Our SLI test rig
If you're not familiar with all the basic concepts of SLI, please take a second to read our technology preview, which explains the basics. The heart of our SLI test system is the Asus A8N-SLI motherboard, an nForce4-based board with a pair of PCI Express x16 graphics slots.
This board is loaded with all the latest features, including a pair of Gigabit Ethnernet slots and lots of tweaking options, but the big news, of course, is the dual graphics capability. Between the two PCI-E x16 slots, the A8N-SLI has a card that plugs into a connector that looks much like the SO-DIMM connectors used for laptop expansion RAM. The mobo ships with the card in the single video card position, where it routes all 16 lanes of PCI Express connectivity to graphics slot one. However, for SLI, one may turn the card around 180 degrees and reinstall it. In dual video card mode, eight PCI Express lanes are directed to graphics slot one, and eight lanes to slot two.
The cards we used for testing were PCI Express versions of the GeForce 6800 Ultra. These NVIDIA reference cards came with the core clocked at 425MHz and memory at 1.1GHz. That's a 25MHz faster core than some of the GeForce 6800 Ultra cards on the market, but Ultra cards from BFG Tech and Asus are currently selling with 425MHz cores. Although these are Ultra cards, we were able to take them down to 350MHz core and 1GHz memory speeds during our testing, so we could also show you how a pair of GeForce 6800 GT cards would perform in an SLI config.
The two GeForce cards are connected together via an SLI, or Scalable Link Interface, connector. This link transmits key data between the cards, but I haven't yet pumped NVIDIA for all the technical info on how it works. Of course, the PCI Express connections to the rest of the system are very important, but the SLI link is necessary to coordinate between the two cards. NVIDIA says SLI can happen only between two cards of the same brand and model, and the company has spawned a certification program for SLI graphics cards, motherboards, and applications, to ensure basic compatibility.
Beyond the trick reversible card on the motherboard, the SLI connector, and the acquisition of a suitably beefy power supply unit, setting up an SLI system is relatively easy. NVIDIA's drivers recognize the presence of the second video card upon boot and suggest enabling SLI mode. Do so, and after a reboot, SLI is ready to roll.
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