Radeon 7970 PCIe power load sharing

Discussion in 'Mac Pro' started by Daze513, Sep 30, 2015.

  1. Daze513 macrumors newbie

    Joined:
    Sep 22, 2015
    #1
    Ok so I got a flashed MSI r7970 2PMD3GD5 for my Mac Pro 5,1. I've installed it and it runs perfect. Boot screens, PCI 2.0, all ports. Etc. As some of you may know the card requires a 6 pin and 8 pin power connectors. I have this installed with a mini PCIe 6pin to 8 pin cable (not an adapter) and it all works. I run Luxmark, openwave, all those benchmarking tools and have no problems. No shutdowns, glitches, nothing. Fans all work and the expansion Hovers around 900 at idle.

    So why do I bother to say this or post? I know the whole 150w vs 75w thing and the tracers possibly burning out and stuff. I've read all those threads on here. But I've had an idea that I'm not sure will work and maybe some of you can help me if you have already tried, or know better. Can I do the following cabling to achieve a load distribution amoungst both motherboard PCIe plugs?

    Dual 6 pin mini PCIe > single 8 pin PCIe > single 6 pin & single 8 pin (or 6+2) ??

    So my thought is by bridging both supplying connections this way, the card will receive the full power, and when the card needs more power and pulls from the 8 pin it will be load distributed among the 2 6 pins on the Motherboard.

    Makes sense? Maybe the wattage would be to high for the bottleneck at the single 8 pin? I figured if I'm going to pull more than 75w on a single mini 6 pin I rather they both share the load.

    Let me know what you guys think.

    Another option would be to mod the card to accept the 2 6 pin connectors, but so far I have only seen MacVidCards offer this. I have no idea what this requires or what is done. And I fully understand why he wouldn't want to publicize what he mods. Duh. It's a business. I own one. I know what's it's like.

    Thanks guys.
     

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  2. h9826790 macrumors 604

    h9826790

    Joined:
    Apr 3, 2014
    Location:
    Hong Kong
    #2
    IMO, it's very safe to do so, even though technically it may pull more than 75W from a single 6 pin. However, after this load sharing, I think it can't pull too much over 75W.

    I don't have 7970, instead, I installed two 7950 with internal power only in my 4,1 (each card power by a mini 6pin -> dual 6pin). This is more risky than your setup, but still run flawlessly so far. Even Furmark won't trigger the self shut down protection.

    Anyway, HWmonitor is your friend. Just open it and run some tests, if all reading within your comfort zone, then you can forget about the power draw and enjoy to use your computer.
     
  3. Daze513 thread starter macrumors newbie

    Joined:
    Sep 22, 2015
    #3
    Nice. Dual 7950 must be nice.

    Ok so I'm going to try it like that and see how it goes.

    See the following are my wattage draws based on HWmonitor while running Luxmark 2.1 Sala (scored 2295 btw)


    PCIe slot 1: 32.95w
    PCIe Booster A: 91.5w
    PCIe Booster B: 43w


    Surely I can redistribute the Booster Power draws to bring them both within the 75w limitation that so many say is safe. Let's see what kinda cable I can come up with that won't be very long either.
     
  4. h9826790 macrumors 604

    h9826790

    Joined:
    Apr 3, 2014
    Location:
    Hong Kong
    #4
    Ya, the dual 7950 works quite well so far. For your info, this is the Luxmark 2.1 Sala's result and power draw.
    V2 10.11.1b2 850.jpg Screen Shot 2015-10-02 at 06.31.01.jpg
    Obviously the ident is wrong. The Slot 1 and 2 Lines are the 6pin sources, each of them is powering a single 7950. The power draw of Luxmark 2.1 is not very bad, right at the 75W range. However, the cards can draw up to 100W without any problem during stress test. And I quite often allow them to run at the 85W range for a prolonged period of time.

    Of course, if you can share the load, do it! There is no point to keep overloading one of the 6pin if the other one only working at the 50% range. And also, as I said Luxmark 2.1 is not that bad in terms of power draw. If that can draw >90W from one source already, some other more demanding software may able to draw more than 100W. Other members report that their 280X (same as the 7970) will hit the self shutdown protection during stress test (Furmark) if they don't balance the load.

    I won't recommend anyone to exceed the published limit. However, from my own experience, 91.5W won't hurt the machine. If you can balance it, do it. If not, I will say still nothing to worry about. It should be OK for normal ops, just don't touch something Furmark, then you should be fine.
     
  5. Daze513 thread starter macrumors newbie

    Joined:
    Sep 22, 2015
    #5
    Thank a lot. I actually ran furmark. And the second 6 pin is the one that ramped up to about 68w. The first 6 pin held a steady 92w and the slot went to 65w.

    I'm gonna try ordering or self soldering some cables to balance and post back in a couple days. Let's see how it goes.
     
  6. h9826790 macrumors 604

    h9826790

    Joined:
    Apr 3, 2014
    Location:
    Hong Kong
    #6
    In this case, I think your card actually doing very good in balancing the load. It will try to pull more from the 6pin and slot rather than keep pulling the 8pin. Unlike my 7950s, they can only pull up to 35W from the slots, and then all the rest are from the 6pins.
     
  7. h9826790 macrumors 604

    h9826790

    Joined:
    Apr 3, 2014
    Location:
    Hong Kong

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