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From: wrs@Apple.COM (Walter Smith)
Newsgroups: rec.music.synth,comp.sys.mac.programmer
Subject: What's in a Mac MIDI interface (no ?)
Message-ID: <171@internal.Apple.COM>
Date: 31 Oct 88 21:33:16 GMT
References: <4765@oxtrap.UUCP> <886@etive.ed.ac.uk> <634@gvgspd.GVG.TEK.COM>
Organization: Apple Computer Inc, Cupertino, CA
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A MIDI interface for the Macintosh consists of two parts.

1. The RS422-to-current-loop converter.  MIDI is a current-loop
   interface, while the Mac uses RS422, so a converter is necessary.  Most
   little Passport-type boxes use a transistor, optoisolator, and a few
   resistors and capacitors to do it.

2. The external clock.  MIDI runs at 31.25 Kbps (1 MHz divided by 32), a
   non-standard (some say bizarre) data rate.  The Mac serial ports cannot
   be run at this rate without outside help, because their clock is not
   a multiple of 31.25 kHz.  Thus, the interface must provide the Mac
   with an external clock, which has been "standardized" by market pressure
   at 1 MHz although most software will also use 500 KHz and 2 MHz.

You can build one of these things yourself in an afternoon, once you
find a Schmitt-trigger optoisolator.  Of course, it won't look as good
as an Apple interface :-).  A usable circuit appears in MacTutor, the
issue before Kirk Austin's MIDI driver code (not a coincidence, since he
wrote the hardware article too).

- Walt
--
Walter Smith				wrs@apple.com, apple!wrs
Apple Computer, Inc.			(408) 974-5892
My corporation disavows any knowledge of my activities on the network.