I had similar issue with one of my Socket 7 motherboards. Some cheapo PS/2 keyboard I had, relatively modern, about 5 years old, […]
I had similar issue with one of my Socket 7 motherboards. Some cheapo PS/2 keyboard I had, relatively modern, about 5 years old, worked fine. I found two older PS/2 keyboards from 15-20 years ago and it was no go with them.
One appeared to work fine when powering on (LEDs turning on), but when you pressed any key it would just blink it's LEDs, unless I turned caps lock LED off on boot, then it worked fine, until I tried to press any of the keys that turn LEDs on.
The other one was just blinking with it's LEDs. Didn't recognize any key press.
Turned out it was motherboard's fault (btw. it was QDI Titanium IB+), it had transistor to controll voltage on AT keyboard (and I guess PS/2 mouse port) pins and it was faulty. Without anything connected it held at 5V, but when keyboard was connected it was sub 4V. And when you actually started to use the keyboard it went below 3V.
Modern keyboard was least power hungry so even below 3V was enough for it to work. For these others it wasn't, first one managed to keep a bit above 3V when LEDs were off so that's why it worked if caps lock was off. But turning any LEDs on would dip it below 3V and it couldn't recover.
After I replaced that transistor all of these keyboards work without issues.