Are all PCI slots made equal?

[PeteSchiffer]

Hi,

I've been wondering about conflicts. (haven't got any, have just been wondering!)

I have heard that if you have a device, such as a sound card, which is conflicting with something, but it won't allow you to change its IRQ, then you can try switching the card to another PCI slot. It will then be assigned another number, which although probably sharing, may not be such a conflict as the previous one. Is there something in this?

Pete

[andyswork@beci.net]

Yes there is something to that.
I have had that problem.
Changing PCI slots solved the problem for me.

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One day I will be a pro, Thanks Andy

[iisbob]

i think i understand what you are asking.

You're wondering why it makes a difference when you move an expansion card from one slot to another, when they are all on the same bus, right?!

It does seem at first glance to be a little asinine; however this is where understanding the BIOS and how an OS assigns IRQ's comes in.

The PCI bus is also the same IRQ as the original Serial bus ( actually it's the same old serial bus , just increased in width and speed ). It's IRQ 11 if you're curous, and it's also the same bus USB uses { hence the name universal serial bus }.

Here is a web site that will help you to understand IRQ's; Webopedia-what is an IRQ? (http://www.webopedia.com/TERM/I/IRQ.html) and also PCGuide's IRO explanation (http://www.pcguide.com/ref/mbsys/res/irq/num.htm). You should really browse Charles site { PCGuide }; it contains pretty much all the info you need to understand these personal confusers of ours. http://www.PCGuide.com/ubb/wink.gif



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iisbob

"It is by the fortune of God that, in this country, we have three benefits: freedom of speech, freedom of thought, and the wisdom never to use either."
-Mark Twain { Samuel Clemen's }

[Paul Komski]

A naive question maybe - but if you were to remove the card and then replace it in the same slot wouldn't this have the same effect as replacing it in a different slot? Or is each slot recognised as unique - if you see what I mean.

My understanding had been that on a new installation a unique or non-conflicting IRQ would be allocated if possible. I seem to remember, long ago, solving a conflict this way with an ISA card for an old scanner, when there was only one slot available. But maybe I am wrong and maybe the allocation for ISA works differently from PCI.


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Take nice care of yourselves - Paul
"There are old pilots and bold pilots but there are no old bold pilots."

[Whyzman]

Hi Pete,

There's also an issue that I ran into first hand building my first computer...it's known as Hardware Sharing. On my ASUS A7V133 there are PCI slots that Hardware Share, and in my research discussing with iisbob and his use of ECS K7S5a it appears all MBs do so. What appears to be the most universal example is the AGP and PCI Slot 1 Hardware Sharing.

Since your Video Card is going to hog resources putting a...say Sound Card in Slot 1 would probably not allow you to get through POST.

On my MB there's a page in the manual that actually shows which slots Hardware Share, i.e., The Interrupt Request Table. PCI 2 actually Hardware Shares with the Promise Controller. If you're using the ATA 100 Promise Controller...since your HDD will be accessed plenty http://www.PCGuide.com/ubb/rolleyes.gif , you'd want to refrain from putting anything into Slot 2.

Although PCI architecture was designed for the sharing of IRQs some devices seemed to have not gotten with the program...and if you put them on a "party line" even if you've got an emergency, sometimes the other party just needs to finish up on some juicy piece of gossip and will not relinquish the line! http://www.PCGuide.com/ubb/frown.gif http://www.PCGuide.com/ubb/biggrin.gif

Playing musical slots appears to be a trial and error way of finding PCI slots that are compatible not only with sharing IRQs, but also with keeping slower traffic moving to the right...since if the slots Hardware Share one device might not want to get out of the fast moving lane! http://www.PCGuide.com/ubb/mad.gif http://www.PCGuide.com/ubb/biggrin.gif

Hardware Sharing appears to be in the actual "Hard Wiring" of the MB and is a different issue than IRQs. So it appears that we are actually fighting on two fronts. My guess is that as iisbob pointed out, some IRQs are pre-assigned as IRQ 11...and some devices probably have favorites adding to the confusion?

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May all your dealings in life be win/win!


Whyzman
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Reserved for Punishing Humor...A Pessimist's blood is always B-negative!


[This message has been edited by Whyzman (edited 03-09-2002).]

[PeteSchiffer]

Hi,

Thanks for the information. I haven't been to the links yet. But will.

I probably put the questions poorly initially. Paul kind of summed it up when asking whether each slot was unique, this uniqueness was what I was getting at. It appears that the slots are unique, at least in some aspects. I'm not quite sure if I understand all that has been written so far, but it does seem to me that the hardware sharing aspect of this is something that is little discussed, and is certainly new to me.

It's possible then to have devices which don't have conflicting IRQs, but are competing for the same part of the bus. Not exactly a conflict, rather a serious bottleneck, which, if two devices were heavy users of the bus, would result in serious problems. These two devices are heading for different places (their IRQs), but unfortunately are taking the same route to get there. Or am I way off target here?

Pete

[iisbob]

just thank your lucky stars you're building a PC in the modern PnP era!

My GOD, as i remember the jumpers and manual IRQ"s i had to set up on old system's-it makes me wonder i didn't go ballistic! http://www.PCGuide.com/ubb/biggrin.gif

Originally there were only 8 IRQ's-finally with the advent of the newer 386 CPU's and busmastering there had to be more IRQ's added to be avaiable for all the expansion crard option's and onboard motherboard accessories.

Basicly you have 16 IRQ's, but only 3-4 are ever really open for you to use.

Here's a brief table of IRQ's for you and you'll kinda get the picture; 0=system timer 1=standard keyboard 2=the bus to the new 8 IRQ's ( to no# 9 actually ) 3=logical COM ports 2/4 4=logical COM ports 1/3 5=generally a sound card, can be used otherwise 6=floppy disk controller 7=your LPT1 port ( this is your parallel port ) 8=real time clock 9=here's the link from #2 that starts the new 8 IRQ's 10=generally open, most time your NIC will have this one 11=PCI/Serial bus ( also yor USB bus ) 12=Good ol' mouse 13=your math co-processor 14=your primary IDE controller 15=and finally your secondary IDE controller

As you can see there are a no# of IRQ's that you don't want to change, or you'll lose some vital equipment.

Your OS ( 95-XP ) makes a determination as to which slots on the mother board are configured for which IRQ's ( with help from the system boards BIOS ), this is why it can be disasterious if you flash the BIOS incorrectly; you could screw up the detection of the correct IRQ's for your motherboard.

Since 98se, IRQ sharing has really come a long way, a good example is the USB ports, they've gone a long way to making your PC truely a plug&play unit for the first time since it's development.

Trust me, you've got it made with today's PC's-you want a real headache, go find an old 286 legacy system and try to install a sound card/modem/hard drive controller! http://www.PCGuide.com/ubb/rolleyes.gif



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iisbob

"It is by the fortune of God that, in this country, we have three benefits: freedom of speech, freedom of thought, and the wisdom never to use either."
-Mark Twain { Samuel Clemen's }

[Whyzman]

Pete,

I think you put your finger on the operative word as far as Hardware Sharing = "Bottleneck" http://www.PCGuide.com/ubb/eek.gif ! Along with iisbob's usual clarity explaining the IRQ situation...the poor bus is a war zone! http://www.PCGuide.com/ubb/frown.gif http://www.PCGuide.com/ubb/biggrin.gif

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May all your dealings in life be win/win!


Whyzman
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Reserved for Punishing Humor...A Pessimist's blood is always B-negative!