NVIDIA's Scalable Link Interface: The New SLI
by Derek Wilson on June 28, 2004 2:00 PM EST- Posted in
- GPUs
Scalable Link Interface
As we first saw during Computex this year, the enigmatic NV45 had a rather odd looking slot-like connector on the top of the card. We assumed that this connector would be for internal NVIDIA purposes, as companies often add testing and diagnostic interfaces to very early hardware. As it turns out, this is NVIDIA's Scalable Link Interface connector.
Notice the gold connector at the top of the card.
In order to make use of NVIDIA's SLI technology, two NVIDIA cards are placed in a system (which requires 2 PCIe x16 slots - more on this later), and the cards are linked together using a special piece of hardware. Currently, this communications hardware is a small PCB with a slot connector at each end. No pass through cable is needed, and one video card acts as the master (connected to the monitor) and the other is the slave.
SLI PCB top view.
SLI PCB bottom view.
When asked whether it would be possible to connect the cards together with something along the lines of a cable, NVIDIA indicated that the PCB approach had afforded them superior signaling qualities, but that they continued to look into the viability of other media. As this is new technology, NVIDIA is slightly weary of sharing some of the lower level details with us. We asked whether their SLI uses a serial or parallel interface (usually fast parallel interfaces are more sensitive to signal routing), but we were told that they may or may not be able to get back to us with that information. Either way, this is going to have to be a very high bandwidth connection as it's over this path that the GPUs will communicate (this includes sending framebuffer data for display).
As previously mentioned, this setup requires having 2 PCIe x16 slots available on one's motherboard. Not only is this going to be difficult to come by in the first few months of PCIe motherboard availability, but currently, none of Intel's chipsets support more than 24 PCIe lanes. The current prototypes of motherboards with two PCI Express x16 slots are actually only using one PCI Express x16 interface and one x8 interface, simply with an x16 connector (so it's physically an x16 slot, but electrically, an x8 slot). This reduces the bandwidth available to the 2GB/s up and down (which is still more than AGP 8x can handle). That's not to say that PCIe bandwidth is necessary for gaming at the moment. The real problem is that there would be no other PCIe slots available for expansion cards. But x1 and x4 PCIe expansion cards haven't been making many waves, so until chipsets support more than 24 PCIe lanes and more PCIe expansion cards come out, it might be possible to get away with this.
NVIDIA Quadro connected in SLI configuration.
Until now, we've just mentioned NV45 as supporting this, but NVIDIA seems to be indicating that all their PCIe cards will have the capability to run in SLI configurations. This includes the Quadro line of workstation graphics cards. This is very interesting, as it shows NVIDIA's commitment to enhancing performance without degrading quality (CAD/CAM professionals can't put up with any graphical artifacts or rendering issues and can always use more graphics power).
But let's move on to the meat of the technology.
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Falloutboy525 - Tuesday, June 29, 2004 - link
acutally I wouldn't be surprised if one of the board manufactures puts 2 cores on one card. but man just thinking aboutt he physical size of the card gives me nightmares.Pumpkinierre - Tuesday, June 29, 2004 - link
The last of the SLI Voodoo2s had a dual gpu on a single board for one PCI slot. I cant see why the same couldnt be done for a dual 6800 gpu board on a single x16 PCIe slot which is nowhere near saturation with current gpus. Load balancing would be accomplished on board. In fact, they could do it on AGP 8x as well. They could extend this to multiple gpu (also possible on a 3x 16PCIe slot mobo (+ 3slot bridge) if it ever came out. Just think of the cooling with a Prescott cpu thrown in! Put a Vapochill to room temperature!Backward daisy chaining of components is a great idea but I doubt whether the greed of manufacturers will let it happen. The concept should not be limited to gpus but extend to mobo/cpus as well. A high speed link bus(Hypertransport perhaps, but not I2B) should allow systems to act as multiple processor system albeit with a little added latency. With parallel processing and multithreads around the corner, it would be useful to those who detest the enormous waste in the IT industry.
quanta - Tuesday, June 29, 2004 - link
Actually, NFactor, GeForce 6800's dedicated video codec is a step behind from ATI's videoshader. It adds transistor counts for things that can already be done by 3D core. As for power consumption goes, we only have NVIDIA's word for lower power requirement, but consider ATI also use videoshader for mobile parts, I suspect NVIDIA's claim only applies to NVIDIA's own products rather than ATI's.As for multiprocessing goes, ATI better catchup. After all, not every gamer can affort Evans & Sutherland simFUSION cards.
