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Com ports & IRQ's

XMAN asked
Medium Priority
Last Modified: 2013-11-15
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COM1 and COM3 share the same interrupt ( IRQ4)
and COM2 and COM4 share the same interrupt (IRQ3)

If this is a hardware board that you are installing it probably will only work on COM3 or COM4 if you can insure that the IRQ is not in use.

Most MODEMS have the ability to use an additional IRQ lik 5,7, or 9. You can set your modem to one of these IRQ's and use the TAPI capability in WIndows95 to manage configure the modem and then install your hardware board for COM4.

I would not try to use COM3 due to it being your modem and it is attached to a port probably off your motherboard and it usually doesnt have the ability to use different IRQ's.

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I would try the suggested answer here, if you're not happy with it you could reject it to give the chance to other techs to answer your question.

I would also need to know what kind of modem do you have in that machine, and if it's a PNP (PLUG & PLAY) or not??


what kind of mouse do you have that requires a com port?

What kind of a comment is that?? All serial mouses needs COM port.

on a new computer it should have the mouse port in the motherboard.

Beautiful, now that mouse port as you said on the motherboard will work using what?? an LPT or BUS??
Come on, if you don't know computers that's fine (you could learn), but making fool out of yourself that's bad. (no heart feeling)

My computer has a mouse port ps/2 to be exact and it is installes on the motherboard with an irq of 12 witch should not conflict with the com ports.

Beside an IRQ the mouse still need a COM. I hope you understand that.

well if that is so how do I have a mouse and yet have a modem/soundcard in com 1 and a scanner in com2 and com 3 and 4 open?

Sorry for taking space to dedate here. No more comments except for you.


Sorry I haven't replied sooner.  My boys are in the midst of collage application time as well as term papers, and with work and... you know.  Anyway - I haven't had an opportunity to investigate the answer from Cpippin.  As far as the comments go...
The mouse is a Logitech serial mouse and is attached to the 9 pin connector (serial card) which also has a 25 pin connector next to it;  this is the port I have connected my new device to.
To Magigraf: the modem is a USR 33.6 Fax/Modem and was installed at purchase.  I assume it is p&p.  
One further note.  I also have a scanner but it is attached to it's own scsi (?) card.  Although, I don't think this has anything to do with the original problem.
I think I will have to move the modem to another Com port but an afraid that someware down the line W95 will hose me up.
Any comments are appriciated.
Thanks for taking the time to help out.


Please do the following:

1) Right click on MyComputer Icon on the desktop
2) Select properties
3) Select the Device Manager Tab
4) Click on the Properties button on the lower left side
5) Now take note of all the IRQ numbers here on a paper

Post them here so I can check what IRQ is free or occupied
Will wait to hear from you.


To Magigraf,
00 sys. timer
01 keyboard
02 programmable interupt
03 com port 4
03 com port 2
04 com port 1
05 sound card
06 std. f.d. controller
07 printer port lpt1
08 sys. CMOS (clock)
10 scsi host adapter
11 irq holder
11 video-57p
13 numeric data processor
14 ide contr.
14 primary ide
15 pci bus master
15 secondary ide contr.


O.K. before we troubleshoot here, you would have to reopen the question to other techs if the proposed answer does not satisfy you.
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