Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!sun-barr!apple!well!mccarthy
From: mccarthy@well.sf.ca.us (Patrick McCarthy)
Newsgroups: comp.sys.ibm.pc
Subject: Re: COM3 and COM4
Message-ID: <16433@well.sf.ca.us>
Date: 27 Feb 90 06:58:49 GMT
References: <278.25E7ADD9@uscacm.UUCP>
Reply-To: mccarthy@well.UUCP (Patrick McCarthy)
Organization: Whole Earth 'Lectronic Link, Sausalito, CA
Lines: 43

In article <278.25E7ADD9@uscacm.UUCP> Berry.Mobley@uscacm.UUCP (Berry Mobley) writes:
>
>     I have a 386 clone at work that I am trying to install 2 more serial 
>ports in.  It's running MS-Dos 4.01 (which supposedly supp
>orts COM3 and COM4.)  The serial card appears to be connected correctly, 
>but I can't get anything to access the ports.  When I do a 
>MODE COMn (n is 3 or 4) I get "Illegal device name".  Using n greater 
>than 4 returns "Invalid Parameter".  
> 
>My question:  Has anyone successfully installed COM3 and COM4 without 
>custom drivers under MSDOS 4.01?
>                                            Berry Mobley
>
>
>
>--  
>--
>Berry Mobley == ...!usceast!uscacm!Berry.Mobley

The problem is that DOS's "support" of four com ports is only half the
solution.  In the original IBM PC, two interrupt lines were set aside for
the COM ports (who will EVER need more than two?).  Thus, software by and
large came to expect that each COM port would have its own interrupt
line (hardware); most serial cards have a switch to choose which of the
two standard lines to use (IRQ 3 or IRQ 4). So even though DOS 4.0 has
enough sense to realize there may be more than two COM ports hooked up,
most existing hardware and software doesn't.  There are really only two ways
to provide a total solution. One is to buy a card which allows you to choose
the IRQ level you want to use; the accompanying software must also allow
you to use a nonstandard IRQ.  Qua-Tech (Canton, OH, I believe) makes
such a board.  The other solution is to use something like the DigiBoard,
which allows multiple (totally nonstandard) ports to share a single
IRQ.  Not many general software packages use this scheme, because there
are many similar but incompatible boards on the market.

Basically, unless you have smart software, it really doesn't matter
whether DOS allegedly supports four ports or not, except on
microchannel machines, where the interrupt sharing scheme is documented
by IBM, and therefore standard across all hardware and software.


Pat McCarthy
mccarthy@well.uucp