I just installed a new ASUS Radeon HD 6870 card and the card has only two DVI slots. A VGA to DVI adapter came with it as well, and I have been trying for about five hours to get it to work correctly. When I plug my VGA cable with the adapter on it into the card, my monitor instantly tells me "Entering Sleep Mode" and refuses to wake up. To check if this was my monitor or not I have also tried this on a second monitor and with two different VGA cables, all to no avail. I disabled the Integrated graphics in Windows Device Manager and went into the BIOS to set the priority as PCIe -> PCI -> IGP, which still has not helped. I don't know what to do now and I'm really frustrated. Any help, please?
Which DVI port are you plugging the adapter into? For most 6870s you have to use the DVI port closest to the top of the card, as the second, lower DVI port is digital only, and won't support a VGA analog signal. Are you using a CRT monitor? If not, go look for a DVI cable for your LCD and try that. If it doesn't work with the DVI cable, then you probably have a bad card.
I am running Crossfire Sapphire 6870's with 2x24" asus LED monitors connected. Running dual DVI connection. The second port the one on the bottom the black port, When the computer was shutting monitors off to save power the secondary DVI connection was getting a DisplayPort link error, and therefor was not turning on just staying in sleep. I have a 750 watt PSU but still wasn't doing the job. Well it turns out you need to use the PCI-E power pipes on your PSU. I happen to have 2 dedicated pipes with 2- 6+2 pin PCI-E connectors on them. Low and behold when changing over to those instead of running 4pin to 6 pin PCI-E adapters that came with the card the problem was fixed.
So in short it was a power issue since the second port gets secondary power. Run enough power to your card and the issue should go away.