Chinese MPG and Modbus

Hi All
I am looking for some help please .
I have a 3 axis CNC router table I am building out of 8020 with 3 stepper motors and a Hobbycnc 3 axis control board. I would like to use a MPG with this table and have been given a this Chinese Pendant.
http://i290.photobucket.com/albums/l...ob/CNC/mpg.jpg

I believe that this will run my table motors though Mach 3 via Modbus. What I would like to do is build my own modbus board to use the MPG with. Can someone help please I know nothing about Modbus (yet). I take it that the R232 connetor plugs into the Modbus RTU and the USB plugs into the computer.

Bob