Minimal Mac to MIDI Interface

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From: ace@tidbits.com (Adam C. Engst)
Subject: [*] minimal-midi-interface.txt
Date: Sat, 22 May 93 08:24:20 PDT

(Oops, I specifically wanted this stored in a /report type directory
so that I could refer to it in the final installment of the TidBITS
MIDI series. Thanks! :-)  -ace)

Here's a schematic for creating a simple MIDI interface for the
Mac. It's a bit old, from what I gather from the person who
sent it to me, and the address may not be valid for the author
any more. Of course, I have no idea if these plans work. :-)

cheers ... Adam C. Engst, TidBITS Editor


                        MINIMAL MAC--MIDI INTERFACE
                        ===========================


Well, here it is all you MIDI fans.  THE ultimate in simplicity!!
This is a simple schematic for a serial to MIDI converter.

There are two functions performed here.  One is the conversion from
current loop to RS-422.  The second function is supplying the 1 MHz
signal for the serial chip to sync up with the 31.5K baud rate of MIDI.


Macintosh                                       MIDI
Serial port                                     5 pin DIN
DB9 pin numbers                                 pin numbers

         +-------+     +------+
   4   10|26LS32 |     |7407  |
  >------+       |11  1|      |2    220 ohm     5
   5    9|       +-----+      +------/\/\/-------< \
  >------+       |     |      |                     \
         |       |     |      |                      | MIDI OUT
         +-------+     +------+                     /
                                    220 ohm     4  /
          390 ohm        +5 volts ---/\/\/-------<
+5 volts --/\/\/----+
                    |
         +-------+  |  +------+1    220 ohm     4
   8    6|26LS31 |  |  |MCT2  +------/\/\/-------< \
  >------+       |11| 5|      |                     \
   9    5|       +--+--+      |                      | MIDI IN
  >------+       |    4|      |2                5   /
         |       |   +-+      +------------------< /
         +-------+   | +------+
                     |
                     v
                   Ground

         +-------+     +------+
   7   10|26LS31 |     |1 MHZ |
  >------+       |9   8| OSC  |
   3   11|       +-----+      |
  >-------       |     |      |
         |       |     |      |
         +-------+     +------+

        Power connections:

                        GROUND  +5
                        -----   --
          7407          7       14
          26LS31        8,12    4,16
          26LS32        8,12    4,16
          OSCILLATOR    7       14

Notes:

The optical isolator MCT-2 above can probably be any relatively
fast optical coupler.  Note that the 390 ohm resistor may need to
be adjusted to make sure the output does not saturate.  Using a
scope while feeding a MIDI signal in should show you whether the
signal looks clean.

The diagram has been drawn to show MAC signals on the left and MIDI
signals on the right.  Also note that the MAC pin numbers are for
the DB9 connector **NOT** the 8 pin mini-DIN connector!!

The age old question of where to get power for this always remains.
On the pre-MAC-PLUSs, there was power available from the serial
port connector.  I solved this problem by finding a 7 VDC AC adapter
at a parts store and using a 5 volt voltage regulator IC to bring
it down to 5 volts.  This is left as an exercise for the student.....

The 26LS31 and 26LS32 are the same type of chips which are used in
the MAC for RS-422 conversion.  I got them from a store in Santa
Clara California (Anchor Electronics  (408) 727-3693).  They also
have 1 Mhz Oscillators as well.

I have built several variations of this over the last two years
and have had good success using a variety of music software with
them.


John Hengesbach
(205)772-1669

uunet!ingr!henges
Intergraph Corporation
Huntsville, AL 35807

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