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7950 Question

Last response: in Graphics & Displays
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October 9, 2012 9:37:07 PM

Im running two 7950s in crossfire, They both require 8Pin adaptors, Im running one adaptor and just have a 6 Pin in the second card, The cards fans are spinning up but its not showing i have a second card or even to activate crossfirex, I did order another 8Pin adaptor to 6, i know on motherboards when you have the 6 or 8 pin at the top you can get away with just running a 4 into the board. The same theory doesnt work for Graphics cards? I have to have the actual 8 Pin going into the card for it to actually register on the PC? Also the 6 to 8 Pin adaptor i have is actually 7 Pin, one pin is a blank

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a b U Graphics card
October 9, 2012 9:50:42 PM

what kind of PSU do you have first of all?
October 9, 2012 9:54:08 PM

850W Roswell Gold.
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a b U Graphics card
October 9, 2012 10:11:23 PM

Most 7950 cards requires 2+6 pin connectors for each, though some may have an 8+6. Which models do you own? You make it sound that each one only requires an 8 pin which isnt right.

Anyways you need to have a 6 pin power connected to each card as well as an 8 pin so if your power supply doesnt have an 8 pin power connector you must use an 6 to 8 pins adapter if you want your cards to work.
a c 505 U Graphics card
October 9, 2012 10:43:28 PM

For a system using two Radeon HD 7950 graphics cards in 2-way CrossFireX mode AMD specifies a minimum of an 700 Watt or greater power supply. The power supply should also have a combined +12 Volt continuous current rating of 45 Amps or greater and have at least four 6-pin PCI Express supplementary power connectors. There are some AMD partners that have Radeon HD 7950 versions that use one 6-pin and one 8-pin or some that even use two 8-pin PCI Express supplementary power connectors.

The only Rosewill 850 Watt power supplies that I know of are the two discontinued models (i.e. RX850-S-B and RX850-D-B) in the Xtreme Series. Both of those power supplies had three 6-pin and one (6+2)-pin PCI Express supplementary power connectors. Both of them also have a combined +12 Volt continuous current rating of 64 Amps.

Quote:
... The same theory doesnt work for Graphics cards? I have to have the actual 8 Pin going into the card for it to actually register on the PC? Also the 6 to 8 Pin adaptor i have is actually 7 Pin, one pin is a blank
The same theory doesn't work for graphics cards. The 8-pin connector is required because one of those extra pins (Pin 4) in the +2 portion of the connector is the Sense 1 pin that indicates to the graphics card that it is allowed to draw 150 Watts from the (6+2)-pin PCI Express supplementary power connector from the power supply unit. If Pin 4 isn't connected (i.e. left Open) (e.g. if you were to just plug in a 6-pin PCIe connector from the power supply unit), then the graphics card is only allowed to draw a maximum of 75 Watts from that PCIe power connector. That is insufficient power for the graphics card to run so the card is essentially disabled.
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