AT keyboard interface
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". Focus keyboards 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.
Typematic (also typamatic) refers to the key-repeat feature. Key repeat is driven by the keyboard controller, which retransmits the make signal for the held key periodically. Typematic delay (how long to wait before initiating key repeat) and typematic rate (how often to repeat the key) are set to default values upon initialisation, and can be set by the host on demand. The Focus FK-5001 permits typematic rate to be set directly from the keyboard.
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.
AT passthrough port on a Cherry G80-3700 for connecting a keyboard