At the time Apple published the guidelines for the 5770, they had decided that they only needed to supply the Dual-Link Active adapter, since it worked in every situation, even if it was overkill. The marketing department later decided it was a US$100 device, which should have changed the calculus, but since this Apple, it did not.
In the case of the new Cylinder Mac Pro, we may be seeing another case of Apple recommending only the straightforward solutions that work "first time every time". There may be more subtle arrangements with many more footnotes that Readers will discover over time.
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When card-makers invented DisplayPort (and its mini variant) it had several advantages:
• The constant re-draw of the screen, every 60 to 100 times a second, disappears. Instead, the screen is re-drawn only when it changes. This dramatically reduces traffic on the cable most of the time. But it requires a display with a screen buffer.
• the higher Voltages used for DVI could be replaced with lower Voltages and still work fine.
• When required, "compatibility mode" or "Dual Mode" (to DVI) could be invoked by asserting a signal back toward the computer. Then the port would re-time to DVI compatibility mode, and put out the signals needed (close to, but not quite at the higher DVI levels). A power pin (supplied at the port) could be used by an Active Single-Link adapter to boost the signal levels up for ONE Single-link adapter, and maybe the second one on a card in a pinch.
Dual-Link adapters required three more balanced Drivers each, and the power supplied at the port was just not enough. So the adapter uses a USB pigtail to pick up still more power to run more signal boosters.
-- Apple is saying, "Don't run more than two [DVI and HDMI combined] on a Mac Pro cylinder".
It is not clear whether they are saying this because it will not work, or because of the support firestorm it created for them the last time (since it becomes so complicated).
Those interested may want to read the wikipedia article on DisplayPort, which discusses the History. DVI compatibility is covered under the heading "Dual Mode":
http://en.wikipedia.org/wiki/DisplayPort
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