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Cable Cable Modems and IRQs

Posted on 1999-01-27
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Last Modified: 2013-12-27
I need guidance in juggling IRQs so a cable modem can be
hooked to my IBM box.  All IRQs are currently in use.  The
modem IRQ is an obvious choice, but it (#4) is not one the
cable company will accept. They required an unshared IRQ and
decline all help to me.  I need to know the mechanics, the
pitfalls (Don't change the mouse!), etc.  Must I disable the
current modem so Win98 won't find it and give it an IRQ?  Must I change jumpers and bust the warranty?
  I know this requires a long, involved answer.  Is there a
source of information that will guide me through this?  The
various Win98 texts at Barnes and Noble touch very briefly
on this.  Doesn't this question come up all the time?

             Appreciate your trouble . . . .
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Question by:ehobbs
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Otta earned 200 total points
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A cable-modem is an "external" device,
and connects, on one side, to your cable-company's cable,
and, on the other side, to a network-card, inside your PC.

So, it's the network-card which uses an IRQ.

Most PCs have at least one "free" IRQ, i.e., my computer:
IRQ Level =  0  PCI Pin = NONE  Flg = EXCLUSIVE    TIMER_CH_0
IRQ Level =  1  PCI Pin = NONE  Flg = EXCLUSIVE    KBD_0 Keyboard Controller
IRQ Level =  2  PCI Pin = NONE  Flg = EXCLUSIVE    PIC_1
IRQ Level =  3  PCI Pin = NONE  Flg = MULTIPLEXED  SERIAL_2 Serial Controller
IRQ Level =  4  PCI Pin = NONE  Flg = SHARED       SERIAL_0 Serial Port Controller
IRQ Level =  5  PCI Pin = NONE  Flg = MULTIPLEXED  Sound Blaster 16 driver
IRQ Level =  6  PCI Pin = NONE  Flg = MULTIPLEXED  FLOPPY_0 Floppy Controller
IRQ Level =  7  PCI Pin = NONE  Flg = MULTIPLEXED  PARALLEL_0 Parallel Port Adapter
IRQ Level =  8  PCI Pin = NONE  Flg = EXCLUSIVE    RTC

IRQ Level = 10  PCI Pin = NONE  Flg = SHARED       AIC7870_0  Adaptec SCSI AIC7870

IRQ #9 and IRQ #11 are "free";
unless you have SCSI hard-drives, IRQ #10 will be "free".

IRQ #12 is used by a PS/2 mouse.
IRQ #14 is used with your first IDE hard-drive.
IRQ #15 is used with your second IDE device (hard-drive or CD-ROM).

So, use IRQ #9 or IRQ #11.

Check your computer, and see if either of those IRQs
are available.


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by:ehobbs
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This is a good atempt to figure out how much or how little I understand.  Yes, I do know that much (not a lot more) about IRQs, and I can find them and read them on my Aptiva.  My problem begins there.  All IRQs are taken.  There is a modem currently on board,
IRQ #4.  Since I am going to be replacing this, you might think I could take away its IRQ number and use that.  Unfortunately, the cable company insist on using one of eight specified IRQs -- 3, 5, 7, 9, 10, 11, 12, 15.  It seems to me that my approach is to disable the modem, swap its number with another device, and thereby free up one of the required IRQs for the cable modem.

Is this sensible?  What pitfalls exist?  What devices do you recommend for this swap -- someone recommended against the mouse -- will I have to swith/remove jumpers to keep
the present modem out of play?  What are the mechanics of this kind of IRQ juggling?  

An IBM type I finally reached by telephone said he didn't think IBM supported cable modems, but he considered IRQ manipulation "child's play."  This child would like more information before venturing onto the playground.

Thanks.
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by:ehobbs
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This is a good atempt to figure out how much or how little I understand.  Yes, I do know that much (not a lot more) about IRQs, and I can find them and read them on my Aptiva.  My problem begins there.  All IRQs are taken.  There is a modem currently on board,
IRQ #4.  Since I am going to be replacing this, you might think I could take away its IRQ number and use that.  Unfortunately, the cable company insist on using one of eight specified IRQs -- 3, 5, 7, 9, 10, 11, 12, 15.  It seems to me that my approach is to disable the modem, swap its number with another device, and thereby free up one of the required IRQs for the cable modem.

