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ATI CrossFire
By Vijay Anand
Category : Graphics
Published by Jimmy Tang on Monday, 26th September, 2005

How Does CrossFire Work?

Unlike NVIDIA's multi-GPU technology that levies some strict restrictions, such as requiring identical SLI capable graphics cards, ATI's CrossFire multi-VPU technology gives adopters a certain amount of flexibility to pair cards from different vendors and models. Since there are several concoctions to enable CrossFire, this graphics horsepower-doubling feat is achieved with a master and slave configuration. How they combine the output to form the final output is akin to what we've seen from NVIDIA, but the actual means of achieving it is different.

Actual raw data such as textures, geometry data and everything else is delivered to the cards solely using the raw bandwidth of the PCI Express bus while the partially rendered output of the slave card is transferred out via the DVI output and is sent to the master card's DMS-59 (a type of high density digital connector). The master card has a compositing engine that combines the slave's output along with its own rendered portion to form the final display that's also sent out via the DMS-59 connector. Of course this requires a special CrossFire Interconnect cable, which is basically a dongle sort of cable with a DMS-59 connector on one end and dual DVI connectors on the other end (one of which hooks up to the slave card and the other is for the actual monitor display).

Here's an ATI CrossFire schematic diagram - picture courtesy of ATI.

The CrossFire Interconnect cable dongle. Of the two DVI connectors on the left, one connects to the slave card and the other to your monitor.

Here's a close-up of the DMS-59 connector. The receptacle is exactly that same dimensions as the standard DVI connector, but the pin density is double that with a total of 59 pins to facilitate DVI input and output on the same connector.

Some of you might shudder at the thought of quality loss due to external cables involved but it's nothing like that of the 3dfx Voodoo days that used analog outputs. This is because the signals carried in the CrossFire Interconnect cable are in digital form, from end to end, just like the receivers and transmitters on the cards themselves. True to the fact, we noticed no loss in quality from our actual tests. Since the master card will need to receive data from the slave card, it also has a DVI receiver chip in addition to the customary DVI transmitter chip. Both of these DVI components interface via the master card's DMS-59 connector that basically has half the pins dedicated for DVI input and output respectively.

Now that you know the gist of how it works, we detail next on the three different rendering techniques that CrossFire employs to speed up games, which is directly related to how the master card composites the slave card's output with its own.

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