Very Large Touchpads For Very Old Computers

Way back when most of our demographic was in diapers, engineering workstations had huge touchscreens for plotting drawings in CAD programs, drawing, and just about everything a Wacom tablet does today. Finding one of these touch pads now is a fool’s errand, more so than finding the computer it was attached to, but [Daniel] figured out a way to relive those days of large touchpads and old computers with a resistive touchscreen and an MSX computer (portuguese, google translatrix).

[Daniel] is using a touchscreen normally used for a monitor, and with the right bit of code on a PIC16F micro, pressure on the touchscreen can be translated into X and Y coordinates. Using the PIC was a great choice in this instance: it’s possible to multiplex ports on an ADC pin with a PIC, making the entire system extremely efficient and easy to calibrate.

After that, it’s just a matter of plugging the output of the microcontroller into the touchpad connector of the MSX and writing a few lines of BASIC to draw a point on the screen. Video below.

27 thoughts on “Very Large Touchpads For Very Old Computers

  1. “Finding one of these touch pads now is a fool’s errand”

    That’s because they weren’t touchscreens in the first place.

    They were recording the timing of the electron beam of a CRT monitor as it reaches the photosensor on the tip of a pen. That’s why the screen flashes whenever the user clicks the button on the pen.

          1. BTW first generation touchscreens either used an IR based break-beam bezel (very coarse grained) or SAW (surface acoustic waveform) transducers on the crt glass itself. These could tell how ‘hard’ the screen was being mashed; we used them to build command and control screens for emergency service responders (together with a custom built PBX switch which allowed for radio patches as well as PSTN connection) in the early to mid 80’s.

          2. I remember playing with those in my youth at a local assisted living facility in Nevada. I remember being perplexed as to why everyone keeps thinking touch screens are new fangled crap every fifteen years or so. I’m even more perplexed as to why developers, especially on platforms whose sole interface is a touchscreen, still keep trying to emulate the keyboard/mouse paradigm.

          3. I think the SAW displays are somewhat more recent, at least a lot newer than infrared and resistive. IR touch units are still pretty popular in industrial settings because of their ruggedness. I have one that is USB and touts a 4kx4k resolution, I dont believe it, but thats what they say. Bad thing about them is touches register before you actually touch the screen.

      1. -“How is that not a touchscreen?”

        Because it doesn’t sense touch.

        The Nintendo gun in the NES game Duck Hunt works on the same principle. It has a telescope that zooms in on a small spot on the CRT glass and upon triggering it flashes the screen white. By timing how long it takes for the gun to see the flash, the game computes what part of the screen the gun is looking at.

        The only difference to the light pen is that you hold the pen up to the glass.

        1. IIRC the gun simply sensed light, period – no timings. When you triggered, the scene turned black and the duck(s) turned into a while square, so if the gun was focused on a lighted area, the game though it was a hit. You could cheat by pointing it at the sun/lights.

          1. I’ve done experiments with this, it’s not quite that easy to trigger the sensor in the Zapper. In actual application it can be possible with certain games (Duck Hunt in 1-duck mode) where you can set another TV to a pure white display and get constant hits, but I couldn’t ever replicate this behavior with a light bulb or the sun.

          2. The Zapper includes a demodulator circuit—it’s just an ordinary IR remote receiver with its demodulation frequency changed from the standard 38kHz to 16kHz, and the IR photodiode replaced with a wideband one.

            More or less any pulsing light source will trigger it, but whether any given false input will work varies depending on the game. You might look into Tepples’s ZapRuder test ROM.

        1. “A digitizer is not a touchscreen. It’s a digitizer, although modern ones often contain a monitor.”
          And a screen is not a display. It’s a screen, altough media displays are often called screens.

          Look, I don’t want to get into a semantic argument about it, I just want the pixels, all right?

          1. A digitizer is a flat plate besides the screen that uses induction to locate the digitizer pen when it comes close enough. It doesn’t sense touch and it doesn’t (usually) display pictures, so it isn’t a touchscreen.

            There’s a switch in the tip of the pen and the signal from the pen changes depending on whether the switch was pressed or not. One doesn’t actually need to be touching the pad to activate it, so you can place paper between the pad and the pen, and trace shapes from the paper into the computer that way – which is why it is called a digitizer. Its function is to digitize drawings and diagrams.

  2. Okay, so I look at the photo above and thought…
    So somebody put a sheet of plastic on a keyboard, and when they press on the sheet, one of the keys below maps out where the sheet was pushed. Not much resolution with that, and wouldn’t pushing near the center of the sheet trigger a bunch of keys?

  3. Yup called a light pen built one for a TI-99 4/a back in the 80’s out of a bic pen a cd-s cell and a transistor into the joystick port.

    Although I find the size they are talking about as “very large” quite amusing.

  4. I meant to say, this is NOT light pen and it does NOT emulate light pen. Believe or not, there was touch pads already over 30 years ago and touch pad support is built in to BIOS in MSX. Light pen is much more simple technology, but it requires exact timing unlike this.

  5. Yup, most of this demographic was in diapers.

    I remember and have actually used the device in question that was used on engineering stations, which was properly called a digitizer table.

    It was not a light pen as many have assumed. It was not a touchscreen either, as it would neither register touch, nor did it have a screen or other form of display.

    It was a large horizontal surface, big enough that you could lay full sized blueprints on it. A mouse-like device was used with it, which had transparent crosshairs surrounded by a wire coil. You’d center the crosshairs on every point/corner of interest on the blueprint (map, or whatever) and click; converting the exact dimensions as originally printed into digital form.

    Funny, a Microstation digitizer table just like the one I used to use showed up in a thrift store a few months ago. I doubt they had any clue what it was. It was priced like common furniture.

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