I too am trying to get one of these figured out, I've just spent the last few hours very carefully looking at the PCB and the 48 pin T1004 ASIC. I can conclude the following:
pins 1-5 on the ASIC appear to go nowhere
pins 6 and 7 are the Left and Right button inputs
pins 8 and 9 appear to go nowhere
pins 10 and 11 go to pin 3 and 2 (red and brown) on the electrical interface
pin 12 appears to go nowhere
pin 22 goes to pin 1 (black) on the electrical interface
27 of the 48 pins are dedicated to X and Y
The other pins go off onto other (mostly unused) circuitry, either with resistors and capacitors terminating one another, ultimately ending up either back at the ASIC, at ground or at what appears to be an unused interface labelled Y1
Now, back in terms of the electrical interface,
pin 4 (orange) appears to go nowhere.
So we have a 4 pin cable for a 4 pin interface (PS/2) and only 3 of the pins appear to be in use. So from this I currently have two theories:
1. That the Orange cable is for a circuit that is useful to PS/2 but not to the touchpad. Considering that the 4 PS/2 circuits are +5V, DATA, CLK and GND, I'm going to go ahead and say that if this is the case, it is CLK that isn't required by the touchpad. I assume that if this were the case, the T1004 would be running at a standard clockrate or sync'ing up in some other way, such as via DATA.
Therefore, if this is the case, it's likely that red and black are +5V and GND respectively, leaving brown as DATA.
The more likely theory at the moment however is this:
2. Looking at the PCB, it's apparent that this is an overglorified capacitor... that's what touchpads are - capacitive, which is why yours got really hot when it was miswired - it was storing charge like a small battery.
The PCB is primarily a big GND, and uses current passing tape to electrically ground to the chassis of the E500, which goes all the way around to where the ports are, including the PS/2 port. The important thing to note is that this potentially gives us our fourth electrical circuit!
So in this theory, red is +5V, brown and black are DATA and CLK (I'm undecided on the order) and anywhere on the PCB's plentiful ground rail is GND. Either way, this reduces the number of possible combinations on the cable.
Seeing as Synaptics seem to have plenty of useful documentation for interfacing to their equipment, I've tried to get in touch with them to see if they can shed some light on this, but there's been no response yet.
I'm also trying to get another dead E500 so that I can really rip one of these apart - I need to shave the touchpad layer off the other side of the PCB to check out the circuitry on that side to get a more complete understanding, but as it is, I'm not willing to do that to my one and only device.
Hope this helps someone. If someone is willing to guinea pig one of their touchpads to test the above two theories, please do so and let me know how you get on.
Standard disclaimers apply - if the touchpad explodes, taking off your arm and killing your pets, I take no responsibility
Keyword for Google crawlers - Compaq SPS 135227-001
Synaptics Touchpad TM41PUK324
My multimeter threw a wobbly while I was repairing a 3kva UPS, and today's a public holiday in NZ so there's no chance of getting a replacement. It's visual + brain matter for the time being, and as I said, I'm not willing to guinea pig this one. All I can offer at this point is my observations in the hope that someone else can prove/disprove them.
Linear Mode
