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Dangling Issues About Dongles

Part 2: AMD's Eyefinity Technology Explained

This first question that might spring to mind as you look over AMD’s dongle chart is why different dongles support different resolutions. According to AMD’s Shane Parfitt, most DACs within passive dongles can only support up to 1920x1200. In looking at VGA monitors, this is fine because most VGA monitors stay at this resolution or lower. True, there are some 2048x1536 CRTs in circulation, and there’s no hard limit on VGA resolution, but there’s no point in designing a dongle product for only a handful of users. People with 30" LCDs capable of 2560x1600 are a lot more likely to use the monitor’s DVI or HDMI port rather than its VGA connection.

If it’s 2560x1600 that you want, then it’s active Dual-Link DVI output you need. Single-Link passive dongles will only get you to 1920x1200. The wiring scheme actually determines a large part of why passive dongles can’t reach the top display resolutions.

Active DisplayPort-to-DVI dongle.Active DisplayPort-to-DVI dongle.

“If you look at the passive dongles,” explains Parfitt, “you’ll notice that we’re using the same output pins as on the DisplayPort connector. Manufacturers actually map that to a TMDS output pin configuration. So right now, TMDS is RGB. That’s four pins, and DisplayPort also has four—the four main links. Yet DisplayPort carries two times the bandwidth of DVI. If you look at the Dual-Link DVI connector, that can support 25x16, but that requires twice as many pins and cables as Single-Link DVI and as DisplayPort.”

AMD splits its dongle validation program into two tiers: Validated and Ready. The company created a kit to send to dongle vendors, enabling them to do much preliminary testing themselves. The kit helps them assess EMI output levels and perform quality signal testing, making sure the dongle correctly passes signals and doesn’t block any data. If the dongle survives the kit’s testing, then the unit is classified as Ready. The vendor can then submit the dongle to AMD, which will then test the device according to Validated criteria. This level is more focused on Eyefinity scenarios and real-world testing. Does it work properly with games? Does it show all expected resolutions? Will it work in three- and six-panel output situations? If so, then it gets the Validated seal of approval.

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