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MODBUS
Modbus has been around for years and is utilised comprehensively by Industry

The following technical information is from Moore Industries-Pacific, Inc.
Using MODBUS for Process Control and Automation - MODBUS is the most popular industrial protocol being used today, for good reasons. It is simple, inexpensive, universal and easy to use. Even though MODBUS has been around for nearly 30 years, almost all major industrial instrumentation and automation equipment vendors continue to support it in new products. This white paper discusses how MODBUS works and a few clever ways it can be used in new and legacy plants.

Modbus Protocol - This is an excellent Modbus Reference from Modbus.com
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

Modbus Interface Tutorial - Covers History of the Modbus protocol, Modbus message structure, Modbus serial transmission modes: Modbus/ASCII and Modbus/RTU, Modbus addressing,  Modbus function codes - Thanks to Lammert Bies. 

Modicon Modbus Protocol Reference Guide - 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 -from eecs.

What you should know about Modbus - A good description from Modbus Driver.com

Frequently Asked Questions about Modbus - The following questions are answered by "Simply Modbus" 
What is Modbus?
What is it used for?
How does it work?
What is hexadecimal?
How is data stored in Standard Modbus?
What is a function code?
What is a CRC?
What are the formats of Modbus commands and responses?
What are data types?
What is byte and word ordering?
What is a Modbus Map?
What is the difference between Modbus ASCII and Modbus RTU?
What are extended register addresses?
How does 2-byte addressing work?
How can you send events and historical data?
What is Enron Modbus?

How Real and 32-bit Data is Encoded in Modbus RTU - This article discusses some of the typical difficulties encountered when handling 32-bit data types via Modbus RTU and offers practical help for solving these problems - from chipkin.com.

The September 2010 and October 2010 CAS newsletters have some Excellent Modbus Information - Included in these two parts is;  MODBUS - Introduction, 4 types of data, There are (were) a Max of 9999 points of each data type, 5 Digit vs 6 Digit Addressing, What about Scaling In Modbus, Floating Point Numbers In Modbus, Byte/Word Order – An ambiguous nightmare, Bit Order - Sometimes it’s a problem too, Modbus and Gateways, What about errors / exceptions, There can only be one master on a Modbus Serial Trunk, Multiple Clients of a Modbus slave, Old device – slow processors – limited capability, Modbus Ascii, JBUS, Enron and other Variants, Modbus RS232, RS485 and TCP/IP, Modbus on RS232, Modbus on RS485, Modbus Resources, Testing and Trouble Shooting, What to take to site with you, Trouble Shooting Modbus TCP/IP, Using the CAS Modbus Scanner, Converting Modbus 16 bit numbers to 32 bit numbers, How Real (Floating Point) and 32-bit Data is Encoded in Modbus RTU Messages, The Importance of byte order, Determining byte order, Practical Help and Hubs vs Switches – Using Wireshark to sniff network packets

Modbus Specifications - Download the current versions of Modbus specifications and implementation guides - from modbus.org.

Modbus Technical Resources - from modbus.org

Ricardo Saat's Modbus page - Some useful information here.

A Modbus Users' Community - a useful Forum

Free Modbus Software - From Wikipedia, the free encyclopedia