MIDI interface
The box or card that translates between a computer's buses and MIDI's five-pin DIN sockets.
Classic MIDI is a 31,250 bit/s current loop with an optocoupler on the receiving end, and no computer shipped with such ports at first. Roland's MPU-401 (1984) became the PC standard — so much so that sound cards emulated its registers for years. The Atari ST had the ports built in from 1985, which made it the studio computer of choice for a decade.
A three-byte message takes just under a millisecond to send, and one cable carries only sixteen channels. Multiport interfaces got around this by giving each instrument its own cable, and the better ones timestamped events so a chord wasn't smeared across several milliseconds. With USB MIDI most instruments now have the interface built in, but the boxes live on for hardware that only has DIN — including gear that relies on MIDI Thru.