Official Mac Pro PCIe Power Limitations

Discussion in 'Mac Pro' started by darkcoupon, Dec 17, 2012.

  1. darkcoupon, Dec 17, 2012
    Last edited: Dec 17, 2012

    macrumors regular

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    Aug 8, 2012
    #1
    Since some useful threads regarding graphics cards and the PCIe auxiliary power plugs have been closed, I'd like to open this one up for discussion and information on the actual power limitations for the Mac Pro PCIe expansion bays.

    Apple's official rating (seen on the current Mac Pro tech specs page, and any other Apple literature regarding PCIe expansion) only indicates that a combined 300w is the maximum limitation on all PCIe slots. Whether this includes the aux PCIe power or not has yet to be determined.

    If the aux power is piggybacked from the PCIe expansion slots, then running a high-end graphics card with 250w+ TDP would theoretically be possible, assuming the rest of the slots are empty or don't contain cards that draw more than the allowed spec. If the aux power is not included in the 300w it would be advisable to limit your graphics card choices to cards that draw under 225w (75w from PCIe slot, and 75w presumed limit of each aux plug) for stable operation until the exact limitations of the logic board traces to the aux plugs have been determined.

    Any info/links/stories regarding the actual Logic Board trace layouts or limitations would be a great contribution towards finding the exact limitations that we should be setting for Graphics Card use and other PCIe expansion.
     
  2. macrumors 6502

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    #2



    Here we go again.......... :D
     
  3. thread starter macrumors regular

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    Aug 8, 2012
    #3
    :) I think it's a conversation worth having. Some people would rather argue about it or be offended over the questioning of collective logic, but until we have solid facts on the issue there's no reason to fully buy into it.
     
  4. macrumors 603

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    Apr 15, 2004
    #4
    Lemme grab some popcorn... This should get interesting. :)

    I'd guess that 300w is without the aux connectors. It doesn't make sense to include them in case you're running all cards without aux connectors on them. But I don't know anything for sure, so don't quote me on that... or flame me.. or anything...
     
  5. Wardenski, Dec 18, 2012
    Last edited: Dec 18, 2012

    macrumors 6502

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    #5
    4 PCI slots: 4 x 75 W = 300 W
    2 Aux 6 pin: 2 x 75 W = 150 W

    So that allows two 5770s which are offered on the Mac Pro page plus two PCI cards (e.g raid) which seems reasonable. I believe the 300 W does not include the auxillery inputs but how the power is routed between them is another matter which is the big issue IMO.
     
  6. macrumors 6502a

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    Oct 4, 2008
    #6
    Each PCI-e slot is rated for 75W. 75W * 4 slots = 300W, which is what you're seeing on the Apple page.

    Each booster cable is rated for 75W. The booster cables are not counted towards the 300W, they are routed separately on the logic board even though everything congregates at the primary DC power connector behind the optical drive bays anyways.

    That being said, the theoretical limit is as you say 225W for a single CPU in slot 1 with both booster cables attached. Personally, I would say it would be ill advised to exceed that limit even if the rest of the system can handle it simply because the booster cables are pretty thin (wire wise) and the connectors on the logic board aren't very beefy (they look like they're rated for 75W/piece, no more).

    At the same time, I've seen people split both booster cables and shove two 5870s in there. Exactly how stable a Mac Pro would be with 2x5870s, two 12 core CPUs, 64GB of RAM, four disk drives and a RAID card... Well, I would expect that would tax the PSU.

    Fun fact: The Mac Pro SMC monitors voltages and currents all around the logic board, including the voltage and current running to each PCI-e slot. However, the booster cables do not have their own dedicated sensors, and don't show up as current utilization on the PCI-e slot. They do, however, count towards the total current leaving the power supply.

    This makes me think that you probably could exceed 75W on the booster cables without SMC tripping and shutting down the system, but I still don't think it's a good idea since the connectors and wires are so goddam tiny.

    -SC
     
  7. macrumors 6502a

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    #7
    This is exactly where we stand. Some very good points that people have said in some form or another. I guarantee that this thread is not worth another protracted discussion. It's a test that we need.
     
  8. macrumors 604

    theSeb

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    #8
    I agree with your post, except this bit. If we sum all of that equipment, then I don't think this would tax the PSU itself. I think the problem is specifically how much the booster cable headers on the motherboard can handle.
     
