Which second GPU for a G5 Quad?

Discussion in 'PowerPC Macs' started by tevion5, Jul 17, 2015.

  1. tevion5 macrumors 68000

    tevion5

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    Ireland
    #1
    It's a great problem to have, but I need help deciding whether to put my 6600LE or 7800GT in slot 3 of my new Quad.

    While I've heard extra GPU's simply act as extra out ports for your main GPU, I don't believe that's the whole story. This is because when I used a 7800GT in slot 1 and a 6600LE in slot 3 on my DC 2.0, monitors plugged into the 6600LE had much choppier HD video playback. I would merely drag the browser from one monitor to the other, and the difference was very noticeable. Night and day.

    So I suspect this would not be the case with a second 7800GT. However, this card requires PCIE power, and the G5 only has one cable. I have ordered a splitter, but would this perhaps put too much strain on the PSU? (These cards are so old though, I doubt they use that much power surely?)

    This is now a silly question, but while I know the SLI ports on the 7800GT are useless in a G5, would there be any harm in bridging them with a spare SLI cable? It looks much nicer IMO than having two open ports facing up. Would this cause any kind of harm? o_O

    This is not about cost, so if there is even a slight advantage to having two 7800GT's instead of one 7800GT and a 6600LE, I will do that. Assuming it's possible at all!

    If you are anyway knowledgable on this very specific topic please share all you can! :)
     
  2. Intell macrumors P6

    Intell

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    #2
    The reason why the playback was not very good on the 6600 is because the 6600 isn't a very good card when compared to the 7800. It was driving and processing the pixels on the displays connected to it, not the 7800.

    Having two 7800s would enable you to have four responsive and power displays connected to one machine. As for the extra power requirement, you'd have to do some research. However, it may be possible to use a SATA or Molex to video power adapter without out spacing your power supply, but at the loss of a drive.

    The SLI cable won't do anything when connected, but as you stated it would look nicer.
     
  3. tevion5 thread starter macrumors 68000

    tevion5

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    #3
    Ahh so the secondary card is not simply extra outputs for the primary GPU, but does in fact do its own processing. Well then that makes sense. I am currently experiencing a performance drop with my secondary 7800gt but that is surely a result of it not having external power currently. I have ordered and was hoping to use a PCIE splitter cable. The 7800gt uses 65W peak I believe, so surely the CPU's wouldn't be using anything near 900W?! O.O

    I will rarely be using multiple displays with this system, but I bought it to have as sort of the ultimate PPC machine. And having two high end GPU's makes it more ultimate IMO. :cool: I couldn't be bothered selling it for now.

    Thanks for the info!
     
  4. Intell macrumors P6

    Intell

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    #4
    The PCIe and CPU buses are different. Each bus has a different maximum power output. Example: The SATA power lines cannot output 120W safely or at all. They can only output what their maximum design is. You may be pushing the PCIe bus beyond its maximum design load by using a splitter on it. I suggest doing so reading on having multiple video cards in Mac Pros and how people have reached the limites of the PCIe bus and how they've over come them.
     
  5. tevion5 thread starter macrumors 68000

    tevion5

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    #5
    Oh, so would I potentially be risking damage to my PSU if I overloaded the PCIE bus?
     
  6. MagicBoy macrumors 68040

    MagicBoy

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    Manchester, UK
    #6
    Different lane PCIE slots output different amounts of power if they run to spec.

    The 16x graphics slots outputs up to 75W via the logic board. Additional power is supplied by the PCIE power connector, either 75W (6 pin) or 150W (8 pin).
    4x and 8x slots output up to 25W max.

    Question is if the 6600LE gets only 25W and if that holds it back in the 8x slot.
     
  7. Intell macrumors P6

    Intell

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    #7
    You could damage your power supply if you overload the PCIe auxiliary cable by splitting it.
     
  8. tevion5 thread starter macrumors 68000

    tevion5

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    #8
    Ahh now I'm cautious to use the splitter.

    But if the 7800GT only used 65W, and the PCIEx16 port provides 75W...then couldn't I just use the cable for the secondary GPU and the first one should be okay without it?

    Might give that a shot now!
     
  9. Intell macrumors P6

    Intell

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    #9
    You'll have to do more research to find out if the 7800 actually needs the additional power to run at full speed.
     
  10. tevion5, Jul 21, 2015
    Last edited: Aug 3, 2015

    tevion5 thread starter macrumors 68000

    tevion5

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    #10
    From my experiments today, I believe that it does indeed require the power cable to run at full speed. I swapped the power cable over to the x8 card, hoping the x16 card would be getting enough from it's slot. Both cards appeared to play 720P video in VLC just fine, with the unpowered x16 card being ever so slightly more choppy at parts, but I can't be certain. I then ran Battlefield 1942 at 1920x1200 with the monitor connected to the secondary card. It ran completely smooth. I then restarted the game on the same settings but I plugged the cinema display into the unpowered primary card. It had a very harsh performance drop. Possibly even half the FPS.

    I turned off the quad and put the cable back into the x16 card. Battlefield 1942 now ran perfectly on the x16, and choppy as hell on the x8.

    From this, I conclude that the PCIE power cable is required for the 7800GT to perform at full capacity, whether it is in an x16 or x8 slot. This means in order to run a second 7800GT, I am going to have to power it somehow.

    However, I also no for certain now that an x8 slot 7800GT can perform just as well as one in an x16. I assumed it would, but it's nice to have evidence.
     
  11. val1984 macrumors member

    Joined:
    Jan 26, 2015
    #11
    Maybe removing one hard drive to use its SATA power connector with this kind of adapter would work? I haven't tested myself so it might melt your Quad power supply and I wouldn't know.
     
  12. tevion5 thread starter macrumors 68000

    tevion5

    Joined:
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    #12
    -- Update --

    I took the plunge and hooked up the Y-Splitter after doing a good bit of research on the topic. I deemed it was safe enough to try it, seeing as one card is always going to virtually be on idle all the time anyway. I am never going to be using BOTH cards to run a high end game at the same time.

    Having got both cards hooked up with the splitter, I bravely fired the system up. It booted without issue as usual. Then I launched Battlefield 1942 at 1920x1200, with the Cinema Display on the primary card. It ran perfectly smoothly. I then restarted the game but this time with the monitor hooked up to the secondary card. Eureka! It ran just as smoothly as the primary card. So clearly the extra power makes a big difference, and so far my system seems to be running along flawlessly. I know there is quite a bit of headroom on this PSU, so as long as I don't go mad and hook up 4 monitors all each running a separate 3D game, I think I will be safe.

    I will let you know asap if anything goes wrong, but if it doesn't, I may be one of the first (only?) documented instance of somebody running two powered GPU's in a G5 Quad.
     

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