Question about IRQ Channels

TruckStuff · Post by **TruckStuff** » Wed Dec 04, 2002 9:17 pm

When my motherboar manual says that the IRQ channel for the AGP bus is "INTB/INTC", what does this mean? Does this mean that it resides on both IRQ channels? Either? Maybe? What?

I am trying to pin down some flakey behavior and am lookint at my card orientation. Right now, they are arranged as follows:

AGP = INTB/INTC = TNT2 Ultra
PCI1 = INTA = Adaptec 19160
PCI2 = INTB = Empty
PCI3 = INTA = Empty
PCI4 = INTB = SB Live Value
PCI5 = INTC = USB card (I have an older AMD SMP board with teh southbridge bug)
PCI6 = INTD = Empty
Onboard LAN = INTD = Integrated 3Com

I am thinking I might pull the USB card since the only USB peripheral is my mouse and I don't game anymore. This would free up the INTC channel, but this goes back to my original question about the AGP card. Anyone have any thoughts or tips (ahem, FP

)?

Post by **FlyingPenguin** » Wed Dec 04, 2002 11:56 pm

There are only four PCI Interrupt channels (A, B, C and D). These are NOT the same as Interrupts (which are numbered). All PCI, AGP and onboard hardware needs to share these 4 channels.

All slots are assigned interupt channels - actually they're assigned TWO because some cards use 2 interupts (pretty rare nowadays).

When you see it listing INTB/INTC for the AGP slot it means that the video card will use Interupt channel B for the first IRQ and C for the 2nd (the TNT2 only uses one like almost every video card I know of, so it's only using channel B).

It's rare for your motherboard to tell you what the channel assignments are, but it's nice to know. It helps you to avoid IRQ channel sharing problems.

Some cards do not like to share interupt channels and will cause all kinds of bizarre problems including lockups and random reboots. Some PCI sound cards (the SBLive in pariticular) have this problem, as well as some PCI NICs and busmaster SCSI controllers.

If at all possible you want to arrange your cards so NONE of your cards are sharing IRQ channels (keep in mind that not all cards use IRQs and it's fine to have two cards in slotsa sharing the same IRQ channel if one of them is not using an IRQ). However in practice it's pretty damn difficult to do.

Best you can do is avoid sharing IRQ channels between cards that are fussy about it.

Right off the bat I see a problem with your system because the SBLive is sharing a channel with the video card. From long experience I can tell you that you do NOT want the SBLive sharing a channel with anything.

You have no choice with the Vid card (INT B) and onboard LAN (INT D) - you can't move those devices.

I would recommend moving the USB card to PCI6 so it's sharing with the onboard LAN, then put the SBlive in PCI 5 so it's on INT C by itself.

If you have problems with the USB or the NIC after doing that then try putting the USB in PCI 3.

I'm betting that the NIC and the USB are the least likely to have sharing problems so they should be okay together. The vid card and the SBLive will be a lot happier on their own channels by themselves.

If you still have problems then pull ALL the cards except the video card and then install each card ONE BY ONE starting with the SBLive in PCI 5. Test each card (hopefully the problem is easily repeatable) and then install the next one. If at any time you have a problem whith a card, move it to another free slot.

And yeah, if you can live without the USB card it would certainly help.

Hope this helps...

TruckStuff · Post by **TruckStuff** » Thu Dec 05, 2002 9:26 am

I'll take those recommendations and give it a shot. When I rearranged my cards last, I chose this orientation because I was hoping that the "newer" TNT2U would be more polite about using the IRQ channel to make up for the SB Live. Guess not.

Are there sound cards out there that do not have this problem? I realize that I want each card as isolated as possible on an IRQ channel, but in a situation like this where something is going to have to share, is there a sound card that is more "freindly" with its neighbors on teh same channel? Or is it just inherent to all PCI sound cards (realizing that some are better than others)?

I would like to be able to hold onto the USB card just in case I get a digital camera or something down the road. But if something has to go, I will remove it. I will test today and find out.

Out of curiosity, are there certain characteristics that you look for in a card to know if it will be good/bad at channel sharing? Or is it just expereience? For example, what makes you think that the USB card and LAN won't have any problems sharing? Just curious.