Help Mining with 2x 7990 is the same hastrate as mining with one!

[user0244]

Hi,
Been mining over the past few days with one 7990 card
I was getting 1.3Mh/s without breaking sweat
So i buy another 7990 hoping to get 2.6Mh/s however when i fire up both cards
The overall hashrate is 1.4Mhz!

What happens is i start card 1 and it hashes at approx 670, so i start "side b" of the
card and that hashes at approx 670. So i start card 2 and again that hashes at approx 670
So with 3/4 running my hashrate is approx 2.0Mh/s So i start up card 2 "side b" and boom
the hashrate across all the cards drops. Some only hash at about 200. As i say
overall the hashrate is 1.4Mh/s

Anyone got any ideas whey this happens. It reminds me back in April when i had 3 x7950 and same thing happened

Has anyone got 2x 7990 hashes at 2.6 in the same PC

Thanks

[EntropyExtropy]

PCIe bottleneck?

[hendo420]

What power supply do you have?

If memory serves me right I believe the 6990 is 2x 6970 cores. a 6970 at stock voltage/clocks runs about 250-300w. This means that a 6990 should run around 500-600w and 2 of them would be upward of 1200w not including the main system. Maybe you don't have enough umph in your power supply to run both cards at 100%

[user0244]

I have 1000watt PSU. The system draws about 950w from the wall
I do have another PSU, can i just attached this PSU to the other card?

Thanks for the replies!

[OGskunk]

yes you can also what's the command line you are using for cgminer?

[user0244]

I am using GuiMiner with the standard setting

I tried attaching another psu to the other card, but the card doesn't switch on!
Don't know what i am doing wrong

Cheers

[OGskunk]

I am using GuiMiner with the standard setting

I tried attaching another psu to the other card, but the card doesn't switch on!
Don't know what i am doing wrong

Cheers

Is your PSU plugged into the wall and also turned on?

1000W PSU should handle 2x 7990 no problem, I have 3 7950 running on a 1000W and no problems at all

[user0244]

Yes the PSU is switched on
As i say if i attach both cards to the 1000w PSU both cards power up and spin
But if i detach one of the cards from the 1000w and attach it to my 850w PSU
The card attached to the 850w does not turn on!

Cheers

[hendo420]

Yes the PSU is switched on
As i say if i attach both cards to the 1000w PSU both cards power up and spin
But if i detach one of the cards from the 1000w and attach it to my 850w PSU
The card attached to the 850w does not turn on!

Cheers

You have to jumper the 850w psu to get it to turn on.
http://www.youtube.com/watch?v=iVt3nYv4zlU