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General Dongle Education

Part 2: AMD's Eyefinity Technology Explained
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As we mentioned in the prior Eyefinity story, there are two kinds of dongles for Eyefinity: passive and active. As with any other kind of dongle for your PC, these adapters convert one type of connection into another. Conventional video port types are based on Transition Minimized Differential Signaling (TMDS) and use either DVI or HDMI interfaces. TMDS is substantially different than DisplayPort. For example, TMDS uses raster scanning, whereas DisplayPort is packetized. The protocols are different. TMDS typically runs at about 5V while DisplayPort is only 3.3V.

Passive DisplayPort-to-DVIPassive DisplayPort-to-DVI

Passive dongles drive non-DisplayPort signals through the DisplayPort connectors by shifting signals from one format to the other. (This is why you might sometimes hear dongles referred to as level shifters.) The graphics card is able to sense that a passive dongle is attached to the DP connector. At that point, instead of passing a 3.3V DisplayPort stream, the card outputs a 3.3V TMDS signal through that port and the passive dongle shifts the voltage level up to meet the TMDS spec.

Active dongles are composed of a DisplayPort receiver, which attaches to the graphics card, and a TMDS transmitter, which integrates a digital-to-analog converter (DAC) for VGA output. That’s really the main difference between the two types. Power is often supplied by a cable that connects to a USB port. With an active dongle, the adapter looks like a DisplayPort to the graphics card, so the card transmits DisplayPort signals natively. In the passive case, the card outputs TMDS for HDMI or DVI displays.

Active DisplayPort-to-DVI dongleActive DisplayPort-to-DVI dongle

Is Your Dongle Flashing?

The critical thing to know about Eyefinity and dongles is that there’s a hard limit of two TMDS output streams, period. So, if you want to use Eyefinity to set up a 2x1 “array” (yes, dual-monitor Eyefinity sounds a bit silly, but that’s how the driver sees it), it doesn’t matter what you throw at the card. Two VGA screens? No problem. You could use a VGA adapter on a DVI port and an active VGA dongle on a DisplayPort connection. Just keep your legacy output stream count in mind as you scale beyond two monitors.

“If you’re already using two DVI connectors on the board, you can’t use a passive dongle because, in theory, that would be a third TMDS signal stream,” says Roger Quero, technical manager at AMD’s GPU Technologies unit. “You can have two passive dongles, and the rest have to be active. Like, if you’re thinking about the six-output card, that’s six mini DisplayPorts. Two of those connections could be passive, putting out TMDS over those ports, then the rest have to be active so that we think it’s a DisplayPort monitor.”

There’s a catch with active adapters, though. A lot of early buzz flew around Web forums, noting that some active dongle-attached displays were “blinking.” There are only a handful of potential causes for this. One is that you’ve simply got a dud dongle. A more common cause is that the dongle isn’t getting enough power through the USB connection, specifically 5V at 500mA. This could be the result of improper wiring, bad power management, a low-power USB setting in the BIOS, not having external power to a USB hub, and similar factors. The easy test is to switch to a known high power USB port or powered hub. If that fixes the problem, you have your culprit.

The third possible cause pertains to dongle quality. As with so many other components in the PC space, there are reputable brands that pride themselves on quality and no-name brands that only seek to sell high volumes at rock bottom prices, regardless of whether the product works as it should or not.

Passive DisplayPort-to-VGA dongle.Passive DisplayPort-to-VGA dongle.

“With inferior dongles,” says AMD’s Roger Quero, “you could end up with your panels flashing or flickering. It might appear to be working fine, but then you can’t set up an Eyefinity large surface. You could run into problems with EMI emissions. Some are not FCC-certified. It comes down to if you’re buying from a reputable vendor versus from Bob’s Dongles and Tackle Shop.”

“This is why we operate a dongle validation program, frankly, to help separate the wheat from the chaff,” adds Bruce Gasson, AMD senior technical marketing specialist. “There are a lot of dongles out there and more arriving every day. Some do not work. That’s a fact. That’s one of the reasons for launching this program in the first place. We want to make sure that the dongles we promote are the ones that work.”

You can see the lengthening list of approved dongles here.

Currently, you’ll find  active adapters available in the market for DisplayPort to VGA and DisplayPort to Dual-Link DVI. You’ll also find passive dongles for DisplayPort to HDMI and DisplayPort to Single-Link DVI. Keep watching, though, and you should soon see a DisplayPort to Single-Link DVI active dongle appear. With this, Eyefinity will be able to support three Single-Link DVI monitors, side-stepping the present two-TDMS-connections-only rule.

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