I'm sure many of us can remember the days of 3dfx, the first Voodoo Graphics back in 1996 and about a year later the introduction of the Voodoo2. Voodoo2 actually made sure that 3dfx reigned supreme for quite some time as two cards could be combined in something called an SLI, Scan Line Interleave, configuration. Each card rendered half of the image scan lines which resulted in double the performance of a single board and the ability to play OpenGL games such as Quake 2 in a 1024x768 resolution. To date no manufacturer has come up with a similar concept simply because modern graphics accelerators are all AGP based, there's no dual AGP motherboards and PCI simply doesn't have the bandwidth to handle modern graphics accelerators. With the arrival of PCI-E things have changed though,
Whereas Voodoo2 SLI used a ribbon cable to be connected between two Voodoo2 cards internally and a pass through VGA cable externally to distribute the analog signal Nvidia's implementation is all done in the digital domain. Both 6800 series PCI-E cards are connected by means of a SLI, Scalable Link Interface, dubbed the MIO port, a high-speed digital interconnect which connects to a connector on top of both cards. This connector is actually available on all PCI-E GeForce 6800 series graphics cards. Through this MIO port both cards communicate to each other and distribute the workload which is accelerated by dynamic load-balancing algorithms. In essence the screen is divided vertically in two parts; one graphics card renders the upper section and the second graphics card renders the lower section. The load balancing algorithms however allow it to distribute the load across the graphics processors. Initially they'll both start out at 50% but this ratio can change depending on the load
