Didn't think I'd see SLI again, but today has been full of surprises. For those who don't know/remember SLI was created a long time ago by 3dfx. This technology works by using multiple graphics processing units (GPUs) in a single system. This technology was first showcased with 3dfx's Voodoo2 video cards (still have a Voodoo2 card).
SLI (Scan Line Interleaving) created by 3dfx six years ago was all about pure performance and higher resolutions. Back in those days whoever had the fastest graphics solution won. The idea was that if you could add multiple 3D accelerators in one system you could speed up 3D rendering on that PC. 3dfx conquered this challenge by creating SLI or Scan Line Interleaving technology. SLI technology from 3dfx worked by having one card render the even number of lines on the screen while the other card worked on the odd number of lines. This basically "doubled" frame rate performance and allowed resolutions up to 1024x768.
This SLI technology was introduced on their Voodoo2 line of video cards. The Voodoo2 was a 3D only accelerator and required that a 2D video card already be installed in the system. With SLI technology not only could you add one Voodoo2 3D accelerator, but you could add two to your system. You connected the two cards via an internal ribbon cable as well as an external pass-through cable. Once the drivers were installed it did everything "automagically". All you had to do was sit back and enjoy much faster frame rates in your games.
News source: [H]ard|OCP