Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!uunet!motcid!koch
From: koch@cell.mot.COM (Clifton Koch)
Newsgroups: comp.sys.ibm.pc
Subject: Re: COM3 and COM4
Summary: COM3 & COM4
Message-ID: <1419@navy22.UUCP>
Date: 28 Feb 90 21:55:17 GMT
References: <278.25E7ADD9@uscacm.UUCP> <16433@well.sf.ca.us>
Organization: Motorola Inc. - Cellular Infrastructure Div., Arlington Heights, IL 60004
Lines: 33

> >     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".  
> > 
> 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

That's still only part of the story.  The BIOS has to support COM3 and COM4
also.  If it doesn't you have to install a driver.  I've used a shareware
program called PORTFINDER that pokes around memory and looks for COM and LPT
port hardware, if it finds hardware, but no drivers, it loads in drivers.
It worked great with DOS3.3.  It allowed me ways to init modems via the
command line (i.e. echo AT&C1 S0=0 etc >COM3; it was a stupid modem).  Let
me know if you can't find a copy of the program (usually called PFxx.ZIP
where xx is the version, I think mine was PF22), let me know, and we'll
work out some way to get it to you.

Also, the 'standard' way I've seen com ports configured is that COM1 and COM3
share IRQ4, and COM2 and COM4 share IRQ3.  The drivers sort out who really
caused the interrupt.

Cliff Koch