NVIDIA's GeForce 6 SLI: Demolishing Performance Barriers

SLI – The Requirements

There’s been a lot of confusion as to what is required to run a SLI configuration, so we put together a quick list of the things you’ll need:

• Everything necessary to put together a working system, including SLI motherboard
• Two graphics cards with identical GPUs from the same manufacturer.  Video BIOS revisions must also be identical. Note that if the cards run at different clock speeds, the driver will run both cards at the lower clock speed of the two.  NVIDIA has announced their SLI certification program, which means that two SLI certified cards should have no problems working in tandem. Currently only NVIDIA cards will work in SLI mode although ATI plans on introducing SLI technology in 2005.
• A power supply capable of supplying adequate power to the system as well as both graphics cards.  Note: you may need one or two 2 x 4-pin to 1 x 6-pin PCI Express power adapters if you are using two 6800GT or 6800 Ultra graphics card with a power supply that either has no or only one 6-pin PCI Express power connector.
• A SLI video bridge connector.  This connector should be provided with your nForce4 SLI motherboard. 
• NVIDIA drivers with SLI support.  Currently the 66.93s are the only NVIDIA sanctioned drivers with SLI support, however NVIDIA is working on rolling in SLI support to all of their drivers, including the newly released 67.02 driver. 

It’s no big surprise that you can’t use different, GPUs; in our tests we tried combining a 6800 Ultra with a 6600GT, but NVIDIA’s driver wouldn’t even let us enable SLI on the combination.  When we tried to combine two different 6600GTs (non SLI certified) we could enable SLI through the driver, but there were tons of stability problems.  Accessing the NVIDIA Control Panel would cause the system to lock up, presumably because the control panel had issues reading from two different video BIOSes.  If we didn’t bother with the NVIDIA Control Panel and just tried to run a game we were met with video corruption issues and lockups.  Right now it seems like the only option for SLI is to have two identical cards; in theory they can be from different manufacturers as long as the video BIOSes and all of the hardware specifications are identical.  In order to make upgrading easier, NVIDIA introduced their SLI certification program which is designed to ensure compatibility between all identical-GPU cards going forward.  Only time will tell whether or not this actually pans out to make upgrading to a SLI configuration easy.

One thing to make sure you have are sufficient power connectors coming off of your power supply.  If you are using two 6600GTs then it’s not a big deal, since the cards themselves don’t require any external power.  However, with two 6800GTs, each card is outfitted with a 6-pin PCI Express power connector, which must be used for proper/stable SLI operation.  Since most power supplies only include one (or no) PCI Express power connectors, chances are that you’ll have to use a 4-pin molex to 6-pin PCI Express power adapter, which takes two regular 4-pin power connectors and combines them into a single 6-pin PCI Express connector.  You should, in theory, use two separate power cables with the adapter (in order to avoid pulling too much current off of a single cable and violating the ATX spec) but in practice we had no issues with using two connectors off of a single cable to power one of the graphics cards.  If you have no PCI Express power connectors on your power supply then you’d need four separate power connectors just to power your graphics cards, add another one for ASUS EZ-PLUG and then you can start thinking about powering up things like your hard drive and DVD drive.  While purchasing a SLI motherboard will pave a nice upgrade path for you in the future, you may need to enable that future by upgrading your power supply as well.