WARNING: This code is no longer being maintained and it's NOT compatible with Arduino IDE 1.x.
We recommend you check out this improved version instead.
For the Modbus RTU Master click here.
For the Modbus RTU Slave click here.
For examples using the arduino together with a Linux box click here.
Additional helpful links (thanks to Samuel Marco):
QModbus, a modbus RTU master with a Qt-based GUI.