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Thread: Modem & IRQ

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    Modem & IRQ

    OK, I'm about ready to install a new 56K modem from USR. I ran the installation assistant (setup) CD-ROM, which told me I have no free IRQs. Now what? Setup showed me a list of what's in my system now, including the IRQs. That list was a bit incomplete, however--I checked in with my Device Manager and found a much more thorough list, which shows several instances where an IRQ has two or more things using it. My existing modem is on COM2 and uses IRQ 3. I suppose I need to manually change something...? Suggestions?

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    Re: Modem & IRQ

    irq sharing is common as some devices preclude the use of others or are quite happy to use the same interrupt.
    if you're installing a modem i assume you won't be using com2 (or 1 if you use a PS2 mouse) so you could disable the port and use its IRQ

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    Re: Modem & IRQ

    How do I "disable" com1 or com2? I think I want to keep both modems--the information on the box for my new one says nothing about "voice," so I assume it won't let me make phone calls. I use my computer with an Intel Webcam to call my daughter and her family, and I want to continue to do that. (Or should I look for another modem...one with voice capability?) From your information, it appears that I could have one modem on com1 and the other on com2 and each have its own IRQ...si/non?

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    Re: Modem & IRQ

    I think there's a bit of confusion here. Com1 and Com2 are physically the same : the difference lies in the resources they use. (Similarly for Com3 and Com4). What your helper may have meant is that you can disable a bit of hardware called Serial Port B. This will free up resources, and can be done through the BIOS (press DEL or F2 as you boot up, but before Windows starts).

    You are able to manually allocate IRQs and other resources using Safe Mode. I would not advise this : Windows is already doing the best it can with what you've got to offer. However, when in SM, you might have a look in Device Manager and see if there are any instances of multiple identical devices. If you find any, remove them all. On next boot Windows PnP will re-install a single instance of the drivers for the hardware, and at the same time may well shift IRQs around.
    If, on the other hand, you're feeling adventurous, you could upgrade to Windows 2000. This does not use IRQs as such, and spreads resources much more efficiently.

    Rgds

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    Re: Modem & IRQ

    Um. I though it was 3 and 4 that were the same as 1 and 2...
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    Re: Modem & IRQ

    I'm not that adventurous. I've already put Win98 over Win95 and I'm very happy with the way she runs...at least for now. You suggested Win2000--any thoughts on Windows ME? It seems to be getting mixed reviews. BTW: Thanks for the information!

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    Re: Modem & IRQ

    It is true that there are two ports and they are allocated the names com 1 and 2, they do share IRQ's with com 3 and 4 but not base address. I seem to have missed the bit which states that this is a second modem. If they are plugged into real com ports i.e they are external modems then they will obviously use the IRQ for that port, if one is internal (or indeed both are)you could disable the port in bios and use its IRQ

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    Re: Modem & IRQ

    It always been my understanding that 1 = 3 and 2 = 4 ... <img src=/S/yep.gif border=0 alt=yep width=15 height=15>
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    Re: Modem & IRQ

    Oops! Shouldn't have had that second G&T. <img src=/S/blush.gif border=0 alt=blush width=15 height=15>
    Everyone (except me) is absolutely correct Com1=3, 2=4. However, it doesn't alter the main thrust of the argument : you can have as many Com ports as your system will handle, depending on the resources being used, but you can do them all with only Serial Port A active. Deactivating Serial Port B saves on IRQs.
    From what I read, ME is only needed if :
    a. you intend to get into multimedia in a big way or
    b.if you're upgrading from 95.
    If you already have 98SE there is no need to shell out for ME. A bit like the Office XP : upgrade if you have only Office 97, but if you already have Office 2000, there's no real need.
    I still think Windows 2000 is the brightest kid on the block.
    (These are my personal opinions, open to argument. Don't take them as Gospel. <img src=/S/brainwash.gif border=0 alt=brainwash width=15 height=15>)

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    Re: Modem & IRQ

    I opened my Device Manager/Computer/View Resources (IRQ). Some interesting stuff here, e.g., IRQs 14 and 15. IRQ 14 and 15 appear twice. IRQ 14's first listing: Intel 82371SB PCI Bus Master IDE Controller; second listing: Primary IDE controller (dual fifo). For #15, first: SAME AS #14's FIRST LISTING! Second: Secondary IDE controller (dual fifo). Why would that Intel whatchamajigger need to use both IRQs? If I changed anything in this line-up (re: #14 and #15), would it help me with my IRQ problem and the modem installation? BTW: In the IRQ listings, I see COM3 and COM4 mentioned, but never COM1 and COM2. Gee, I'm so confused. Maybe I need that G&T!

