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Why do dual GPU cards only need 2 cables?

Adrianr

Adrianr

Adrianr

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How can a 4870X2/3870X2 (And presumably the 260GX2 if it ever decides to exist) be powered off one 6 pin and one 8 pin PCI-E power cable, when to run two of the single GPU cards in Crossfire normally requires at least 4 x 6pin PCI-E power cables, 2 per card..

Are single cards being drastically over compensated? Or are dual GPU cards thrashing the 2 power cables? Or both?

Anyone care to explain? :)
 
an interesting question..
something to do with efficiency of having both gpu's sharing the same power controllers perhaps?
 
Probably because one card needs more juice than 1 connector can reliably provide but nothing like the power potential from having both connectors - whereas the X2/GX2 cards use much more of the power potential from the 2nd cable and are probably closer to the max from 2 cables.

As a general rule I think 6pin connection on average can supply about 150watt and most high end cards spun up use 140+.
 
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But 8pins can provide up to 300w's of power which is more than the 4870 needs, so why not 1x8pin on the 4870? Thats what i think the OP is trying to ask.
 
Good question, an x2 can draw a total of 225 watts through the connectors (6+8 pin) while a single 4870 uses 150 watts (6+6 pin).
I guess the 4870 isn't using the full amount, someone will have a better answer!

Edit:
To the post above, not many psu's have an 8 pin connector (150 watt max) more have two 6 pins (75 watt max) I presume this is why most have the two 6 pins rather than a single 8 pin
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CALIFORNIA MEDICAL MARIJUANA
 
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Don't forget the slot contributes 75W (Pci-e 1.1).

The 4870X2 to need 6+8pin puts the potential supply over 225w, but under 300w max.
The 4870 to need 2x6pin puts the potential supply over 150w, but under 225w max.

Iirc TDP's are 4870=160w and the X2=230w, so real world a bit less at stock.
 
The power needed can be supplied down the 2 cables and the two cards are interconnected so no need for an extra pair of cables.
 
Very Simple, 2 Cards in Crossfire do not run at 100% on each Card/core at the same time, they will use about 160% of a single card.
Just as well really as they don't give 200% of the performance :)
 
JAKUS said:
Very Simple, 2 Cards in Crossfire do not run at 100% on each Card/core at the same time, they will use about 160% of a single card.
Just as well really as they don't give 200% of the performance :)
Click to expand...

That's more bottlenecks and overheads than much else, I can get each X2 core to 100% load no problem. And even less demanding applications (games), the loading is the same on both cores. More the exception than a rule, but on the odd result 100% scaling is not impossible either.


 
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Yeah they May show 100% with the sample rate on GPUZ but they are Rendering Alternate Frames so not individually running 100%, 100% of the time
 
If it doubles the frame rate, the net load is still double. The VDDC core currents are the same in single mode as double, its not 20% out. In AFR each core dumps the frame buffer as soon as it can, if its waiting sure the net load would decrease. Crossfire power consumption is less proportional with each successive card, 3rd and 4th cards can't be doing much, always seemed like a waste to me.
 
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JAKUS said:
Yeah they May show 100% with the sample rate on GPUZ but they are Rendering Alternate Frames so not individually running 100%, 100% of the time
Click to expand...

Same problem applies to both situations though.. How much each core gets used isn't a power problem, it's a game coding problem. The 4870X2 running at absolute FULL whack would not exceed the power draw given by 1 x 6pin and 1 x 8pin, or the manufacturer would put more inputs on it, likewise 2 x 4870s in Crossfire running full chat wouldn't exceed the power provided by 4 x 6 pins.

If 75W is provided by the PCIE slot, 75W per 6pin and 150W per 8 pin, surely no single card should need more than a single 8 pin? Except maybe a GTX280, which i'm sure has a 6pin+8pin for safety sake, as the 260 obviously gets by fine on 2 x 6pin.

I presume the only explanation for this is as mentioned a few posts above, it's easier to put on 2 x 6 pins rather than 1 8pin, as more power supplies are likely to have that configuration. And it costs more to add on a single 8pin extra if you ever go Crossfire/SLI and want to just use 2 cables. Maybe there could be some sort of splitter device that, akin to an SLI/Crossfire bridge, links the 4 6pin connectors together and provides a single 6 pin + 8pin, for the cable tidy freak in us all.

I think my curiosity is quelled, thanks folks :)
 
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Well your curiosity shouldn't be quelled Adrianr, because thats not the main reason :p

While a 4870 and others could be powered by a single 8pin PCI-E connector, the reason two 6pins are used is because it provides more accurate consistent power. Theres a lot of cheap PSU's that spike up and down on the output rails (the more powerful the card, the more variation possible) or cant quite produce as much power as advertised because of poor efficiency, especially under the stress of high end cards - This can reduce the life of the card and cause instability/crashing, so to help out they use 2 connectors. Even on a high-end PSU it's still going to be more stable to use 2.
And even cards that could technically be powered by one 6pin, but approach it's power limits, will also use two 6pin.

This is the reason a lot of high-end cards have two connectors, even when one 6pin/8pin would technically be enough.
 
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We did miss the more obvious point about all this.....whether the Cards require the potential power available or not by adding multiple 6pin and 8pin connectors, does encourage people to use a more suitable power supply, thus reducing the headaches for the manufacturers
 
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MR.B said:
Well your curiosity shouldn't be quelled Adrianr, because thats not the main reason :p



This is the reason a lot of high-end cards have two connectors, even when one 8pin would technically be enough.
Click to expand...

That's not quite true. 75W 6pin divided by 12V = 6.25 A, split over 3 wires is 2.08 A per wire/pin..I don't think that's really pushing the wire/connector, More likely the PCB tracks
I still feel they do it for the reasons in My above post also 2 connectors could help with any shortfall if the PSU has separate 12V rails
 
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