Dual video cards question

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Dual video cards question

Postby Beefy_Suavo » Mon Nov 26, 2007 6:59 pm

This question has been bugging me for a while and I haven't been able to find a good answer.

If a computer has two PCI Express slots, and I want to have two video cards linked together (either two nVidia cards using SLI or two ATI cards using CrossFire), does the chipset matter?

I remember reading an article on Tom's Hardware a while ago (which of course I can't find now) that said for a single video card it doesn't matter what chipset you have (ATI, nVidia, Intel, etc.) It also said that for dual video cards, you need a chipset that can support the configuration you want, but didn't go into any details as to why.

So if you have two PCI Express slots and two video cards connected using a bridge or a cable, why does the chipset matter? Why can't the cards do all their needed communication through the bridge/cable?

If I want to have two ATI video cards connected together in CrossFire configuration, but have a nVidia chipset, will it still work?

Oh, and if your wondering why I don't just use two nVidia cards, it's because my stupid motherboard doesn't like the 8800 series or newer.
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Postby Kariudo » Mon Nov 26, 2007 8:18 pm

*side note: sli and crossfire aren't worth the money for the second card.

sli and crossfire are developed by 2 completely different companies...so that means that the way that they go about doing things is different. Think of it as speaking different languages.

the [northbridge] chipset is the link between your cpu and gpu (and a few other things).
I think that the drivers are responsible for allowing sli or crossfire (the "languages" if you will)...so following that, it should be possible to allow crossfire on an sli chipset
supporting link
but it doesn't appear to work the other way around

at any rate, it would take a lot of digging and a fair bit of risk to make crossfire work on an sli chipset (involving driver un/installation and possibly bios flashing as well)

as long as the motherboard has pci-e x16 it shouldn't matter what pci-e x16 card you have. My guess is that your power supply you have can't deliver enough power to the 8800
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Postby Joe88 » Tue Nov 27, 2007 3:26 am

well SLI chipsets are only supported by nvidia
todays would be the 570i (AMD cpu board)
and the new 650i and 680i (intel CPU boards)

as far as ATI goes
they have support for ATI chipsets (ATI/AMD [AMD owns ATI])
and also this includes Intel chipsets as well such as the new P35 and X38, there was also support for the 975X
(I do find it funny though intel support ATI crossfire on their chipsets even their competition AMD owns it...)

atm the Radeon 3870 is a very good buy (if you can find one that is) if your computer doesnt like 8800 series cards, even 2 3850 outpower a 8800GTX in benchmarks for about $350

there are hacked drivers that will allow SLi an CorssFireX on motherboards not made for it via hacked driver
however be warned, the drivers are horrible, your a lot better just staying away...
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Postby bobbias » Tue Nov 27, 2007 4:23 am

The only time I could think of actually needing a second GPU is either needing insane amounts of rendering power, or some system that needs a LOT of VRAM. (Example, a system used to render and design 3D scenes in movies and such. I know a bit of what 3D rendering takes, since my dad works as a 3D solid modeling designer [not movies, he designs assembly line stuff for car companies and whatnot.])
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Postby Beefy_Suavo » Wed Nov 28, 2007 10:43 am

Kariudo wrote:as long as the motherboard has pci-e x16 it shouldn't matter what pci-e x16 card you have. My guess is that your power supply you have can't deliver enough power to the 8800

I have a 1000W power supply, but it only has two 6-pin PCIe connectors. so if one card needs two power connections, then I won't have enough for a second card.

I guess what I want answered the most is: how much of the communication between two video cards is handled using the cable/connector between them and the video drivers, and how much of the communication has to go through the chipset on the motherboard.

If it's all done between the video cards and they don't have to go through the motherboard, then wouldn't any motherboard with two PCIe slots enable you to have two matching video cards of whatever make/model you want?
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Postby Kariudo » Wed Nov 28, 2007 12:26 pm

If it was all done by the video cards then it should be able to work on any motherboard (with 2 or more pci-e x16 slots)...but that's not the case
All of the information has to be sent through the northbridge to the gpu's

I'd guess that the drivers are responsible for identifying which frames/parts of frames are to be sent to each gpu (and maybe how to recombine the video information as well)...but that's just my guess.

the bridge is used for communication between the two gpu's (because all of the video information has to go through one of the cards to your monitor)

it's possible to have sli without the bridge (on lower end cards), but you can't take the northbridge out of the equation. AFAIK, Crossifre will not work without the dongle (for older crossfire cards) or the bridges (newer cards, x1950pro and later)

as for your psu, how many 12v rails does it have and what's the amperage on each rail?
although it's usually not advised, there are cables out there that take 2 x 4-pin molex and give you 1x 6-pin pci-e (there's probably an 8-pin one too)
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Postby bobbias » Wed Nov 28, 2007 12:44 pm

Well, as far as determining which frame is done by which card, it's pretty simple.

With SLI, you have 2 modes. One where each card does alternating frames, which tends to increase lag between input, since now 2 frames are finished when originally only 1 was, however, the increase in framerate more or less negates that issue. The other way is by splitting the frame in half, and having 1 GPU render the top, and the other render the bottom. This doesn't have the same lag issue as the way, but increases driver overhead. Neither one of these is "better" than the other, since it depends on the rendering requirements of the game.

Crossfire has more modes:
Crossfire can do alternate rendering like mentioned above, or "Scissor mode" which is similar to the split mode in SLI, but is can split the image vertically or horizontally, as well as splitting on either a 50/50 scale or a 60/40 scale. Then there is the "supertiling mode" where each image is split into a whole bunch of "tiles" which are rendered alternating between the 2 cards. This is a good way to split the work load between them, since the tiles are so small that it's hard to create a pattern that would cause more stress on only 1 of the GPUs. Crossfire also allows for super high quality anti-aliasing and the ability to do both anti-aliasing and HRD lighting processing at the same time.
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Postby Beefy_Suavo » Thu Nov 29, 2007 5:12 pm

Kariudo wrote:as for your psu, how many 12v rails does it have and what's the amperage on each rail?
although it's usually not advised, there are cables out there that take 2 x 4-pin molex and give you 1x 6-pin pci-e (there's probably an 8-pin one too)

One rail, 66A


If I do decide to try two video cards, it won't be until after DirectX 10.1 is released.
Worst case is the CrossFire or SLI won't work and I'll just have two non-linked video cards.
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