Text Lite mm500 keyboard design

The first time I started playing around with a display, is a mm500 from Text Lite. I bought this display without cables, software or even a proprietary keyboard. It’s a display previously used at PTT Post, the former Dutch Postal Service.

I’ve tried to connect it via RS232 and a USB to serial converter with no luck.

The next option was to see if there are keyboards available, found this image on the internet:

It used a ribbon cable that connects to the micro processor bord. It should be a matrix type keyboard, so I started measuring signals with my oscilloscope and logic analyser. This way I could figure out which lines were on the X axis and which on the Y axis.

button test

After long testing and trying I figured out pretty much all keys and functions. After that the shift+key special functions started to take shape. They appear to be somewhat different than on the keyboard display I found online.

I then translated that into a schematic and eventually in a PCB (needed one, but order minimum was 5 @ JLCPCB 🙂 )

Trying it out…


Since I wanted to keep the costs to a minimum, a keyboard case was not really an option. So I ended up having a sheet of plastic cut up on a large laser cutter.

Good enough for when we need a new text loaded on the display.
Also mixed up two keys (pauze and backspace) so maybe redo the laser cuting once more and also make a back side and screw it all together. Or even a printed sheet on top…

This display is now used in the Makerspace facilities in our town.