SLI - Nvidia's Multi-GPU Geforce is Seeing double

Those of you that can remember the old Voodoo 2 card created by 3DFx will know all about SLI. It was in its day, the day of the Voodoo 2 a great feature for the gamer. The basics are that it allowed two voodoo 2 cards to be linked together by an external cable. This would allow the user to utilize the power of both graphics cards at the same time. This made the Voodoo 2 an instant hit and was the most popular graphics card of its time. However SLI never featured again, that is until now. The new Geforce 6800 series from Nvidia has re-launched this technology which they acquired when the bought out 3Dfx. Now SLI is back with a vengeance and ready to power many top end systems to unbelievable frame rates.

Why is It called Scan Line Interleaving?

SLI was named Scan Line Interleave because of the way that the original system worked on the Voodoo 2. The way the Voodoo 2 worked in an SLI situation was quite a complex one. You may be aware that the Voodoo 2 cards had to have a separate 2D graphics card as well in order to display the normal pictures on screen. The Voodoo 2 was simply an accelerator. This means that the system with a Voodoo 2 SLI configuration would have 3 graphics cards inside the case. Once the 2D graphics card determined that the information it received needed to go to the 3D graphics card for processing it would send out the data to the Voodoo 2 cards. In an SLI configuration the cards would separate the data between them.

The first graphics card would only process the odd lines of the image 1,3,5,7 etc and all the even lines would be send to the second card for processing. this would then be send back and merged or "interleaved". This is how the technology got the name Scan Line Interleaving.

Nvidia's SLI solution

Nvidia, although having bought the knowledge of how SLI works from 3Dfx all those years ago, have decided to use there own solution, or if you like an improvement on the old solution. The New technology will be known as Multi GPU as it is not truly SLI by nature. Firstly instead of the old external cable the connection will be internal in form of a small circuit board, known as a bridge (pictured below)

Connectors that the bridge fits onto will be built into the Geforce 6800 boards (check with manufacturer before buying as some may decide not to implement this feature). Not only is the connection different but the way that the cards split the data between them is also different. The old Voodoo 2 style way of doing things was the true Scan Line Interleaving. one line to card one and the second line to card 2 etc. Nvidia has gone about it a different way. The have decided to split the image horizontally and send the top portion of the image to card 1 to process and the bottom portion to card 2.

The clever part to this idea is in the actual way it is split. The image is split on a 50/50 basis. The graphics cards can determine how much processing power will be needed to render the image at any one time. Often in games the top half of the image is more static than the bottom as it usually renders things such as sky. This would normally mean that the Geforce 6800 assigned to render the top half of the image would have an easy ride so to speak and the second Geforce would be lagging behind with a more complex image to render. However Nvidia's new technology has the ability to calculate the load on each GPU in order to make them finish each frame at the same time.

Nvidia's load balancing technique will make sure that the maximum frame rate is always achieved as one Geforce 6800 is not waiting around for the other to finish a frame. As the image changing the split of the image between the two Geforce cards is altered to suit on the fly.

Hardware Requirements

Firstly and most obviously you will need 2 Geforce 6800 cards. It is very important that these are the same type of Geforce 6800, for example you cannot put a Geforce 6800 ultra in an SLI configuration with a standard Geforce 6800. You do not have to have the cards from the same manufacturers but we would recommend that you do if at all possible in order to remove any potential compatibility problems. You will then need the connecting bridge which will likely be sold separately at retailer where you can by the Geforce 6800. You will then need a Motherboard with 2 PCI-Express x16 slots. This technology will only work with the PCI express versions of the card, as the AGP bus will not support the configuration of 2 graphics cards, nor do motherboards using AGP have 2 available slots.

Overclocking

Overclocking the Multi-GPU configuration is possible. However It's unlikely that you will be able to overclock one card more than another. So if one of your Geforce 6800 cards is not a good overclocker than you will be stuck at core speeds. The driver will be able to do this overclocking and will overclock both cards equally for you when you overclock the first.

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