Modbus Protocol Specification

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modbus protocol

Modbus Protocol
PDF format version of the MODBUS Protocol
The original was found at
(In case of any discrepancies, that version should be considered accurate.) 
Hope you find this useful! 
Spehro Pefhany, January 2000
3-1750 The Queensway Suite 1298 Toronto ON Canada M9C 4H5
(905) 271-4477
fax: (905) 271-9838 e-mail:

Modbus Protocol
Chapter 1 
Modbus Protocol
Chapter 2 
Data and Control Functions
Chapter 3 
Diagnostic Subfunctions
Chapter 4 
Exception Responses
Chapter 5 
Application Notes
Chapter 6 
LRC / CRC Generation [1/11/2000 10:32:59 PM]

Chapter 1
Modbus Protocol

Introducing Modbus Protocol

Two Serial Transmission Modes

Modbus Message Framing

Error Checking Methods
1.1 Introducing Modbus Protocol
Modicon programmable controllers can communicate with each other and with other devices
over a variety of networks. Supported networks include the Modicon Modbus and Modbus Plus
industrial networks, and standard networks such as MAP and Ethernet. Networks are accessed
by built-in ports in the controllers or by network adapters, option modules, and gateways that
are available from Modicon. For original equipment manufacturers, Modicon ModConnect
partner programs are available for closely integrating networks like Modbus Plus into
proprietary product designs.
The common language used by all Modicon controllers is the Modbus protocol. This protocol
defines a message structure that controllers will recognize and use, regardless of the type of
networks over which they communicate. It describes the process a controller uses to request
access to another device, how it will respond to requests from the other devices, and how errors
will be detected and reported. It establishes a common format for the layout and contents of
message fields.
The Modbus protocol provides the internal standard that the Modicon controllers use for
parsing messages. During communications on a Modbus network, the protocol determines how
each controller will know its device address, recognize a message addressed to it, determine the
kind of action to be taken, and extract any data or other information contained in the message. If
a reply is required, the controller will construct the reply message and send it using Modbus
On other networks, messages containing Modbus protocol are imbedded into the frame or
packet structure that is used on the network. For example, Modicon network controllers for
Modbus Plus or MAP, with associated application software libraries and drivers, provide
conversion between the imbedded Modbus message protocol and the specific framing protocols
those networks use to communicate between their node devices.
This conversion also extends to resolving node addresses, routing paths, and error-checking
methods specific to each kind of network. For example, Modbus device addresses contained in
the Modbus protocol will be converted into node addresses prior to transmission of the
messages. Error-checking fields will also be applied to message packets, consistent with each
Modbus Protocol (1 of 5) [1/11/2000 10:36:08 PM]

network's protocol. At the final point of delivery, however-for example, a controller-the
contents of the imbedded message, written using Modbus protocol, define the action to be
Figure 1 shows how devices might be interconnected in a hierarchy of networks that employ
widely differing communication techniques. In message transactions, the Modbus protocol
imbedded into each network's packet structure provides the common language by which the
devices can exchange data.

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