Path: utzoo!attcan!uunet!nuchat!sugar!karl
From: karl@sugar.uu.net (Karl Lehenbauer)
Newsgroups: comp.sys.amiga
Subject: Re: Multiple Serial Ports (Re: vt100 v2.9)
Summary: use NEC 16550A chips and support MIDI, too
Message-ID: <3241@sugar.uu.net>
Date: 8 Jan 89 04:13:23 GMT
References: <8812150227.AA09671@postgres.Berkeley.EDU> <14049@oberon.USC.EDU> <554@sunkisd.CS.Concordia.CA>
Organization: Sugar Land Unix - Houston, TX
Lines: 22

To anyone who might be developing a multiport serial board, two things:

1) Consider using the NEC 16550A's rather than 8250's or whatever, because
the 16550's have a 16-byte typeahead buffer.  At high speeds, it is very
difficult to get out and get all the data on the Amiga.  For example, at
MIDI speeds, bytes come in every 333 microseconds.  It's hard to catch
them all, indeed impossible, as the exec seems to sometimes leave interrupts
off for longer than that.  With 16550's, the minimum time the system has
available between the UART receiving a data byte and the system reading
it before getting an overrun is widened by a beefy 16X, for MIDI to about
5 ms.

2) Please support MIDI as well as RS-232.  Supporting MIDI requires only
a couple of things:  One is that you need to be able to produce data at
31250 kilobits (1 megahertz / 16) per second.  The other is that you produce 
0 and 5 volts for logic 0 and 1 rather than the -12 and +12 for RS-232 and 
that you use an opto-isolator capable of a few microseconds switching times
on the input data wire.
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