I will dig up the code and post the relevant portions to make it talk to mach3 (I will do it after Christmas as I have a house full for a few days 
). There was only bits of information online that pointed me in the right direction, the Arduino code is only half of the solution. The configuring of Mach3 is also essential to make this work right (creation of brain files). I used the Mach3 modbus protocol for communication between the Arduino and Mach3. I posted some info that may get you started in the Mach3 forum (no code, just info) it is here:
http://forum.machsupport.com/forum/index.php/topic,33265.20.html
Again, give me a few days and I will post it.
Merry Christmas!