA/TIA-485
(or EIA/TIA-232)
MODBUS Master / Slave
MODBUS Application
Layer
Client / server
