Looking for some help with resistive touch screen

I have an Eonon DVD playing head unit for my car. It has a 7" touch screen and accepts video input. What I would like to do is tap in to the touch screen 4 wire output and drive a USB touch screen interface to let me use it as a touch screen for a carpc.

My thought is to piggyback the 4 wires of the touch screen and drive another touch interface to USB. It has what I am assuming is a little sort of SOC and some kind of 16 pin touch interface but i don't know how it interfaces to the built in program. Presumably rs232. I have backtraced the 4 touch screen pins to be 2,3,4, and 5 and i believe power is on 9. No idea about a data sheet but it seems to be a clone of another chip sold on eBay from china as well.

I will use the composite video input for the carpc video output, the head unit amp for the speakers, and will retain the "host" software for the built in radio, DVD, backup camera, the aux input will be the carpc. The only thing I haven't quite figured out is the two touch interfaces being on at the same time. The old touch interface is off except for a small strip of commands at the bottom of the display. I could simply avoid placing carpc buttons there. The new interface will of course be active at the same time and I will have to turn that off for sure. Perhaps a toggle switch on the faceplate to power up one interface or the other to limit input to a single controller.

Mostly my question is...can i solder in the 4 pins of the new controller on top of the existing controller and have both work? Also, i have seen one controller state it is for 300-600 ohms between pins on the same layer... My screen looks like it does about 600 to 1200 ohms from left to right extremes, measuring from pin 1 to 2, Is that going to be a problem?

You cannot expect to be able to piggyback two controllers onto one resistive touch screen panel. The controller alternately provides bias across the X direction and uses the Y surface to sense the touch point kind of like the wiper on a pot. It then switches over to putting bias across the Y direction and uses the X surface to detect the touch point in the other direction. When a layer is acting like a "pot wiper" the controller generally bridges the two two electrodes of that layer together through a switch component. You will have to disconnect one controller before getting the other one to work.

The touch panels & controllers I've worked with detect resistance in a ratiometric manner so that the absolute resistance of the panel is nulled out in the measurement. This should allow the you some leeway in getting various panels to work with a particular controller. There may be hard limits of usable range of resistance however for a particular controller.

-- mkaras