Unlike it's big brother, the BBC Micro, this is a fairly standard matrix keyboard. The unusual flat flex cable plugs directly into one of my USB keyboard controllers. The round black object on the left of the board is a piezo sounder. I couldn't resist recreating the power on sound of the Electron or BBC on these keyboards.
The keyboard mapping is fairly straightforward, function keys are func+ numbers, keys with multiple symbols on have shift + key for the left hand symbol, and the ctrl + key for the right hand key.
When I was looking around for a candidate for conversion, I found a prototype I had made early on. This one had a damaged flex, and so I had fitted a ribbon cable, wired in this case to an arduino shield. That was plugged into an Arduino Leonardo, I was investigating using the Arduino HID library, but it didn't work too well in this application.
From what I remember, you sent it characters and it converted those to USB key codes. Which was fine for the letters and numbers, but I needed to have a lookup table of what I needed to send to get the right key pressed, and couldn't get several keys I needed. Shortly after that, my made the first PCB versions, with custom firmware.
I think black solder mask PCBs were on offer that week. The new versions are a bit smaller, and a bit greener, now all surface mount, but essentially the same circuit.
You can order a conversion kit below. The kit includes the controller board, mounting pillars and a USB lead:
I can now supply these with the keys mapped for normal desktop use, or for use with RISC OS Pico