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From: aswNS@hamp.hampshire.edu (Albert S Woodhull)
Subject: Re: com 3
Message-ID: <b47cb$91627.36f@news.hampshire.edu>
Date: Sat, 04 Nov 1995 14:22:39 GMT
References: <46rum4$65s@atheria.europa.com> <a1c7cb$15312f.2b7@news.hampshire.edu> <470lpe$dh8@news2.cts.com> <951030211.201302@drinkel.ow.org>
Organization: Hampshire College, Amherst MA
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On Mon, 30 Oct 1995 20:13:02 +0100 (MET) Miquel van Smoorenburg
(miquels@drinkel.ow.org) wrote in comp.os.minix:

: I tried this with Minix 1.5, but it didn't work. This is because
: of a bad design decision of IBM when they designed the ISA bus.
: You just CAN'T have two seperate devices sharing the same interrupt,
: because the interrupt line is either high or low. This basically
: means that if an interrupt comes in on COM3 but not on COM1, COM3
: will try to raise the interrupt line but COM1 keeps it low ->
: short circuit! This has even DESTROYED ISA cards.

Thank you, Miquel. This is very interesting and thought-provoking.  I
had never given a thought to the hardware, I always just assumed that
interrupt lines would be driven by open collector or tri-state
outputs. I *know* I have used shared interrupts on various PCs in the
past, usually they worked, and where they didn't I just assumed that
the drivers weren't written to properly daisy-chain the service
routines. 

Is it impossible for hardware designers to work around the limitations
of the IBM bus?  I have observed that a lot of internal modem cards
for ISA machines come with jumpers for four COM ports and frequently
are set up with COM3 as the factory default. I presume that the
manufacturers assume that purchasers will already have a standard
COM1/2 serial port installed and will want to install a new peripheral
without disturbing anyhting already installed. Indeed, it was putting
such a modem into one of my Minix machines where I was already using
both serial ports that made me start thinking again about increasing
the number of Minix serial ports. The modem works OK on com3/tty3 with
MS-DOS and Linux, I was hoping I could make the same hardware
configuration work with Minix.
--
Albert S. Woodhull, Hampshire College, Amherst, MA
awoodhull@hamp.hampshire.edu
woodhull@shaysnet.com
413-549-2962

