Mac Pro GPU Power

Discussion in 'Mac Pro' started by davidmikucki, Apr 20, 2017.

  1. davidmikucki macrumors newbie

    Joined:
    Jun 30, 2016
    #1
    After the Pascal drivers came out, I bought a 1080 TI (8-pin, 6-pin) to upgrade from my 980 TI (I do a lot with Octane Render, so I'm always looking for more power). It didn't work and I spent a few hours trying to figure out why. I discovered it was that the card wasn't getting enough power. I'm driving it with the two PCI Express power sources from the motherboard combined to an 8-pin, and two of the SATA power sources combined for the 6-pin. That wasn't cutting it.

    So I found a 500 watt power supply I had lying around, plugged that into it using the ol' paperclip trick and now it works. But I have to have the side of my case off and an unwieldy power supply plugged in from outside, with a scary-looking paperclip hack on the motherboard leads.

    Does anyone have any suggestions for how to clean this up? I don't think there is a way to get sufficient power to the card from Mac itself, so I'm stuck with a secondary PSU. But maybe there is a neater way to run the power into the machine? Sadly, I'm using all my PCI card slots, so I don't think I can squeeze them in the back that way.

    Thanks!
     
  2. ActionableMango macrumors 604

    ActionableMango

    Joined:
    Sep 21, 2010
    #2
    Hmmm, something is wrong. I looked up the specs.

    Each 6-pin PCIe power source is rated for 75W, so two of them together is 125W, which is exactly what an 8-pin socket on the GPU is expecting. So that is in spec, in fact the 2x6-pin to 1x8-pin is a normal solution.

    SATA spec for the 12v line is 54W, so two of those should be 108W, and the 6-pin socket on the GPU is expected to pull 75W. So that should be well within spec.

    I can only lamely suggest that maybe you have a bad cable or connection somewhere.

    An alternative is the Pixlas Mod. The MP's power supply has far more than sufficient capacity, you're just not able to easily tap into it. The Pixlas Mod helps you get power to GPUs.

    Another alternative, but one I don't like nearly as much, there are secondary PSUs that are designed to fit in the optical bay. Unfortunately you have to do case mods to get the power into the PCIe bay, and use an empty slot cover to get AC power into the 2nd PSU (or tap into the AC power coming into the main PSU). I really don't like this solution myself, but a few people have done it.

    And finally going a completely different route, there's a fair chance that MVC will have GPU mod in the future, allowing 1080ti to fit in the power envelope of a MP, with little or no performance loss. He has done so in the past for other power hungry cards.
     
  3. davidmikucki thread starter macrumors newbie

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    Jun 30, 2016
    #3
    Thanks for the reply. This is really helpful.

    To your first point, that's interesting. My solution powered the 980 TI, and I double, triple, and quadruple checked the connections—so unless I have a bad cable, but I would expect that to have affected the 980. I wonder what's going on…

    As for optical bay PSU's, I could only find one, and it seems to be eternally out of stock. Do you know where I might find one that is available? Also, how does one turn these on/off?

    Thanks.
     
  4. ActionableMango, Apr 20, 2017
    Last edited: Apr 20, 2017

    ActionableMango macrumors 604

    ActionableMango

    Joined:
    Sep 21, 2010
    #4
    Sorry I have no idea where to get them normally. After a quick look I can only find them on Ebay.

    The secondary power supply usually has a SATA or molex connector that you attach to SATA or molex coming off of the primary power supply. The secondary PSU senses voltage on that connector so that it turns on and off together with the primary PSU.

    There is a thread about one here:
    https://forums.macrumors.com/thread...cal-bay-for-graphics-cards-build-log.1472518/

    I never really cared for these because they seemed cheap and poorly made. I seem to remember them being around $25 new and prone to failure.

    One thing that's confusing me here is that even a 1080ti shouldn't be pulling much power unless its under heavy load. But you can't even get it to display the OS? I can't imagine it exceeding the power requirements just trying to do that.
     
  5. h9826790 macrumors 604

    h9826790

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    Apr 3, 2014
    Location:
    Hong Kong
    #5
    How do you know it's because not getting enough power?

    Everything is within spec, and the same setup used for 980Ti. That's just doesn't make any sense to me.

    Of course I trust you, there is something wrong. However. I doubt if that's not "enough" power.
     
  6. ActionableMango macrumors 604

    ActionableMango

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    Sep 21, 2010
    #6
    Yes, that is why I think something is wrong. Both cards have the same 250W TDP. And the 1080ti shouldn't be using going anywhere near that just trying to boot up and run without a heavy load.
     
  7. davidmikucki thread starter macrumors newbie

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    Jun 30, 2016
    #7
    It's bugging me, too. What I know is that if I plug in an external PSU, it works great and I can use Octane Render. If I try to plug it in the same way I had my 980 TI plugged in, it won’t even boot (except to Windows—oddly). But when I plug in my 980 TI the same way, it works no problem. I am wondering if there is something wrong with one of the connections, but the 980 TI isn't drawing enough power for it to matter.

