IBM TrackPoint Mouse with PicAxe

[Mad Professor]

Good Day All.

I have just started working on a new project to make a CNC Touch Probe made from an old IBM laptop TrackPoint mouse.

I had the idea then I found this video on YouTube: Find Center Of Slot - Edge Probe Mach 3.

In the above video, the TrackPoint mouse is apart of the interface pcb and the guy has just connected a PS/2 cable to it and running some scripts for the Mach3 CNC software.

I am looking to do something along the same kind of lines, but I am not looking at interfacing it to a computer using PS/2 or USB, I am needing a simple 5volt logic output to connect to the computers printer port.

The TrackPoint mouse I happen to have is from a dead donor laptop that very old (100Mhz CPU), this version of the TrackPoint mouse is just the TrackPoint Stick, all the interface is apart of the mainboard, so I have to start from scratch.

The TrackPoint mouse in question is just a strain gauge it has four connections, Ground, Power, X Axis, Y Axis.

here are the ohm readings of the pins left to right.
1&2: 747 ohm
1&3: 973 ohm
1&4: 747 ohm
2&3: 713 ohm
2&4: 972 ohm
3&4: 713 ohm

I am 99.9% sure pin 2 is the ground pin.

So far I have been able to find the following datasheets that are related to the TrackPoint mouse.
TPM749 Microcontroller with TrackPoint microcode from IBM
TPM754 Microcontroller with TrackPoint microcode from IBM
TPM754A Microcontroller with TrackPoint microcode from IBM
LMC6036 Low Power 2.7V Single Supply CMOS Operational Amplifier

I will be doing all my testing on an 18x, but I would like to use the smallest possible chip for the finished project, so something like the 08M.

I would like help and advice with the best way to go about interfacing this TrackPoint so when ever any of the Axis are moved the interface will output 5 volts.

Thanks for your time.

Best Regards.

 

[djc1]

MP,

Have a look at the LHS of the diagram on page 9 of TPM754A_3.pdf

Then Google 'Wheatstone Bridge' and see if the two look similar (to be fair, the diagram in the PDF could be better drawn).

Pin outs don't really matter at this stage as long as you choose a pair with the higher resistance as V+ and GND and the other pair as 'output'.

You know 'R', so you can choose a suitable V+ for reasonable current draw.

The output pins should give a _current_ that varies in magnitude and direction according to which way the probe is deflected.

And to turn a variable current into a digital signal, use a transistor.