  9. macrumors 6502a

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    #9
    2 x Xeons X5670 : 2 x 95 W = 190 W
    2 x 5870 : 2 x 190 W = 380 W
    1 x Apple RAID Card = 80 W Maximum?
    5 x 2 TB WD Caviar Black (4 HD bays + optical) = 5 x 12 W = 60 W
    8 x 8GB RAM DDR = 8 x 10 W = 80 W

    Giving a total of 790W. So, 190W is still available to power the motherboard and other components; however you never know in advance how limits may have been set for each power cable.

    This is why we need a physical test or a member of the Apple engineering team. Failing those, an Apple store Genius armed with a powerful card and a Mac Pro out the back to tinker with.
     
  10. macrumors 68030

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    Oct 15, 2008
    #10
    A physical test is of limited value. If the result is that one particular year model, or one particular sample can run outside of spec (or can't) then that's all we'll know.

    It'll be just like overclocking CPUs... When you start running outside official design specs then YMMV.
     
  11. macrumors 68000

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    Arizona
    #11
    Here is a conversation worth having; Order a Mac Pro with a 5870 video card and live with it. Need more horsepower and flexibility? Get a Dell or an HP.
     
  12. macrumors 6502

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    #12
    :d
     
  13. thread starter macrumors regular

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    Aug 8, 2012
    #13
    I think that's likely the case as well, but you'd think after 3+ years of having the same back plane board in the Mac Pro that Apple would update the limitations to include the aux 6-pin plugs. That's the only thing that makes me wonder if they draw power away from the other PCIe slots.

    ----------

    That's great advice. Why would I want to get a faster video card anyway? Apple already sells a two year old card that's way more expensive and is just "good enough" for most.
     
  14. macrumors 6502a

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    Dec 28, 2009
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    UK
    #14
    Well not quite, traces and over clocking are two different beasts. The traces power draw and overhead can be quite accurately measured and repeated as the traces will be the same tolerance across the board. Processor design is a tad different in that certain batches will have better clock speed.

    As for model specific testing - I don't doubt that the information will be limited to certain model of testing but I wouldn't say 2009-2012 is of little value to the community.

    ----------

    The only thing with less value than this comment is the 5870 Apple offer.
     
  15. macrumors 603

    Joined:
    Mar 10, 2009
    #15
    This leaves out some substantive power consumers; the sockets. Firewire and USB 2.0 are essentially power supply sockets also. Firewire 10+W for each bank (or socket?). USB 2.0/3.0 a bit less so.

    The other assumption here is that the 900W is evenly distributable.
     
  16. All Taken, Dec 18, 2012
    Last edited: Dec 18, 2012

    macrumors 6502a

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    #16
    "however you never know in advance how limits may have been set for each power cable."

    It's 980w not 900w

    Already covered that part. You can say for certain that each component can get at these figures under maximum draw.

    I didn't include USB and Firewire in these figures as they're external. Calculate them in and you don't have much more.

    We know that the draw for most components is distributable, take a processor upgrade.... 16GB DIMMS.... 15,000rpm drives.... Splitting off the SATA connectors for added power....

    The SMC allows power to be drawn as required until safety limits are reached, as said above the only component that isn't regulated, at least smartly is the 2x 6 pin connectors. That isn't to say that you wouldn't trip the PSU with an over draw?

    I asked a question before regarding power drain from other PCIe slots, it seems the specification for PCIe 2.0 allows a single slot to draw no more than 75w, so, with this in mind is there a card out there that can be used solely for taking power from a PCIe slot and feeding it to something like a 6/8 pin connector?
     
  17. thread starter macrumors regular

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    Aug 8, 2012
    #17
    I've been looking for a device similar to that but haven't seen anything yet.
     
  18. m4v3r1ck, Jan 8, 2013
    Last edited: Jan 29, 2013

    macrumors 68000

    m4v3r1ck

    Joined:
    Nov 2, 2011
    Location:
    The Netherlands
    #18
    Wiring Mac Pro PCI-e mini to PCI-e PC NVIDIA GRAPHICS

    [SOLVED!]

    Hi all,

    I'm looking for a wiring diagram to juice a EVGA GTX 670 FTW 4GB in a Ma Pro 3.1 from the two onbooard PCI-e connectors.

    2 cables Mac Pro 6 pins mini PCI-e to 2x 6 pins PC EVGA (Y-split)
    Going to use a crossed cable setup for load balance to both the Gt8800 (bootscreen) and the EVGA!

    Thanks in advance!!!

    Cheers
     

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