    Thanks for all the help, guys.
     
  8. Prince134 macrumors 6502

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    Aug 17, 2010
    #8
    On board dual pcie for 8+6 on to my Titan X pascal has no problem to run FCP, Luxmark, and Octane benchmark. Your Mac PRO PSU or even cable could be defective.
     
  9. JesterJJZ macrumors 68020

    JesterJJZ

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    Jul 21, 2004
    #9
    I did this too. But I drilled a hole for cables next to the PCI slots. I have my external PSU sitting on my tower under my desk. Works great.

    Oh and for everyone claiming these cards work with internal power, it's true to an extent. If you have a fairly vanilla machine, I suppose it's no problem. In my tower I have full slots, 96GB of RAM, upgraded processors and 6 hard drives. It's too much with the GTX in there, my machine would randomly turn off. External PSU solved that.
     
  10. h9826790 macrumors 604

    h9826790

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    Apr 3, 2014
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    Hong Kong
    #10
    Dual CPU 260W max
    6 HDD 100W max
    logic board + RAM etc, 200W max

    Unless your 1080Ti can pull 450W, I don't see how a 980W PSU can't deliver enough power to the card.

    It's been discuss many times. That's all about the mini 6pin shutdown protection, not the PSU's ability. The proof is that few members here can fix the cMP by Pixlas mod, which allow them to use exactly the same hardware without shutdown.
     
  11. JesterJJZ macrumors 68020

    JesterJJZ

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    Jul 21, 2004
    #11
    Numbers on paper vs real world. Guys at MacVidCards confirmed a fully loaded system like mine needs an external PSU for the 980ti.
     
  12. h9826790, Apr 21, 2017
    Last edited: Apr 21, 2017

    h9826790 macrumors 604

    h9826790

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    #12
    There are also plenty members here have fully loaded system do not required external PSU. Number is numbers, won't lie. If the numbers shows marginal, then yes, base on different condition, some user may require external PSU. However, with just few HDDs and dual CPU, and single 980Ti or 1080Ti is not a problem at all.

    As I said, 980Ti's demand can be very marginal on the mini 6pins, but not the PSU. That is the real world limitation. Extra PSU can very effectively solve the mini 6pin auto shutdown issue, same as the Pixlas mod.

    If you mean this post confirm that extra PSU is required. Please read the comments again. The Mac shut down because they only get power from the mini 6pins. By utilising the SATA power, or Pixlas mod, no shutdown occur anymore. It's nothing to do with the PSU limit.

    http://www.macvidcards.com/blog/titan-xp-might-be-too-much-for-cmp#comments
     
  13. davidmikucki thread starter macrumors newbie

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    Jun 30, 2016
    #13
    So the weird thing is that the problem when I have the 1080 TI plugged into internal power (PCI-E + SATA power), the machine does boot. I just get a black screen from the video card. If I also plug in a GT120 (old video card, came with the machine) and plug my monitor into that, I see in System Information two cards, the GT120 and something called "NVIDIA Chip Model" that reports as having 256MB RAM.

    But everything works 100% with the external PSU.
     
  14. ActionableMango macrumors 604

    ActionableMango

    Joined:
    Sep 21, 2010
    #14
    This is super confusing. The fact that it works on internal power just fine for Windows but won't even boot OS X would indicate to me that it is not a power problem at all and sounds more like an OS X driver or other software problem. Especially since the 1080ti uses very little power at idle. It seems like it simply cannot be power related.

    But the fact that it all works fine in OS X once you add external power seems to indicate that it is in fact a lack of power.

    I don't really know what to think. Maybe OS X does something when initializing the card during bootup that causes a power spike, so it fails at that point, and maybe Windows doesn't do the same thing. That's the only theory I have that fits all your symptoms.

    I have a 1080ti myself, but it's an overclocked 2x8-pin version. I don't have all the cables necessary to try to power it in a Mac Pro.
     
  15. davidmikucki thread starter macrumors newbie

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    Jun 30, 2016
    #15
    This was exactly my theory, but it sounded crazier than I wanted to say out loud. Glad to hear someone else is thinking it. I think now the only thing I have left to test is the cables.
     
  16. Executor macrumors member

    Joined:
    Mar 1, 2008
    #16
    I posted on the Macvidcards thread. The Titan Xp with power coming from the SATA ports to 6 pin and combining the internal dual mini 6 pin to 8 pin works perfectly fine. And yes my Mac Pro is FULLY loaded hard drives PCI express cards and dual Xeons.
     
  17. Macschrauber macrumors member

    Joined:
    Dec 27, 2015
    #17
    If it comes to reach the upper specs of the psu simple things as temperature and mains voltage is something to think about. It makes a difference if a psu runs from 110 or from 230 volts...
     

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