This came in as a request from a previous buyer of my Commodore 64 USB Keyboard, could I make a TI99/4A USB keyboard? I don't see why not. This first step was to locate one, I didn't have a TI99/4A in my ever expanding collection of vintage computers, so I looked around and managed to get hold of one which was 'untested'.
So here it is, the first commercially available 16-bit home computer, powered by the impressing looking 64 pin TMS9900NL chip.
And true to form, untested means broken. In this case, it looks like a memory fault, not in the massive 256 bytes of 16 bit RAM, but in 16K of 8 bit DRAM hung off the video controller that was actually also the main system RAM.
These are 4116 chips a common source of failures these days. Oddly, these are NEC brand chips. TI had its own massive memory production arm. Many a Spectrum is full of TMS4116 chips, no wonder TI were making a loss on these things if they bought their competitors DRAM chips!
This spectrum not only has 8 TMS brand 4116's, but a further 8 TMS 4532's. That's more TI chips than TI's own computer.
The memory fault in this case is good because I can put that aside to fix later. What I need here is the keyboard and case to become the USB keyboard.
The keyboard PCB is separate from the main board and connected via a short ribbon cable, so once the main board is carefully removed, the ribbon can be reconnected to the USB keyboard controller PCB.
Here I'm using the latest version of my USB keyboard PCB, all surface mount this time (don't look too closely at the soldering!).
A fairly easy keyboard to map, most of the major keys you would want are there, apart from delete. There is a handy 'function' key bottom right, which I have used to select the functions on the front of the keys, and other missing keys such as F1-F10 and delete across the top row.
So another keyboard type added to the repository, you can buy a complete converted TI-99/4A from my Etsy store.
Update: I've updated the USB keyboard controller now, and this is available as a kit f you have a dead TI-99/4A with a good keyboard. The kit has with all the bits you would need, the USB controller, power LED, USB cable and mounting pillars.
USB Keyboard Controller Kits are available from my Sell My Retro store (no longer including the LED)