Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!zaphod.mps.ohio-state.edu!rpi!dali.cs.montana.edu!masscomp!ocpt!tsdiag!davet
From: davet@tsdiag.ccur.com (Dave Tiller N2KAU)
Newsgroups: sci.electronics
Subject: Re: MIDI Interface
Keywords: MIDI
Message-ID: <1138@tsdiag.ccur.com>
Date: 16 Nov 90 15:55:10 GMT
References: <3757.658193903@lemsys.UUCP> <10268@uhccux.uhcc.Hawaii.Edu> <16585@mentor.cc.purdue.edu>
Organization: Concurrent Computer Corp. Oceanport NJ
Lines: 23

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.
-- 
David E. Tiller         davet@tsdiag.ccur.com  | Concurrent Computer Corp.
FAX:  201-870-5952      Ph: (201) 870-4119 (w) | 2 Crescent Place, M/S 117
UUCP: ucbvax!rutgers!petsd!tsdiag!davet        | Oceanport NJ, 07757
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