The 5-pin DIN keyboard connector used with the first IBM PC (1981) and second-generation IBM AT (1984). It was subsequently replaced with the smaller 6-pin Mini-DIN connector on the IBM PS/2 in 1987.

It’s not too much of a chore to find a keyboard that’ll talk to a PC-AT, but the PC-XT had an entirely different protocol, not to mention that chunky 5-pin DIN connector. As with many things ...