I'm not a keyboard expert but I know that XT and AT keyboards have different scancodes and protocols. While an AT keyboard might work on an XT system (if it has an XT/AT switch or if it's autosensing), it's also true that not many XT keyboards, if not any, will work on AT systems unless the said systems support the XT scancode. AT motherboards that support XT keyboards in their keyboard controller are rare but they do exist and they were available during the 286 and 386 era. I kinda wish there was a cheap ready-made adapter that would convert an XT keyboard scancode in order to be used on an AT system. There's one made by Hagstrom Electronics that does exactly that, but unfortunately it's very expensive. Using an XT Model F or a decently made clone on my RDD is a pipedream of mine.

The XT has an 8255 on the motherboard which handles keyboard, dipswitches, and the cassette port (kind of like MSX...). On ATs that chip is gone and the keyboard is handled by a microcontroller instead.

https://wiki.osdev.org/PS/2_Keyboard

OSDEV doesn't seem to have an article about XT keyboard, maybe this will help https://hackaday.com/2017/01/21/attoxtkeyboard/

Also, communication with XT keyboards was one-way only, so no lock LEDs on the keyboard itself. AT was bidirectional, so you can have LEDs indicating stuff.

Note that by the time the AT standard came along, basically almost everyone stuck to it, but that that wasn't the case in the PC/XT era, so any given XT clone might do things a little differently.

The XT protocol was simpler, but in some ways more elegant also.
I don't know how to explain it properly, but in simple words, the XT keyboard simply sent scan codes to the PC.
The AT keyboard required a lock/un-lock mechanism for each keypress..

Jo22 wrote:

The XT protocol was simpler, but in some ways more elegant also.
I don't know how to explain it properly, but in simple words, the XT keyboard simply sent scan codes to the PC.
The AT keyboard required a lock/un-lock mechanism for each keypress..

Lock/unlock - do you mean key-down / key-up?

If the XT keyboard does not have it - how does it differentiate between a single tap versus a key held down?