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    Re: Modem & IRQ

    What's a "real com port"? You mean a "real com port" is something I can see and touch when I look at the back side of my tower? There's one or two available connections that I never use, like a serial port. MerC is saying I can go into the BIOS to disable Serial Port B, and that will free up resources. So...if I disable Serial Port B, then re-boot, I might gain a free IRQ? And that Serial Port will be inactive and useless, but I don't use it anyway (who does?) and I'll never miss it. Is that kinda how it works?

    Oh, yes: Both modems are internal. Since I'd be using only one at a time, is there any reason the two couldn't play with the same IRQ?

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    Re: Modem & IRQ

    I would shutdown and then power back on to be sure that all of the Power On Self Test routines run after you disable the port. Plug and Play should then be able to reassign the interrupts.
    Legare Coleman

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    Re: Modem & IRQ

    There are several IRQ's that ar classed as system, that is they are reserved for use by default.
    3 and 4 are com ports,7 is printer port 1 etc. 14 and 15 are the IDE disk handler, the individual ide channels are 14 and 15 and so their parent device, the disk controler also is listed as using them, 'cos they are all the same device.
    The physical ports at the rear of the machine are the com ports, they are "assigned" IRQ and address by the motherboard bios to give you some flexibility. If you are not using them you can disable them in the bios and, even though they may still be listed as allocated to the ports, you can use them. This is common with modems when they switched from using the ports to being internal.
    The other commonly available IRQ's are 9 and 5. 5 is the second lpt port that is rarely fitted but this is generally picked up by sound cards.
    So, if your original modem is on com1 irq 4 then disable com2 (irq3) and use that for a second modem.
    It would definately be a bad thing to reassign the IRQ's for the disk controller, even if it is possible.

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    Re: Modem & IRQ

    I'd like to contribute a bit to the IRQ business...

    The "ComX" is just a name or label. There are standard settings:
    Com1= I/O 3F8, IRQ 4
    Com2= I/O 2F8, IRQ 3
    Com3= I/O 3E8, IRQ 4
    Com4= I/O 2E8, IRQ 3

    These settings can be changed in Win95+. Therefore Com1 can be set to any combination of I/O port and IRQ that you like.

    Standart com port settings can be used pseudo-simultaneously. That is to say, a device can be plugged in to com1 and com 3, but with the standart settings the two cannot be used at the same time. Change the IRQ of com 3 to an unused IRQ and *bingo!* everything will play nice together.

    IRQ sharing is thanks to a technology called PCI Steering. This is a function performed by your bios. Some PCI devices will support this, others will not. It is not unheard of to have 5 devices sharing one IRQ. The motherboard and bios will play traffic cop. You will usually see windows put a placeholder for PCI steering in your IRQ list from the device mangler.

    Now for the kicker...
    Winmodems requiring an IRQ will consume 100% of that IRQ's time. You cannot share an IRQ with anything else.

    Caesar3, can you go to device mangler, double-click Computer, and give us a list of your IRQ usage? Maybe we can help better with more info.

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    Re: Modem & IRQ

    Important announcement: My new modem is installed and working like a champ! My thanks to everyone who contributed to this discussion! FYI: I yanked out the old modem, then disabled COM port B, as suggested. I ran setup again, and it all went like clockwork from there. I'd buy three rounds for all of you if I could! (And another G&T for you, MerC!) Thank you, everyone! <img src=/S/cheers.gif border=0 alt=cheers width=30 height=16>

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