I have this one:
http://llg.cubic.org/tools/sonyrm/
the RM-X2S and RM-X4S is mainly a variable resistor: different resistance according to the button pressed.
the way to get the proper reading includes simply to pass the output to an analog to digital converter.
you have one in the joystick port, or you can use an external processor and convert the signal to serial pulse of your making... I prefer the easiest way