NT: COM 3&4 available somehow ?

Posted on 2000-05-02
Last Modified: 2012-05-04
Running NT4 on an unknown MB.
I wondered if there's the possibility to have COM3 and 4 available on that machine ?

I saw some machines with only two COM plugs (1 & 2) but with 4 COM ports listed under NT.
I quite sure they didn't have a multi I/O card installed.

Is there a way to activate COM 3 & 4 so I can get our internal modem to work ?
I don't use COM1 or 2 but this stupid thing needs to have COM 3 or 4 available.

Maybe it would be helpful to fiddle around in machine's BIOS or in NT itself ?

In other words, without having a multi /IO card installed, is there a way to have COM 1-4 available under NT without any conflicts or maybe to change COM 1+2 to COM3+4 ?

If you need further details, let me know...

Question by:mitrakis
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Expert Comment

ID: 2770983
try booting up to BIOS, Find comm ports and enable them

Expert Comment

ID: 2771639
No, you dont understand... the internal modem has a comport already installed. It is built in to your modemcard, so fiddling with bios wont help. You must determine what the internal settings for the internal IO are, IE com 3, and then just add it. If you are using additional IO cards, you will only make things worse. If you want to do it the easy way, enter BIOS, disable one of the com ports, and try to add a modem in winnt using com1 fist, if it cant find anything, it will work on com2. If the card is not plug and play. The question is, do you have the driver for this modem? if not, try one of the standards. ***winnt will almost always show all 4 comports, regardless if they are physical there or not*** just pick one at the time.....

Author Comment

ID: 2772935
? *Who* didn't understand *what* ?

NT doesn't show me COM3+4 at all

I'll try to add COM3+4 manually with standard settings (IRQ and base address)

If 2.) won't work, I'll try to remove either COM1 or 2 and look if COM3 or 4 shows up then

If none of the above will work, I'll get back to you again later

BTW, the modem is PnP and I do have drivers available but NT didn't detect any modems since it's looking only on COM1+2

Best regards

Author Comment

ID: 2772942
..and welcome to EE "ware933267".

Accepted Solution

tandy earned 50 total points
ID: 2773157
Sounds to me you may have to disable COM1 or COM2 in the Bios.

COM1 and COM3, share IRQ4
COM2 and COM4, share IRQ3

If you have a mouse on COM1, Try Disabling COM2 in the Bios.
This may enable the BIOS and NT to use COM4 and thus be available
to your modem. Hope this helps,

Also, Make sure There is a memory address available for COM4 to use.
Here are some typical addresses used by COM4 on most PC's:


Good Luck


Expert Comment

ID: 2773362
It just seem that you were planning to install a IO card. Thanks for the welcome!!
To make it easier for you, i looked up my old bible from "the old days"
The IO adresses for the 4 comports are:
Com1: 3f8-3ff
Com2: 2f8-2ff
Com3: 3e8-3ef
Com4: 2e8-2ef
These are useful if you are fiddling with the internal ports. Make sure they dont share the same IRQ, because of the way NT uses the hardware. If you disable both internal Comports, you KNOW IRQ 3 and 4 are free. Then you may enable one at the time to see if you may have at least one of the internal COM ports enabled. (I understand that you use PS-2 mouse, since both comports are free?)

Expert Comment

ID: 2773462
Have you tried going to the ports icon in the control panel and adding two ports.  Even if you select the default settings when adding the port you should be able to add com 3 and com 4.  Then reboot and try to detect modem.

If this doesn't work you may have to fiddle with the settings a bit.  NT installs defaults com3 to the addres 3f8.  You can try changing this to 3e8 and adjust the irq.
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Author Comment

ID: 2773524
THX for your suggestions.
Since I've posted this Q for my colleague, I'll forward him your comments and let you know how everything goes as soon as he checked it out on his NT machine.

He won't install an I/O card; he just needs to get COM3 or 4 working so the modem will work properly.
He read in the manual that the modem wants to be installed on COM3 or 4...COM1 + 2 are not used, since he uses a PS/2 mouse.

I'll let you know what worked, soon !

LVL 12

Expert Comment

ID: 2774132
My NT 4 Workstation recognizes a modem on COM4 -- I use COM1 for my mouse, I use COM2 for my digital-camera.

I never use COM2 and COM4 at the same time -- so, both COM ports can use the same IRQ!

I agree with everybody else: reboot into BIOS-setup mode, and disable the COM1 and COM2 ports.

Then, when you reboot, your computer's  Plug-and-Play BIOS will assign COM1 to the modem, and you can boot NT, and then configure NT to use the modem as COM1.

Expert Comment

ID: 2775994

I never use COM2 and COM4 at the same time -- so, both COM ports can use the same IRQ!

Newer Bioses can Juggle two Serial Devices on the same IRQ by a method
called polling. It is inefficient as it uses system resources  to identify the device  for each IRQ call. That is why manufacturers are moving toward USB.
Glad it is working in your case, credit your chipset manufacturer for that one.

Older bioses, as I assume is the case here(Unknown MB) do not have that
ability. In fact, if the board is not plug N'  then the OS must use older
bios standards to identify each device.


Expert Comment

ID: 2777441
Do you want to setup a modem in NT? or just enable those COM ports for the fun of it?  I don't quite follow.  Most modems will let you set themselves up on pretty much whatever you decide.  If you're using a PS/2 mouse and don't have a modem installed currently, then you have both COM ports free, which should be no problem to install a modem.

LVL 12

Expert Comment

ID: 2777589
> Glad it is working in your case, credit your chipset manufacturer for that one.

I've used that "trick" on several motherboards, with different chipsets (some without PnP), and with different Operating Systems (DOS, OS/2, Windows 95/98/NT), and it's worked, every time; so, I don't think that credit goes _ONLY_ to the manufacturer of the chipset.

Expert Comment

ID: 2779330
I've used that "trick" on several motherboards, with different chipsets (some without PnP), and with different Operating Systems (DOS, OS/2, Windows 95/98/NT), and it's worked, every time; so, I don't think that credit goes _ONLY_ to the manufacturer of the chipset.

Your point is well taken. Credit also goes to otta :)

I have not been as fortunate with the systems I have worked with.
They were very "unstable" when juggling Coms using common IRQ's.
(Lots of Blue screens) . Good software(or OSes) can prevent conflicts from
occuring when "highlevel"  input is being processed. When device
level procesess occur, it is the Motherboard bios and the "firmware" in
the chipset that must contend with the requests before the "OS" ever
gets to respond....

Anyway, you are right, if it works okay , then go for it..


Author Comment

ID: 2830581
It worked !
He had to disable the COM ports from within BIOS.
Since he's using a PS/2 mouse, COM1 and COM2 weren't mandatory.
After rebooting, NT showed COM3 and 4 and the modem worked.

Best regards

I've accepted tandy's comment as answer based on the statement of my colleague, who folowed tandy's instructions and got it to work.

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