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From: waksman@eniac.seas.upenn.edu (Adlai Waksman)
Newsgroups: comp.sys.mac,rec.music.synth
Subject: Re: DIY MIDI interface?
Summary: converting TTL clock
Keywords: MIDI Macintosh
Message-ID: <7248@netnews.upenn.edu>
Date: 21 Jan 89 17:52:15 GMT
References: <817@ttrdf.UUCP> <12456@dcatla.UUCP>
Sender: news@netnews.upenn.edu
Reply-To: waksman@eniac.seas.upenn.edu (Adlai Waksman)
Organization: University of Pennsylvania
Lines: 22

In article <12456@dcatla.UUCP> dlmnm@sunc.UUCP (Michael N. Moran) writes:
>On a related topic, I'd like to know if there is a "correct" way to
>setup a serial port to use external clocking.  ...

Most MIDI software expects a 1 MHz external clock on the Mac's HSKi line
(also called Ext Clk or TRxC; pin 2 on the Mini-DIN 8-pin serial port, or
pin 7 on the pre-Mac Plus DB-9 serial port).  This is not a TTL clock; it
must swing both above and below 0 volts.

To convert a TTL clock signal (0-5v) to a bipolar one, Kirk Austin's method
from his October 1985 MacTutor schematic has worked for me.  Couple the
signal capacitatively:
	                        /------------o  to Mac HSKi (-2.5 - +2.5v)
	1MHz TTL clock ----||---+
	                 0.1uF  \---/\/\/---+
	                             1K    _|_ GND (Mini DIN #4, DB-9 #3;
	                                   ///	and TTL circuits' ground)

___________________________________________Adlai Waksman(@eniac.seas.upenn.edu)
     ...The Universe is thronged with fire and light,	
 And we but smaller suns, which, skinned, trapped and kept
Enshrined in blood and precious bones, hold back the night.  (Ray Bradbury)