MIDI is short for Musical Instument Digital Interface.

 The hardware consist of a 31.25 kb/s asynchronous link using a start bit,
 8 data bits and a stop bit. The link itself is a current loop type on which
 the logical 0 is formed by current on. The receiver side must be opto-
 coupled using a device that requires less than 5 mA to turn on and have
 rise and fall times of less than 2 .s. The connectors on the devices must
 be female 5-pin DIN (180.), pin 1 and 3 are not connected, pin 2 is
 connected to ground at the transmitting side, pin 4 and 5 are the current
 loop. The cable should not exceed 50 feet and is a shielded twisted pair
 cable.

 Input: (MIDI IN)

             /---\      +--------+                +-----------------+
 4 --------------|------| 220 .  |----------------+---+             |
             |   |      +--------+       |        |   |             |
             |   |                       -        |   .  .          |
             |   |                       .        |   -  .          |
             |   |                       |        |   |             |
 5 --------------|--------------------------------+---+             |
             \-+-/                                +-----------------+
               |
             Shield

 Output: (MIDI OUT & MIDI THRU)

                       5V
                        |                 +--------+          /---\
                        +-----------------| 220 .  |--------------|---- 4
                                          +--------+          |   |
                                                              |   |
                                                              |   |---- 2
             |\             |\                                |   |
             | \            | \           +--------+          |   |
      -------|  >O----------|  >O---------| 220 .  |------------------- 5
             | /            | /           +--------+          \-+-/
             |/             |/                                  |
                                                              Shield


 (Output of the optocoupler is directly connected to TTL-IC driving the
  MIDI-THRU output!)