I have a P7P55D-E Premium Motherboard. It supports Crossfire and SLI at 8x8.
I have 2 gtx 470's (currently only 1 installed)
I tried 3 different bridges.
The power supply I have is a brand new 1000w
it cost like $160 and it says it can handle 3 way SLI
Most people that have the same problem as me it seems like
people would tell them it must be a power issue. I even setup another
750 power supply on the side to power just 1 of the video cards. You can
hear windows boot up but no video comes out. I tried 3 different bridges
already. I updated the bios to the most recent. There is nothing in the
bios that allows me to change settings on the pci express or sli.
I used both of the cards in a single setup so both the cards work.
I also got it to boot with a single card in each of the pci-express sockets.
So the cards and the pci slots are fine. Something is holding the cards back
from working and im so lost on what to do. To make things even worse
the chances of someone reading this post and helping is very slim.
Then the chances of someone helping and finding the answer are even slimmer.
With one of your cards installed, remove the drivers completely,here is how
1_Download driver sweeper(from guru3d.com)
2_Go to control panel and uninstall the current driver from there
3_Reboot and go to safemode and open driver sweeper and click on Nvidia,then choose clean
4_Boot normally and Install the new driver
Then install the second card, and check if it boots fine or not(if it did,then make sure you download the latest VGA drivers)