The protocol used is Parallax’s AppMod serial protocol.
Here are two links to more info:
http://www.parallax.com/dl/docs/cols/nv ... /nv144.pdf
http://forums.parallax.com/forums/defau ... 5&m=258522
Its also labeled the "open serial" scheme.
I didn't look harder for the actual protocol specs, but it sounds similar to I2C but using asynchronous clocking so there's no actual clock pin. The line floats high and can be pulled low by either end.
There should be more info on the parallax web site.
Daryl