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This idea is similar to the setup I have now, with a PC hooked to a 24" Apple Cinema Display. I'm using a Sapphire Radeon 4890, which has Display Port, and I'm using the Circuit Assembly cable to adapt Mini DP to DP. Unfortunately, I was unable to use a XFX Radeon 5850. It just wouldn't work, and XFX and ATI were unable to help me. I've also heard from others using an Asus 5850 and XFX 5870 and neither of them were able to use the ACD on them either. Perhaps it would work on the iMac, but it doesn't look good right now for the 5800 series and the 24" Apple Cinema Display.

There are some powered converters but none of them say they'll support a resolution higher than 1920x1200.
 
If your ATI card has a Mini-Display Port output, then you are good. If it has ANY OTHER TYPE OF OUTPUT, it won't work, and NO COMBINATION OF ADAPTERS will work.
 
If your ATI card has a Mini-Display Port output, then you are good. If it has ANY OTHER TYPE OF OUTPUT, it won't work, and NO COMBINATION OF ADAPTERS will work.

Huh? That's a little misleading.

Any PC video card with a full size DisplayPort output or a mini DisplayPort output SHOULD work with the 27" iMac's DisplayPort input, but for some reason the ATI Radeon 5000 does NOT work.

The only video card that has been confirmed to work so far is the ATI Radeon 4890 (with a DP/mDP adapter cable).
 
The only video card that has been confirmed to work so far is the ATI Radeon 4890 (with a DP/mDP adapter cable).

Correction

All 3000 & 4000 series with DP work
as does 5900 series with Dp work

all other 5000 do NOt work but for the new 5870 Eyefinity 6
http://www.gigabyte.com.tw/Products/VGA/Products_Overview.aspx?ProductID=3403
or the Asus Eyefinity 6 would work too

Maybe the below 5770 Eyefinity 5 card will also work when it actually ships:
http://www.powercolor.com/au/products_features.asp?id=240#Specification

these all are latest generation with native multiple miniDP ports
 
Any update on this issue? Has ATI fixed isuues from many of its 5XXX series cards not working with Displayport on 27"IMAC?
 
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