Is this sensible?  What pitfalls exist?  What devices do you recommend for this swap -- someone recommended against the mouse -- will I have to swith/remove jumpers to keep
the present modem out of play?  What are the mechanics of this kind of IRQ juggling?  

An IBM type I finally reached by telephone said he didn't think IBM supported cable modems, but he considered IRQ manipulation "child's play."  This child would like more information before venturing onto the playground.

Thanks.
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by:Otta
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> All IRQs are taken.

What are the current assignments?

> Unfortunately, the cable company insist on using one of
> eight specified IRQs -- 3, 5, 7, 9, 10, 11, 12, 15.

The motherboard's COM1 port usually uses IRQ #4.
(A serial-mouse would probably use COM1.)

The motherboard's COM2 port usually uses IRQ #3.

If you have two printers, the second printer uses IRQ #5.

Your first printer uses IRQ #7.

Your sound-card can use IRQ #5 or #9 or #10.

Your network card can use IRQ #5 or #9 or #10 or #11.

A PS/2-style mouse uses IRQ #12.

Your IDE hard-drive(s) and CD-ROM(s) use IRQ #14 and #15.

So, what's your current configuration?
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by:ehobbs
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00 System Timer
01 Standard 101/102-Key or Microsoft Natural Keyboard
02 Programmable Interrupt controller
03 Communications Port (COM2)
04 LT Win Modem
05 Crystal PnP Audio System CODEC
06 Standard Floppy Disk Controller
07 ECP Printer Port (LPT1)
08 System CMOS/real time ckicj
09 SCI IRQ used by ACPI bus
10 Intel 82371AB/EB PCI to USB Universal Host Controller
10 ACPI IRQ Holder for PCI IRQ Steering
11 Crystal PnP Audio System MPU-401 Compatible
12 ScrollPoint (PS/2)
13 Numeric Data Processor
14 Intel 82371AB/EB PCI Bus Master IDE Controller
14 Primary IDE controller (dual fifo)
15 Intel 82371AB/EB PCI Bus Master IDE Controller
15 Secondary IDE controller (dual fifo)

I may not have caught all my typos . . . .




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by:ehobbs
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00 System Timer
01 Standard 101/102-Key or Microsoft Natural Keyboard
02 Programmable Interrupt controller
03 Communications Port (COM2)
04 LT Win Modem
05 Crystal PnP Audio System CODEC
06 Standard Floppy Disk Controller
07 ECP Printer Port (LPT1)
08 System CMOS/real time ckicj
09 SCI IRQ used by ACPI bus
10 Intel 82371AB/EB PCI to USB Universal Host Controller
10 ACPI IRQ Holder for PCI IRQ Steering
11 Crystal PnP Audio System MPU-401 Compatible
12 ScrollPoint (PS/2)
13 Numeric Data Processor
14 Intel 82371AB/EB PCI Bus Master IDE Controller
14 Primary IDE controller (dual fifo)
15 Intel 82371AB/EB PCI Bus Master IDE Controller
15 Secondary IDE controller (dual fifo)

I may not have caught all my typos . . . .




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by:Otta
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> ckicj
> ...
> I may not have caught all my typos . . . .

Indeed.  :-)

> cable company insist on using one of eight specified
> IRQs -- 3, 5, 7, 9, 10, 11, 12, 15.
> ...
> 03 Communications Port (COM2)
> 04 LT Win Modem

Do you have anything physically-connected to the COM2 port?
Your modem seems to be using COM1.

If not, then reboot, and enter BIOS-setup mode,
and "disable" the motherboard's COM2 port.

This will "free" IRQ #3, as per your needs.

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by:ehobbs
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I have tried several times to get through.  Is anybody receiving this?  I am very happy with the progress thus far.

This is a test.
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by:ehobbs
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Several attempts have failed.  Are you still there?


