Tuesday, 22 December 2015

Day 22 - TRS80 Model 4 USB Keyboard

This is day 22 of the Advent Calendar of USB keyboards. Today, a TRS80 Model 4 USB keyboard conversion. I was sent this TRS80 Model 4 keyboard by someone who was part way through a case mod project on their TRS80, and wanted the keyboard converted into a USB keyboard.
The TRS80s with built in screens don't tend to survive well when shipped, and often the monitor mountings will snap off and the loose tube inside the chassis doesn't survive well. Hence I imagine the owner chose to start again. The keyboard was therefore untested, so the first step was to clean it up.
One of the keys (Clear) wasn't moving, and it looked like the key had snapped off at some point and been glued back on. It seems the glue had seeped down into the switch and locked the whole thing up, so I had to replace it.
The TRS80 used all individual mechanical keyswitches, but I didn't have an exact match to hand. It turns out the size is still standard today, and a Cherry MX Black switch fitted fine. The cross at the top was slightly smaller, but the keycaps was full of cyanoacrylate anyway, so I just cleared a suitable amount to fit snugly.
The pinout was different, but it was raised a bit higher, so I was about to wire it in between the metal frame and the PCB. Sorry, the picture isn't very clear, the two pins are the bottom of the Cherry switch, which doesn't reach the PCB, and the green wires are connected to the pads below.
The feel was slightly different, but as it was an ancillary key, this wasn't too noticeable. Had it been one of the more letters or numbers, I would have swapped it with one of the individual keys anyway.
With the key fixed, I was able to look at reading the keyboard. The connector used was a dual row 0.1" header, so a suitable connector was found and fitted to the USB keyboard controller. The keyboard plugged into this and the keyboard was mapped, a simple 8x8 matrix.
There was one extra key, the square orange power button. Although it looks like it should illuminate, it is just a push switch.
That's all reassembled and posted back to the owner, to continue their project.

Tune in tomorrow to open another window on the Advent Calendar of USB keyboards.