Touchpad USB Conversion

Over 20 years ago, when we were first getting into PCs, my dad got a Chicony KTP-7903 keyboard in a 'goody' box from Bull Electrical. These were large cheap boxes of surplus gear and could be anything, but always had some interesting stuff in them. We even got parts of the duff Rabbit phone system in one.

The keyboard was an odd-shaped thing that was supposed to be ergonomic, but it had a touchpad on the removable wrist rest which was pretty unusual. The device had a full-size 5-pin DIN PS/2 connection for the keyboard and a separate serial (RS232) connection for the touchpad.

Pretty soon my dad got bored with the weird keyboard but he had the genius idea of carefully cutting the touchpad off the end of the rest and using it separately. It turned out to be really useful anywhere where a mouse was going to be hard to use.

The only problem was that it had a serial connection and by about 5 years ago it was obsolete as even the older computers I use no longer had a serial port. I tried using a serial-to-USB adaptor, but that didn't work so I threw the touchpad in a box. Then a few months ago I had the idea of putting a more recent touchpad module from a stripped-out laptop into the case.

A bit of research on Synaptics touchpads revealed that even recent laptops are still fitted with PS/2 modules rather then USB, possibly because there are stocks of these available. A check of the serial number on the one I had showed it was a PS/2 module, so it wasn't going to be a straight connection to USB.

I bought an active PS/2-to-USB adaptor on eBay for £2 to see if I could cobble all this together. The original serial touchpad was old enough to have an old-school through-hole PCB whereas the replacement was surface mount. Luckily my skills with the hot air gun and solder paste meant I was able to bodge an adaptor board for the connection strip to the touchpad.

First job (above) was to remove the USB plug from the adaptor and wire a USB cable to it. Opening a terminal in Linux and typing 'lsusb' confirmed that the adaptor was still working at this point when its identification came up.

The next job was to remove the 2 mini 6-pin DIN sockets where the PS/2 keyboard and mouse would have been connected and solder 4 tail wires for +5V, GND, CLOCK and DATA in line with the PS/2 protocol. I soldered these tails to the touchpad and nothing happened. My first thought was that the touchpad was only 3.3v and I'd fried it, but a check with my trusty and very old logic probe showed that when the pad was touched there was activity on the CLOCK and DATA lines, which was encouraging. Then I noticed something weird, the keyboard on the connected laptop had died. Maybe I had wired the touchpad to the wrong PS/2 connection on the adaptor although they were supposed to be interchangeable.

So swapping over the connections worked and the touchpad was controlling the cursor. Two more wires for the button connections, another quick check and all is working. The internals look a bit ignorant with two adaptor boards, but with some hot glue for insulation and the case back together it looks fine if a bit of a weird shape. This will come in very handy with the likes of the Raspberry Pi.