                                                               Elston
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by:Otta
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> Are you still there?

I stopped to eat, but I'm back.
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by:ehobbs
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I have tried several times to answer . . . .

Please expand upon:

1.  How can I find out if COM2 is spoken for by some device?  It surprises me that an IRQ would be assigned to -- nothing??  Can I ask the port directly what is looking through it?

2.  Should I -- must I -- can I disable the existing modem so Windows doesn't find a conflict with the cable modem when it's installed?

  My next question is a little outside the rules of the game.  You can't be expected to hold a beginner's hand forever.  But.  Can you point me to a decent text or other source in BIOS setup, etc?  O'Reilly's "Windows 98 Annoyances" says the BIOS and the motherboard can be reached by "Esc" or "Delete" after power-up before the beep.  This doesn't work on my machine, and I'd like to try turning off COM2 as you suggest.

                                                         Thanks
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by:Otta
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> 1. How can I find out if COM2 is spoken for by some device?

Inspect your motherboard.
Either it has on-board connectors for COM1 and COM2,
or there is a "daughter-board" which has connectors for COM1 and COM2.
Either way, find those connectors,
and see if any ribbon-cable is connected to the COM2 connector.
If there is, trace the ribbon-cable to the other end,
which is likely to be to the back of your computer,
where you connect the other cables (telephone-line, power,
printer-cable, monitor-cable, mouse-tail, Ethernet-cable,
PS/2 mouse-port, keyboard, and USB ports).

Is anything plugged-in to that connector?

> It surprises me that an IRQ would be assigned to -- nothing??

I have a digital-camera (still pictures, not video).
When I want to "upload" the images into my computer,
I temporarily connect from my COM2 port to my camera,
and run the software which came with the camera.
It communicates over COM2, to the camera,
using its own device-driver, rather than any device-driver
supplied by Windows.  As far as Windows is concerned,
I have "nothing" attached to COM2 -- but I'm smarter than Windows.  :-)

> Can I ask the port directly what is looking through it?

Some add-on software can try sending/receiving on COM2,
just to see if any device is "connected" and is "responding".

> 2. Should I -- must I -- can I disable the existing modem
> so Windows doesn't find a conflict with the cable modem
> when it's installed?

No, and no.  My computer has an Ethernet card (connected
to a cable-modem) and a US Robotics modem.
Each device uses a different IRQ, with no "conflicts".

The only "conflict" which could arise will occur
when you try to use both devices at the same time.
When you want to send an IP-packet,
you have to think about how to "route" the packet;
should it be sent to the USR modem, or to the Ethernet card,
since both devices could have connections to the Internet.

> O'Reilly's "Windows 98 Annoyances" says the BIOS and the
> motherboard can be reached by "Esc" or "Delete" after
> power-up before the beep. This doesn't work on my machine,
> and I'd like to try turning off COM2 as you suggest.

First, turn on the monitor, and then let it warm up,
and then turn the computer on.
You should see a "press <something> to enter setup" message.

Or, if you have a COMPAQ Presario computer, press F10.

What type of computer do you have?
Look at the manufacturer's web-site, for more details.

Another thing to try is to press-and-hold any keyboard-key,
and then turn your computer on, hoping to generate a message:
"keyboard error -- press <F1> to enter SETUP"
(or something like it) on your screen.
Then, press <F1>.  



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by:ehobbs
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It looks as though my comments earlier today didn't get through.

Let me repeat that I am totally satisfied, very pleased with this site and its representative, Otta, for the tolerance of naivete and generosity of advice.  I will be back when I've built up some points.

Would someone post me a confirmation if this message gets through?

By the way, I got to the startup page and disabled the serial port, and IRQ #3 is now available.  Everything seems to work.

                                                     Thanks,

                                                            Elston
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> Would someone post me a confirmation if this message gets through?

At the top-right of this page, notice the:

... Reload ? .....

hyper-link?  You can "click" it, and you should
see the "updated" view of your question.

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by:ehobbs
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Thanks all 'round.  You've been great.
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