Re: serial ports

Pat Crean (pat@parrett.net)
Mon, 5 Aug 1996 15:23:32 -0400

Mainly because IBM, in the original PC design made interrupts active high
and drove them with ttl drivers. In other words, an inactive interrupt
signal is actively pulled down to ground which means that a shared
interrupt would have to source lots and lots (in signal terms) of current
to be recognized. Would have been so convenient if they had designed them
to be active low and driven them with open collectors......

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From: Magnus Hiie <Mgn@ekspress.ee>
To: linux-kernel@vger.rutgers.edu
Subject: serial ports
Date: Monday, August 05, 1996 7:17 AM

Hi!

I'm sorry, if this is a newbie question, but:

Why is it impossible for multiple (standard) serial ports (ttyS?) to
use the same IRQs?

If the ports are used one by one (a process opens a port, does smth
with it, closes it), they can be on the same IRQ. But this can't be
done using these IRQ sharing ports at the same time (overrun errors).

If I remember correctly, I once did smth like this when I used DOS.

Is it some hardware limitation or what? When an interrupt occurs,
serial driver could look up which port it was meant for, couldn't it?

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Magnus Hiie
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