Newsgroups: comp.sys.ibm.pc.hardware
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!magnus.acs.ohio-state.edu!csn!kessner!david
From: david@kessner.denver.co.us (David Kessner)
Subject: Re: Why does my modem disable the serial port?
Message-ID: <1991Jun11.050931.4739@kessner.denver.co.us>
Organization: Kessner, Inc.
References: <1991May25.133555.7807@bronze.ucs.indiana.edu> <1991Jun10.153842.12545@midway.uchicago.edu>
Distribution: na
Date: Tue, 11 Jun 91 05:09:31 GMT

In article <1991Jun10.153842.12545@midway.uchicago.edu> valley@gsbsun.uchicago.edu (Doug Dougherty) writes:
>hanj@bronze.ucs.indiana.edu (Jining Han) writes:
>>I have a very standard 286 (AMI bios), with 2 parallel ports and
>>2 serial ports.  The problem is, when I put a modem in, one of the
>>serial ports is disabled (com1 if I set modem to use com1 or com3,
>>com2 if I set modem to use com2 or com4).  
>
>You have to disable one of the on-board serial ports when you install an
>internal modem.  Else you end up with two devices trying to be the same
>COM port.  (I.e., I think the thing you don't realize is that the
>internal modem has a built-in serial port)
>--
>	(Another fine mess brought to you by valley@gsbsun.uchicago.edu)

Ahh, no.

To be more precise, in the standard configurations an AT can support
four serial ports (more in not-so-standard configurations).

Each port has an IRQ# and an I/O port address.  I forgot the I/O port addresses
but that is not what we want to focus on anyway.  The IRQ's are the real
issue.  Here are the standard IRQ#'s for the COM Ports:

	COM1 - IRQ4
	COM2 - IRQ3
	COM3 - IRQ4
	COM4 - IRQ3

As you see, the odd-numbered COM ports use the same IRQ (as do the even ones).
Because of the way the PC's IRQ's are designed, IRQ's cannot be shared 
between devices.  What this means to the rest of us is that COM1 cannot
be used at the same time as IRQ3 (same for IRQ2 and IRQ4).  

This does not mean that one COM port must be disabled-- just that it cannot
be used when the other one is.  Putting a mouse on COM1 makes COM3 just about
useless (since a mouse is used all the time), but you could connect a modem/
printer/plotter/etc to COM2 and COM4 without any problem.

-- 
David Kessner - david@kessner.denver.co.us            |
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