Article 4783 of comp.sys.atari.8bit:
  From: terpin@acsu.buffalo.edu (christophe d terpin)
  Newsgroups: comp.sys.atari.8bit
  Subject: Homemade MIDI Interface -- Part 1 of 2
  Summary: How to build midi interface for Atari 8bit
  Keywords: MIDI, interface
  Date: 28 Mar 91 02:51:18 GMT
  Sender: news@acsu.Buffalo.EDU
  Organization: SUNY Buffalo
  
  
  Homemade MIDI interface for the Atari 8-Bit
  -------------------------------------------
  Parts List:
  1   7404 TTL Hex Inverter OR 7400 NAND gate
  2   220 ohm resistors (10%)
  1   Atari serial port plug for your computer
  2   5-pin Midi plugs (RCA) for your midi keyboard
  1   breadboard/experimenter perf board
  1   project case
  approx. 10 ft. of 3 wire cable 
  twist ties to tie down wires etc..
  
  
  The serial port pins as seen on the computer are:
       
       2    4   6   8   10   12
        o   o   o   o   o   o  
      o   o   o   o   o   o   o
      1   3   5   7   9   11  13
  
       1 - Clock In      7 - Command
       2 - Clock Out     8 - Motor Control
       3 - Data In       9 - Proceed
       4 - Ground       10 - +5V
       5 - Data Out     11 - Audio In
       6 - Ground       12 - +12V       13 - Interrupt
  
  For this project, we will only need to use pin #'s 3,4,5, and 10 of the
  serial port.


























  ---------------
  (CRUDE)                                    Ground (pin 4)
  SCHEMATIC:      7404                         |
                |\    |\                      2|  
                | \   | \    220 ohm       5   o   4
  data out   ---|  O--|  O---\/\/\/---------o     o---/\/\/\/-- +5 (pin 10)
  from computer | /   | /                  3 o   o 1
  (pin 5)       |/    |/                  MIDI OUTPUT cable to synth  
     
              2
           5  o  4
  Ground---o     o------------(optoisolator)------- data in to computer
          3 o   o 1                                 (pin 3)
       MIDI INPUT cable
        from synth
  
  Pins 1,3 of the Midi Output cable and pins 1,2,3 of the Midi Input
  cable are not connected.
  
  The optoisolator is optional, although it is recommended for extra
  protection for your midi keyboard.  I have used the interface 
  hard wired without the optoisolator without any problems (so far).
  
  Also note: the normal serial i/o to the disk will sometimes interfere
  with the midi interface and vice versa.  This sometimes results in 
  a note sounding on the synth when loading or saving a file, or the
  disk will glitch if you try playing the keyboard during disk i/o.
  One possible way to remedy this annoying problem is to replace
  the 7404 with a tri-state buffer which can be turned to a high-Z state
  using the motor control (pin 8) from the serial port. 
  
  Anyway, BE CAREFUL and check all your wiring before you hook the 
  interface up to the synthesizer.  
  
  Please let me know if you have any suggestions for improving the 
  interface.
  
  next::  BASIC Midi Recorder Program
       New & Improved Assembly Source Code
  
  ---------------------------------------------------------------
  Christopher Terpin
  terpin@autarch.acsu.buffalo.edu