One of the many technical aspects of MIDI that is often confusing for those wishing to get into the details is how the connectors work. The many books available on MIDI aren’t much help. Everyone knows that MIDI only travels one way across cabling and that’s why there are both MIDI IN and OUT jacks. Since it travels one way, with one stream of data, MIDI is called a serial connection. Being a serial connection, only one of the five pins on the connector actually transmits data. This is where books get confusing. Some books will say that three pins out of the five are functional and some will say that two pins are functional. Really, only one pin transmits data. Three pins do stuff, but only one transmits data. Beyond the one most important pin, two others have some minor work to do and two of the pins do absolutely nothing. The important, data transmitting pin, is pin 5. Pin 2 is a ground, and pin 4 contains a bias voltage. Pins 1 and 3 typically aren’t connected to anything at all.
So, then, why use a 5 pin connector when only one pin transmits data? The answer is that in 1982-1983 when the MIDI standard was developed the 5-pin DIN connector was cheap and widely available -- two factors that were important to the developers of the MIDI standard.
For the geeks, here’s the schematic of the 5-pin DIN as it’s used in MIDI. The picture comes from the MIDI Manufacturers Association.