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From: bender@oobleck.Eng.Sun.COM (Michael Bender)
Newsgroups: sci.electronics
Subject: Re: MIDI Interface
Keywords: MIDI
Message-ID: <2973@exodus.Eng.Sun.COM>
Date: 16 Nov 90 21:05:36 GMT
References: <3757.658193903@lemsys.UUCP> <10268@uhccux.uhcc.Hawaii.Edu> <16585@mentor.cc.purdue.edu> <1138@tsdiag.ccur.com>
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In article <1138@tsdiag.ccur.com> davet@tsdiag.ccur.com (Dave Tiller N2KAU) writes:
->In article <16585@mentor.cc.purdue.edu> lairdkb@mentor.cc.purdue.edu (Kyler Laird) writes:
->-In article <10268@uhccux.uhcc.Hawaii.Edu> whinery@hale.ifa.hawaii.edu (Alan Whinery) writes:
->->Whoa! Although the serial port and serial device on the mighty Amiga can 
->->handle the 31,250 baud necessary for MIDI, most other machines (for 
->->instance, IBM & compatibles generally cannot. You will find that the 
->->serial handler in MS-DOS is not capable of higher than 19200 baud, not
->-
->-1) Some programs push the serial port to 56K baud, no?
->
->The problem here is not whether the serial port can run _fast_, it's
->whether it can run 31.25 Kbps.  The typical 8250 uarts use a guzinta
->circuit (a programmable divider) to set the bit rate.  Unfortunately
->the accuracy at high speeds just isn't there.  As I recall they use a
->1.8432 MHz xtal to drive the divider and the fixed div-16 prescaler in the
->uart.  Thus, 1.8432 MHz / 31250 / 16 = 3.6864.  Using 3, we'd get 38.4 Kbps,
->using 4, we'd get 28.8 Kbps.  Other values:
->115200 = 1, 19200 = 6, 9600 = 12, 4800 = 24, 2400 = 48, etc..
->This number get stuck in a register on the serial card.

that's true if you use the stock oscillator, but if you replace it
with either a 1 or 2 MhZ oscillator, it works fine.  I believe some
of the early MIDI implementations in keyboards/drum machines used
the motorola 6850 clocked at 1 MhZ.

mike
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