AT keyboard interface
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The AT keyboard interface, often written PC/AT, is a serial keyboard interface developed by IBM for their PC/AT line of computers. The AT interface re-used the same 5-pin DIN connector as the XT keyboard interface, allowing keyboards to be manufactured that could connect to computers with either type of interface. Keyboards so designed were fitted with a switch, typically on the bottom, for selecting between the XT and AT interfaces; this switch this is often marked simply "X" and "A". The Focus FK-2001 notably placed the XT/AT DIP switch on the top of the keyboard, concealed below the flip-up badge.
Failure of a newly-purchased vintage keyboard to function can sometimes be due to the XT/AT switch being in the wrong position.
PC/AT and PS/2
The IBM Personal System 2, or PS/2, adopted a new connector for the keyboard interface, termed the PS/2 connector. The PS/2 keyboard interface is otherwise identical to AT, and AT keyboards can be used on newer PS/2-equipped hosts by way of a simple and cheap female AT to male PS/2 passive adapter. This adapter can of course be connected to a converter for USB connectivity.
