Wasn't sure whether to put this in a new thread or not. Oh well, I will just stick it in here.
Just to let you all know, at least some USB keyboards will work with this converter! Not all. I tried my Unicomp Endura pro USB keyboard, which has a built in trackpoint-style mouse device. It did not work. This would make sense, as a mouse/keyboard combination probably wouldn't convert properly through a passive USB to PS/2 adapter anyway. I assume it could be the same on keyboards with extra USB ports on them, but I don't know.
Now, today I got a new keyboard (entry level mechanical with Cherry Brown switches - the iOne XArmor U9, USB interface) and decided to try, using the AT to XT converter, to connect it to my Leading Edge Model D. I used one of the simple purple PS/2 to USB keyboard converters and connected it to the AT2XTKBD, which in turn (obviously) is connected to the computer. To my surprise, the LEDs blinked and the keyboard worked (almost) fine!
I did notice that, when typing fast, it tends to skip, delay, or mistype characters sometimes. Mostly it just leaves some out. It could seem almost as if the keyboard itself has a buffer or a timing feature that is not quite in sync with the old 4.77mhz CPU. But the problem is worse when Caps, Scroll, and Number locks are on. Specifically when the LEDs are lit (since, on the Leading Edge, a lit LED is not always an accurate indicator of the lock status). So I thought it could be a power supply issue as well. The problem is almost nonexistent when no LEDs are lit. Odd.
Nevertheless, I would consider this a victory, as the keyboard is still usable. Curious to know if anyone else has tried adapting USB keyboards as well.