The AT keyboard interface is bi-directional (they keyboard can talk to the host as well as the host to the keyboard). The protocol for an AT keyboard is 1 start, 8 data, odd parity, 1 stop bit.
Upon powerup, it's usual for a keyboard (AT or XT) to perform a quick diagnostic test on itself and then send a hex AA value saying that the test was completed successfully.
It's a bizarre artifact that some early AT keyboards on powerup send a continuous string of hex AA bytes with the parity bit set intentionally wrong. On the PC AT, this caused to motherboard microcontroller to respond with an FE byte, meaning "Resend the last keystroke", whereupon the keyboard would send an AA with the correct parity and things would continue normally from there.
However, later motherboard keyboard controllers are not prepared for this bizarre behavior and so do not send the FE byte, instead, they complain endlessly. The keyboard and the motherboard are locked in a never-ending loop, with the keyboard endlessly shouting gibberish to a system incapable of understanding it.
Hope this helps.
