Connecting a Keyboard to a Vintage PC-XT, the Hard Way
We’re not sure if there’s any single characteristic that qualifies someone as a hacker. After all, we’re a pretty eclectic bunch, with skills that range all over the map, and what one person feels is trivial, others would look upon as black magic. But there’s one thing we’re sure of: if you find yourself reading the original POST code for the PC-XT motherboard just to get a keyboard working, you’re pretty much our kind of people.
That was the position [Anders Nielsen] found himself in as work progresses on his “PC-XT from Scratch” project, which seeks to build a working mid-80s vintage IBM Model 5160 using as many period-correct parts as possible. The first installment of the series featured the delicate process of bringing the motherboard up, lest the magic smoke was released. After seeing some life out of the old board, [Anders] needed a little IO, specifically video and keyboard. The video side of the equation was relatively trivial, with an early-90s VGA card from eBay — not exactly period correct, but good enough to get something to display.
The keyboard was another matter entirely. It’s not too much of a chore to find a keyboard that’ll talk to a PC-AT, but the PC-XT had an entirely different protocol, not to mention that chunky 5-pin DIN connector. As with many things, the solution involved building an adapter out of an Arduino, but the process was not exactly low-friction. First, there was the question of dealing with the open-collector outputs on the keyboard, which required code changes. Next, [Anders] broke out the oscilloscope to check waveforms, and found a weird 9-Hz square wave on top of the keyboard clock signal. That ended up being a sign that the keyboard was stuck in a manufacturing test mode, according to a read-through of the power on self-test (POST) source code. A tiny tweak of the Arduino code got everything working.
We’ve got to say that we’re loving this deep dive into the internals of a piece of computer history. [Anders] teased the next step, which will use an empty ROM socket on an old network card to load some assembly code onto the XT. We’re very much looking forward to that exploit.
