NVIDIA SLI: What Do You Need for the Multi-GPU Mode?
For the SLI mode to work some other conditions must be met besides the mainboard’s having two PCI Express slots designed as x16. First, either “16 lanes + 8 lanes” or “8 lanes + 8 lanes” configuration must be supported. Second, the mainboard’s chipset must be supported by the ForceWare driver. So far only two chipsets meet this requirement; they are Intel’s i7525 “Tumwater” and NVIDIA’s nForce4 SLI. It is NVIDIA’s chipset that’s the foundation for building SLI-compatible systems as the i7525 is intended for high-performance workstations with the Intel Xeon processor. We don’t think a dual-processor mainboard priced about $350-500 is likely to appeal to an ordinary user, so we will only discuss SLI technology with regard to mainboards on NVIDIA’s chipsets. Theoretically two graphics cards from NVIDIA can also be united into a SLI duo configured as “16 PCI Express lanes + 4 lanes”, but a certain performance hit may be observed due to the low bandwidth of the second slot.
This didn’t prevent some mainboard manufacturers from releasing such products on the nForce4 Ultra and even on the i915P chipset! For example, the PCI Express slot for a second graphics card on the Gigabyte GA-8I915P-SLI mainboard is made by joining lanes intended for independent PCI Express x1 slots, and the DFI LANPARTY UT Ultra-D uses the nForce4 Ultra chipset that doesn’t officially support SLI configurations. Thus, the range of chipsets that can support two graphics cards on NVIDIA’s chips doesn’t include only the nForce4 SLI and the i7525. In fact, the configuration “PCI Express x16 + PCI Express x2” is also possible, but the performance hit would be too obvious and this configuration would lack balance.
NVIDIA didn’t like the release of mainboards like the above-described, and the new versions of the ForceWare driver now block the SLI mode if the mainboard’s configuration doesn’t meet certain requirements. NVIDIA’s position is quite understandable: such systems have a low performance and this may affect negatively the reputation and popularity of the new technology.
Besides an appropriate mainboard, two identical graphics cards are necessary for the SLI mode. They can be either two GeForce 6800 Ultra/GT or two GeForce 6600 GT cards; a GeForce 6600 GT wouldn’t work in the SLI mode with a GeForce 6800 Ultra (or GT). In order to avoid compatibility-related problems NVIDIA recommends that you purchase cards from the same manufacturer for your SLI system.
Don’t forget that two graphics cards will inevitably call for more juice, so you may want to upgrade your current power supply, replacing it with a high-wattage unit with stable output voltages. For example, we powered our test system up with the help of a Cooler Master Real Power RS-450-ACLY unit and enjoyed perfect stability even with two GeForce 6800 Ultra cards inside. The system case should also be roomy enough and should have good ventilation, especially if you’re building a SLI configuration out of two top-end graphics cards.
The next section of this review describes the assembly and launch of a SLI-compatible system by the example of an ASUS A8N-SLI Deluxe mainboard and two pairs of GeForce 6800 Ultra and ASUS Extreme N6600 GT Series (GeForce 6600 GT) graphics cards. We will also examine these products closer in the meantime.