Phiro - Tuesday, June 29, 2004 - link
Yes, but there's the economy of scale. Nvidia has a "single" production line churning out the nv4x chipset and they package them accordingly to their price point - no major modifications required.The 6800U & x800XT don't really qualify as a "halo" products - they are a high-end version of the *same product the majority of users buy.
klah - Tuesday, June 29, 2004 - link
"And the whole "alienware sells 30k systems a year so there is a market for this" - 30k video cards a year is less than a drop in the bucket for the R&D spent on putting this together."The same could be said for the 6800U and x800XT. 99.9% of cards purchased will be sub-$200, so why bother with $500+ units? It's called a halo product. They are not built to make money. They are built for bragging rights and to generate a positive brand image. The 'buzz' this product creates for Nvidia is more substantial than spending the money on magazine ads and lan party sponsorships.
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"Excuse me, but I noticed that one 6800 Ultra takes two slots worth of airspace (due to the gigantic fan). So that means the Ultras would actually occupy the first and third PCIe slots"
No. All pci-e slots are not they same. This setup require two x16 slots. Dual x16 moptherboards do not have any other slots between these. These two slots have about double the space between them as the rest of the x8, x4 and x2 slots.
Nvidia is launching their nforce4 chipset later this year which will support dual pci-e x16. This is probably when this product will become available at retail.
Phiro - Tuesday, June 29, 2004 - link
Ugh what a dumb, dumb waste of technology. Give me dual video cards (for dual directx/opengl displays) but not SLI BS. This is far better served with multiple GPU's on the card, not multiple cards.If Nvidia is really so concerned with people being able to pay for the ultimate in performance or allowing people to "upgrade" without throwing everything away, Nvidia should go with a user manageable socket on their cards and support multi-core GPUs.
And the whole "alienware sells 30k systems a year so there is a market for this" - 30k video cards a year is less than a drop in the bucket for the R&D spent on putting this together.
If this idiotic SLI re-invention cost the release of the nv4x (and prolonged our nv3x agony) a single day, or increased the cost of the nv4x cards by a single dollar, Nvidia is once again crowned king of the dumbshits in my book.
Good choice buying 3dfx, Nvidia. It took a few years but Nvidia proved the old adage "You are what you eat". Nvidia's cards are hotter, larger, more complicated and more proprietary every day.
ScuZZee - Tuesday, June 29, 2004 - link
Excuse me, but I noticed that one 6800 Ultra takes two slots worth of airspace (due to the gigantic fan). So that means the Ultras would actually occupy the first and third PCIe slots (the second and fourth slot would be made useless since it would be blocked by the coolers).So does that means the mobo have to spaced out the two PCIe slots to accommodate the two Ultras?
SpeekinSfear - Tuesday, June 29, 2004 - link
barbaryJust FYI, if you're gonna buy two, the GT model which $399 instead of the $499 Ultra cost can do it too. They're smaller, less hot and power draining, and did I mention cost $100 less. I think the only power difference is that the GT ones have 50mhz less clock speed.
barbary - Tuesday, June 29, 2004 - link
So now I am stuck what to buy.I have a 670 Dell workstation and I was going to buy an ATI X800. But now should I buy a 6800 Ultra??
Question is do I buy two so I know I have a pair??
If I do and this technology doesn't come along for months I have wasted my money.
If I don't buy two a may never get a pair to match and have wasted my money.
Swaid - Tuesday, June 29, 2004 - link
Its not like you *have* to purchase 2 video cards for anything to work, thats only for the big spending enthusiast nuts and the CG/CAD guys. Its already part of the GPU, so its like an added bonus. The hard part in the beginning is getting a motherboard to support 2 PCIe x16.