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Author Topic: 4x Radeon HD 6990  (Read 9426 times)
mrb
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May 19, 2011, 04:22:26 PM
 #21

Yes, they do indeed draw power from the PCIe connector, even though they have additional power connectors.

He is referring to the ATX power spec.  12v power for PCIe slots is provided by only two pins in the ATX power connector, rated for 6 amps each.  He was pulling 7.4 amps over each pin, which was melting the shroud and likely caused permanent damage to his motherboard.  It probably would have started a fire eventually if he hadn't been actively watching it.

His solution was to bypass the ATX plug (and motherboard) entirely by spicing his PCIe extenders to accept power directly from the power supply.

I understand what he did.  What I don't understand is why most people are NOT seeing such issues.

Because few people are running 4x5970 or 4x6990 with the PCIe slots power entirely coming from the 24-pin ATX connector. I spliced my PCIe extenders. ArtForz is doing something similar. foxmulder has extra power connectors on his SR-2. Etc.

You have to understand that specifically the 5970 and 6990 are the cards with the highest current consumption on the slot (I measured 4.1-4.3A at 12V). Most other cards have 2/3rd or less the consumption (eg. the 5870 only draws 3.2A at 12V from the slot).

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minerX
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May 19, 2011, 08:44:32 PM
 #22

will work only on linux as windows has 4 gpu cores limit
basically no different than http://blog.zorinaq.com/?e=42

This is not true.  There is a 4 GPU limit for Crossfire/SLI only.  I was running 5 GPUs last week.  You are essentially limited by your motherboard slots.  Plus power, room, heat, etc.
Inaba
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May 20, 2011, 05:11:12 AM
 #23

Ok, there's some misinformation in this thread Smiley

First off, HD's only draw between 4 to 5 watts.  Raptors and SCSI drives might draw a bit more, but it's not even 10w.  Moving to an SSD will save only a few watts and is completely ridiculous in this application.  If you're going to do that, just make a bootable USB key and boot into Linux that way.

Second people aren't seeing the issue because most boards that designed for 2x SLI, 3x SLI, 4x SLI or Cross/tri/quadfire area designed with the idea in mind that current draw on the board is going to be substantial, so they are beefed up. Usually, (but not always), anyone running such a beefy system will also have a beefy power supply with large gauge wires to handle the current draw.  In the cases we are seeing here, people are using commodity boards and cheap PSUs, which are explicitly NOT designed to handle these kinds of extreme loads... but since this is a fringe area of computing, the problem doesn't crop up that often.

Most low and mid-level boards are going to sag power wise with even 3 cards in the slots.  You can get away with probably 5870's, but if you add a 5970 or two to the board and expect it to run, you're going to have a surprise or flaky operation.  I know this from experience.  Using a high quality gamer board will alleviate this, but has a cost involved.  To solve this, splicing off and using MOLEX reduces or eliminates the draw on the board.  One of the problems I have seen, or at least this is my speculation, is that even if the board works for awhile with 3 5970's in it drawing off the MB, the VRMs will start to heat up and eventually fail if that much current is being drawn continuously.  Again, taking that power draw off the MB and putting it on the PSU, which is designed to handle far more load than 75W @ 4A+ is going to eliminate those problems, extend the life of your board and be far more stable.

The problem I'm trying to work through now is whether or not a common ground is needed and if so, the best way to handle grounding two PSU's to a common ground if you wanted to use two cheaper PSUs to power a mining rig. 

If you're searching these lines for a point, you've probably missed it.  There was never anything there in the first place.
AjaxOfSalamis
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May 20, 2011, 05:37:35 AM
 #24

The problem I'm trying to work through now is whether or not a common ground is needed and if so, the best way to handle grounding two PSU's to a common ground if you wanted to use two cheaper PSUs to power a mining rig. 

http://www.frozencpu.com/products/11742/cpa-167a/ModRight_CableRight_Dual_Power_Supply_Adapter_Cable.html?tl=g2c413s1220&id=qNzMZhNn
SchizophrenicX
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May 20, 2011, 05:38:51 AM
 #25

Hi very insightful stuff written here. I'm building my first rig and this is what I have. I do not have prior experience building computers so could you guys give me some advice?

GPU: 2x Sapphire ATI HD 5970 2 GB
MOBO: ASRock M3A770DE ($60)
CPU: Sempron 140 ($39)
PSU: Durl=http://www.newegg.com/Product/Product.aspx?Item=N82E16817182072]PSU[/url] ($100)
RAM: Kingston 1 GB DDR3 1066
HDD: None/USB ($0)
CASE: None/Cheapassjunk (~$20)
OS: Linux Ubuntu ($0)

$982 for ~1.3 Ghash/s [1.324 Mhash/$]
Not sure about the power consumption but it cost $0.20/kWh

Will I get the problem you described? Should I get a Gold 80+ PSU and/or a better MOBO? Any recommendations on how I can further drive the cost down or any thing that I'm missing on (compatibility or whatever)?

Basiley
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May 20, 2011, 05:41:50 AM
 #26

A PCI-E slot provides up to 75W, each 8-pin connector up to 150W, amounting to 375W for a card. This is one of the reasons why GPUs of 6990 have to be underclocked (830MHz instead of 880MHz in 6970 and 6950) for stable performance.
newer ATX 2.3.1 PSU's had quite marginal 3.3V capacity[listed usually only peak and very optimistic] and 5970/6990 need about 5.2A per each card(?).
and yes, on cheap motherboard you can blow circuity, but not because over-consuming GPU, but inadequate/weak power circuitry on motherboard.
Inaba
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May 20, 2011, 06:22:40 AM
 #27

The problem I'm trying to work through now is whether or not a common ground is needed and if so, the best way to handle grounding two PSU's to a common ground if you wanted to use two cheaper PSUs to power a mining rig. 

http://www.frozencpu.com/products/11742/cpa-167a/ModRight_CableRight_Dual_Power_Supply_Adapter_Cable.html?tl=g2c413s1220&id=qNzMZhNn

This doesn't address the issue we are talking about here. That cable is used to power peripherals, not something directly attached to the motherboard, or at least I don't think it should be.  Using that to power an HD array or other power hungry devices that are NOT connected to the motherboards ground is perfectly fine... but when you try to connect two power supplies with two different ground potentials to the through the same card, you can have some serious consequences, up to and including a burnt out card.

See the other thread where we've been discussing this problem (The PCIe extender sale thread) for more information.

If you're searching these lines for a point, you've probably missed it.  There was never anything there in the first place.
SchizophrenicX
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May 23, 2011, 02:15:02 AM
 #28

Hi tread starter. I hope you don't mind me linking my tread to yours. I'm consolidating build discussions Smiley

bulanula
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May 23, 2011, 05:31:38 PM
 #29

Has anyone actually done this and can you confirm if it is plausible Huh
Tamerz
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May 23, 2011, 05:37:00 PM
 #30

BTW, EVGA makes an accessory called Power Boost. It just takes a 4 pin Molex and feeds it into an empty PCIe slot. Takes some of the load off the 24 pin ATX connection.



http://www.evga.com/products/moreInfo.asp?pn=100-MB-PB01-BR&family=Accessories%20-%20Hardware&sw=4
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