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From: mam@gvgspd.GVG.TEK.COM (Mark A. Matthews)
Newsgroups: rec.music.synth,comp.sys.mac.programmer
Subject: Re: Mac Ports and MIDI (Re:What's between my midi interface and my mac?)
Message-ID: <634@gvgspd.GVG.TEK.COM>
Date: 30 Oct 88 21:11:16 GMT
References: <4765@oxtrap.UUCP> <886@etive.ed.ac.uk>
Reply-To: mam@gvgspd.GVG.TEK.COM (Mark A. Matthews)
Organization: The Grass Valley Group, Grass Valley, CA
Lines: 37


What's between my MIDI interface and my mac is a cable :)

What's *in* my MIDI interface is much more interesting.  Cracking open my
Passport MIDI interface for the Macintosh reveals the following goodies:
2 5-pin DIN connectors (laleled "in" and "out")
1 cable with 9-pin D-connector, suitable for attaching to a Mac
3 1N914-type diodes in various spots about the board.
1 1N752A (zener?  All my good databooks are at work, where I'm not)
3 10 uF 16V capacitors
5 resistors, various values and locations
1 8-pin sip resistor package 4.7K
3 bypass capacitors
1 2N4401 transistor
1 6N138 Opto-isolator (near the "in" connector)
1 4Mhz crystal, with 5 pf capacitor in parallel
1 74HC04
1 74HC74A

Pretty simple.  They certainly make a healthy profit on these puppies.  But
then again, now that I work for a living (as opposed to a starving student)
I don't mind so much.

I haven't tried to create a schematic from it, but it is apparent that the
power is stolen from the Mac.  The 74HC04 is being used as an oscillator for
the 4 Mhz crystal.  The output from the osc. is fed to the clock input of
one of the 74HC74A flip-flops which appears to be wired in a dive-by-4
fashion.  I *think* the output of this is capacitively coupled to the
external clock pin of the D9 connector.

Rats.  Now you've got me curious and I'll just have to draw up a schematic
to see just what this little box is supposed to do.

Oh.  One more note:  On the foil side of the board (it is single-sided), it
has the phrase "McMIDI", "(C) 1986 ADON CORP", "PDI", "UMC-UL", and "06-88".

-Mark (mam@gvgspd.GVG.TEK.COM -or- ...!tektronix!gvgpsa!gvgspd!mam)