Yesterday evening, my Radeon HD 5870 refused to light up either screen (one 30" display and one 27" display). I thought the card had failed outright, and had to swap video cards, then swap back. The same video card is now working flawlessly again. The problem is, it’s flaky. And not just one Mac Pro. And not just one video card.
Plugging/unplugging and swapping cards leads to one inescapable conclusion, especially since the 5770 card in my other Mac Pro has also shown some flakiness problems: there is something wrong with the 2010 Mac Pro video cards that makes them just not work at times, with no apparent rhyme or reasons. I had better luck with one screen than two, but there is no apparent hard and fast rule.
Rob-Art over at barefeats.com had reported this to me some weeks ago, but I figured it was a cable problem, since I saw it only sporadically with my two systems, and swapping cables appeared to fix the problem. But that was apparently just dumb luck.
In short, there is something broken with the Radeon HD 5700 and 5870. Having torn apart my machine yesterday, and seeing no problems with the old NVidia GT120 card, I sure hope Apple gets its act together on this problem.
Letting the screen sleep in sleep mode also shows a failure to wake up the 2nd screen at times. Dual-display systems seem particularly affected. A trend in motion stays in motion: when working, the cards seem to keep working, but once they flake out, it can be a frustrating experience trying to get the card to light up the displays again.
A client of mine reports a similar problem with her hexacore Mac Pro today.
Apple Tech Note
Apple does not address this flaky behavior in Tech Note HT4279, but does state that if three displays are used, then dual-link adapters must be used for DVI display. I’m using one DVI port with a dual-link DVI display, and one DisplayPort—no adapters needed.