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From: lauren@vortex.UUCP (Lauren Weinstein)
Newsgroups: net.micro.pc
Subject: Re: Additional COM ports available for PC's
Message-ID: <421@vortex.UUCP>
Date: Fri, 19-Oct-84 07:44:12 EDT
Article-I.D.: vortex.421
Posted: Fri Oct 19 07:44:12 1984
Date-Received: Sun, 21-Oct-84 15:03:27 EDT
References: <9478@watmath.uucp>
Organization: Vortex Technology, Los Angeles
Lines: 18

Note that unless very specialized software is going to be used,
the practical limit on PC serial ports will usually be 3.
This is accomplished by buying a 2 port serial add-on board
(like the AST cards) to provide COM1 and COM2.  Then, a third
port may be added as COM3, *if* you are willing to steal one of
the (few) interrupts from some other device.  In my case, I
have a modified standard PC async port that is addressed at an
unusual location.  I use the printer interrupt for this third
async line, and let the printer run busy wait instead of interrupt
driven (no big deal -- it all works fine).

However, the critical point is that as soon as you go beyond 3 ports
(unless you are willing to steal yet another interrupt from the
system) you start getting into complex interrupt-sharing techniques
that will be by definition completely non-standard and incompatible
with much (or most) software that uses the async lines.

--Lauren--