While we all wait for further guidance from GWManning :
Under topic area "Networking, Protocols, and Devices" of this forum a post today may be of help as we delve into modbus
MAX 485 & ModbusMaster - Basic Guide
http://arduino.cc/forum/index.php/topic,132177.0.html
Problem with such an active forum like this is there are so many topic areas with info happening.
Enjoy