SimpleModbus is a collection of Arduino libraries that enables you to communicate serially using the Modicon Modbus RTU protocol.
This project was born as an updated version of http://code.google.com/p/simple-modbus/ by Bester Juan because it lacks support for commands other than 3 and 16. More important the code is now on github, so you can contribute more easily.
This projects is actively maintained, so feel free to ask for features or reporting bugs!
This library adds support for command 6 and provides a more extensive support for arduino pins. The goal of the project is to support all usable MODBUS commands on arduino and expose all arduino pins so you can use an arduino as an advanced automation controller for both analog/digital in/out.
NEW: Support for SoftwareSerial, really useful on AtTiny85. You can find both library and an example that works reliable on attiny85 microcontroller.
Simply copy the SimpleModbusMaster or SimpleModbusSlave or both into your Arduino IDE libraries folder. Than restart the ide and open the corresponding example into the example_master or example_slave folder.