[link=http://www.rc-circuits.com]R/Circuits.com[/link]
The above site has all the information you need to understand how these interfaces work. There is schemtaics, PIC code and detailed descriptions on the circuits and PIC programming. You will find all the info you need to build your own interface or you can buy fully assembled interfaces if you prefer.
Dennis

[link=http://www.rc-circuits.com]R/Circuits.com[/link]