107 lines
3.9 KiB
Plaintext
107 lines
3.9 KiB
Plaintext
From: ace@tidbits.com (Adam C. Engst)
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Subject: [*] minimal-midi-interface.txt
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Date: Sat, 22 May 93 08:24:20 PDT
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(Oops, I specifically wanted this stored in a /report type directory
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so that I could refer to it in the final installment of the TidBITS
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MIDI series. Thanks! :-) -ace)
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Here's a schematic for creating a simple MIDI interface for the
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Mac. It's a bit old, from what I gather from the person who
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sent it to me, and the address may not be valid for the author
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any more. Of course, I have no idea if these plans work. :-)
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cheers ... Adam C. Engst, TidBITS Editor
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MINIMAL MAC--MIDI INTERFACE
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===========================
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Well, here it is all you MIDI fans. THE ultimate in simplicity!!
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This is a simple schematic for a serial to MIDI converter.
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There are two functions performed here. One is the conversion from
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current loop to RS-422. The second function is supplying the 1 MHz
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signal for the serial chip to sync up with the 31.5K baud rate of MIDI.
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Macintosh MIDI
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Serial port 5 pin DIN
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DB9 pin numbers pin numbers
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+-------+ +------+
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4 10|26LS32 | |7407 |
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>------+ |11 1| |2 220 ohm 5
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5 9| +-----+ +------/\/\/-------< \
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>------+ | | | \
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| | | | | MIDI OUT
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+-------+ +------+ /
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220 ohm 4 /
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390 ohm +5 volts ---/\/\/-------<
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+5 volts --/\/\/----+
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+-------+ | +------+1 220 ohm 4
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8 6|26LS31 | | |MCT2 +------/\/\/-------< \
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>------+ |11| 5| | \
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9 5| +--+--+ | | MIDI IN
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>------+ | 4| |2 5 /
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| | +-+ +------------------< /
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+-------+ | +------+
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v
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Ground
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+-------+ +------+
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7 10|26LS31 | |1 MHZ |
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>------+ |9 8| OSC |
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3 11| +-----+ |
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>------- | | |
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+-------+ +------+
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Power connections:
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GROUND +5
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----- --
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7407 7 14
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26LS31 8,12 4,16
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26LS32 8,12 4,16
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OSCILLATOR 7 14
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Notes:
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The optical isolator MCT-2 above can probably be any relatively
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fast optical coupler. Note that the 390 ohm resistor may need to
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be adjusted to make sure the output does not saturate. Using a
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scope while feeding a MIDI signal in should show you whether the
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signal looks clean.
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The diagram has been drawn to show MAC signals on the left and MIDI
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signals on the right. Also note that the MAC pin numbers are for
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the DB9 connector **NOT** the 8 pin mini-DIN connector!!
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The age old question of where to get power for this always remains.
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On the pre-MAC-PLUSs, there was power available from the serial
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port connector. I solved this problem by finding a 7 VDC AC adapter
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at a parts store and using a 5 volt voltage regulator IC to bring
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it down to 5 volts. This is left as an exercise for the student.....
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The 26LS31 and 26LS32 are the same type of chips which are used in
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the MAC for RS-422 conversion. I got them from a store in Santa
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Clara California (Anchor Electronics (408) 727-3693). They also
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have 1 Mhz Oscillators as well.
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I have built several variations of this over the last two years
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and have had good success using a variety of music software with
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them.
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John Hengesbach
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(205)772-1669
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uunet!ingr!henges
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Intergraph Corporation
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Huntsville, AL 35807
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