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PCMAG_PF.DOC
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1990-07-07
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Reprinted Article From The October 17, 1989 PC Magazine Advisor
By: Lori Grunin Title: Beyond COM2
LETTER:
I've been stumped by IBM's design limit for serial ports. I'd
like to add a COM3 and COM4 beyond the two already installed in
my 386 clone. I put the new board in and properly set the jump-
ers, but when I attempted to set the communications parameters
via DOS's MODE command, I got "illegal device" error messages.
I've looked for books to tell me how to get around this problem,
but to no avail. Is there a software driver that I must install
to effect valid operation of these ports?
Thomas Emery
Goleta, California
RESPONSE:
IBM's original PC architecture supported only two COM ports, and
although compatible makers, and eventually IBM, added more serial
capability, there are still no standard addresses or IRQ (Inter-
rupt Request) lines for the additional ports.
The addresses for COM1 and COM2 are 3F8h and 2F8h, respectively,
with COM1 on IRQ4 and COM2 on IRQ3. When you install more serial
ports, COM3 usually uses 3E8h and IRQ4, while COM4 uses 2E8h and
IRQ3. Unfortunately, some I/O card makers like to blaze their
own trails; IBM even added uncommon COM port addresses and IRQs
to the long list of PS/2 eccentricities.
The DOS MODE command doesn't work with serial ports above COM2
because DOS gets its I/O information from the BIOS, which finds
out what's installed where in your system during the Power-On
Self-Test (POST). The POST checks only for the first two in-
stalled ports.
To get around this problem, communications software and some
serial peripherals (such as mice) will occasionally support
higher COM ports by addressing them directly, rather than making
DOS function calls. Of course, if your peripherals don't support
these extra ports or you need to redirect data using the MODE
command, that's where trouble arises.
There are a couple of public-domain utilities that allow you to
append your COM port information to the BIOS, making the ports
DOS-accessible. COMSET (COMSET.ARC) and Port Finder (PF2.ARC)
are available in the "general hardware" data library of the IBMHW
forum on Compuserve; Port Finder can also be obtained from its
author, mcTRONic Systems (7426 Cornwall Bridge Ln., Houston, TX
77041;(713)462-7687).
Both packages activate COM3 and COM4 (if they're installed, of
course), and require DOS 3.3 or later. They basically do two
things: activate the extra ports by giving the BIOS the address-
es, and provide utilities for swapping the addresses among the
different ports. Address-swapping allows programs that don't
support COM3 and COM4 to access them. Software that directly
addresses the ports will usually be unaffected.
Keep in mind, however, that you probably won't be able to swap
two ports that share an IRQ (such as COM1 and COM3) if you've got
a mouse attached to one of them, because the system will get
confused when the interrupt triggers.
Port Finder is the more flexible of the two. In addition to the
aforementioned features, it provides the same capabilities for
LPT3, including swapping among printer ports. Furthermore, it
lets you easily restore all your original settings.
Even if you think you've correctly configured the board, it can't
hurt to double check a couple of things. If you've got a bus
mouse, for instance, make sure that you're not stepping on its
IRQ; although interrupt sharing is one of the touted features of
the PS/2, PCs can't do it very well.
Also, don't leave any port holes-if you've got three, they should
be COM1,COM2, and COM3, not COM1,COM2, and COM4. [END]