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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Wonga - Monday, June 28, 2004 - link
Hey hadders, I was thinking the same thing. Surely if these cards need such fancy cooling, they need a little bit of room to actually get some air to that cooling??? And to think I used to get worried putting a PCI card next to a Voodoo Banshee...DigitalDivine - Monday, June 28, 2004 - link
does anyone have any info if nvidia will be doing this for low end cards as well?klah - Monday, June 28, 2004 - link
"But it is hard for us to see very many people justifying spending $1000 on two NV45 based cards even for 2x the performance of one very fast GPU"Probably the same number of people who spend $3k-$7k on systems from Alienware, FalconNW, etc.
Alienware alone sells ~30,000 units/yr.
http://money.cnn.com/2004/03/18/commentary/game_ov...
hadders - Monday, June 28, 2004 - link
Whoops. duh. Admittedly all that hot air is been exited via the cooling vent at the back, but still my original thought was overall ambient temperature. I guess there would be no reason why they couldn't put that second PCIe slot further down the boards.hadders - Monday, June 28, 2004 - link
Hmmm, to be honest I hope they would intend to widen the gap between video cards. I wouldn't think the air flow particularily good on the "second" card if it's pushed up hard against the other? And where is all that hot air been blown?DerekWilson - Monday, June 28, 2004 - link
The thing about NVIDIA SLI is that the technology is part of die ... Its on 6800UE, 6800U, 6800GT, and 6800 non-ultra ... It is poossible that they disable the technology on lower clocked versions just like one of the quad pipes is disabled on the 12 pipe 6800 ...The bottom line is that it wouldn't be any easier or harder for NVIDIA to impiliment this technology for lesser GPUs based on the NV40 core. Its a question of will they. It seems at this point that they aren't plannig on it, but demand can always influence a company's decisions.
At the same time, I wouldn't recommend holding your breath :-)
ET - Monday, June 28, 2004 - link
Even with current games you can easily make yourself GPU limited by running 8x AA at high resolutions (or even less, but wouldn't you want the highest AA and resolution, if you could get them). Future games will be much more demanding.What I'm really interested in is whether this will be available only at the high end, or at the mid-range, too. Buying two mid-range cards for a better-than-single-high-end result could be a nice option.
Operandi - Monday, June 28, 2004 - link
Cool idea, but aren't these high end cards CPU limited by themselves let alone paired together.DerekWilson - Monday, June 28, 2004 - link
I really would like some bandwidth info, and I would have mentioned it if they had offered.That second "bare bones" PCB you are talking about is kind of what I meant when I was speaking of a dedicated slave card. Currently NVIDIA has given us no indication that this is the direction they are heading in.
KillaKilla - Monday, June 28, 2004 - link
Did they give any info as to the bandwidth between cards?Or perhaps even to the viability of dual core cards? (say having a sandard card, and adding a seperate PCB with just the bare minimum, say GPU, RAM and and interface? Figuring that this would cut a bit of the cost off of manufacturing an entirely seperate card.