PCIe card -> 6pin 75W

Longpig

macrumors newbie
Original poster
Apr 30, 2017
5
0
Good morning all,

Like a few others here I didn't check the power requirements before buying the 1080Ti for my Mac Pro 5,1, and have been scrambling to find a solution for powering it which doesn't involve a secondary PSU or overtaxing the 2x6pins.

I still have 1 PCIe slot available, and it struck me that since each slot is rated for 75W one ought to be able to hook a blank PCIe card in there and get a 6pin adapter out of it. But I can't find any such cards (Not that big a market I guess - only Macs route everything through the mobo) so thought I'd check with you guys if this has been attempted or if it's a ridiculous idea.

If it's just a case of hooking up 6 pins from the PCIe slot to a cable it seems straightforward to etch a card or scavenge a dead PCIe card and solder on connectors. If there needs to be a controller on the card to talk to the mobo to even activate power over PCIe, the idea isn't feasible. Any HW engineers who'd care to enlighten me on this?

Cheers!
 

Longpig

macrumors newbie
Original poster
Apr 30, 2017
5
0
If you have SATA ports avail, the easiest way would be

2x SATA -> single 6pin
2x mini 6pin -> single 8pin
I thought SATA were max 18W?

Have a fully populated SATA regardless, but I could move the RAID to external enclosure, or make do with one less internal disk…
 

h9826790

macrumors G5
Apr 3, 2014
12,784
5,596
Hong Kong
There are 3x 12V line in a single SATA port. Each of them rated up to 1.5A. Therefore each of them rated for 12x1.5=18W, and 3x18=54W in total.

But since your SATA ports a full (including the optical bay?) already. So, I think it's not that easy to free up 2 slots for powering the GPU.

Yes, that 75W is very conservative, from testing, the actual shutdown (protection) limit is at about 120W. So, if you can balance the load, It will be very easy for dual mini 6pin to power a 6+8pin card.

Because 6+8 max continue consumption should be at 75+150=225W, and the real world limit of the dual mini 6pin is at about 240W. So, technically it can power any 6+8pin card (again, if you can balance the load), but may be very very close to the edge.
 
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Longpig

macrumors newbie
Original poster
Apr 30, 2017
5
0
Yes, that 75W is very conservative, from testing, the actual shutdown (protection) limit is at about 120W. So, if you can balance the load, It will be very easy for dual mini 6pin to power a 6+8pin card.

Because 6+8 max continue consumption should be at 75+150=225W, and the real world limit of the dual mini 6pin is at about 240W. So, technically it can power any 6+8pin card (again, if you can balance the load), but may be very very close to the edge.
Ok, so what I need is a 2x6 -> 8pin-> 1x6 + 1x8 to distrubute the load? Any hazards in getting the wrong adapters, or are all the pin layouts standard?
 

h9826790

macrumors G5
Apr 3, 2014
12,784
5,596
Hong Kong
Ok, so what I need is a 2x6 -> 8pin-> 1x6 + 1x8 to distrubute the load? Any hazards in getting the wrong adapters, or are all the pin layouts standard?
Yes, it should work.

And pretty hard to get the "wrong" cable, it simply doesn't fit.
 

ActionableMango

macrumors G3
Sep 21, 2010
9,267
6,253
If there needs to be a controller on the card to talk to the mobo to even activate power over PCIe, the idea isn't feasible.
Yes, this needs to happen. By default PCIe cards only get low power unless they negotiate for more.

Someone here in the forums was going to look into designing and testing a card just for this purpose, but I don't know what happened to that effort.
 
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Longpig

macrumors newbie
Original poster
Apr 30, 2017
5
0
Yes, this needs to happen. By default PCIe cards only get low power unless they negotiate for more.

Someone here in the forums was going to look into designing and testing a card just for this purpose, but I don't know what happened to that effort.
Ah, so much for that idea… I'll trawl the forums a bit and see if I can follow up on that. Will also follow up on the 2x6->8->6+8 suggestion above.

Although, since the 5,1 isn't getting any younger I'm considering just getting a Win rig for 3D/gaming and call it a day. That or hope that the 7,1 will be better than the can. (Stings to even consider that – I've been on Mac since -86 so am not keen on switching)
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Yes, it should work.

And pretty hard to get the "wrong" cable, it simply doesn't fit.
Was more thinking that the pin layouts could differ between adapters, not the adapters themselves.
 

ActionableMango

macrumors G3
Sep 21, 2010
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throAU

macrumors 603
Feb 13, 2012
5,284
2,276
Perth, Western Australia
Seeing as you mention gaming...

You could always just step down the NVidia product stack a level or so, and not worry about all this power stuff.

Unless you're running 4k (and even then, i'd rather take 120-144 FPS at 1080p instead of 4k60 or less), the regular 1080 or even 1070 is plenty of power to rip through non-4k content at max details at very good frame-rates.

IMHO, until we get the next generation of cards, frame rate wins. Even the top dog video cards at the moment struggle to push 4k much beyond (or even sustain) 60 fps or more. Whilst those same cards (or even 1-2 tiers down) can drive 1080p at 100-150+, which, if the game is any sort of fast action title, is imho far superior.

If this isn't for gaming, YMMV, etc. And of course the faster cards will obviously be much better for CUDA and non gaming use. But if it IS just for gaming well.... IMHO the 1080 and up are relatively pointless cards right now, unless you want 4k and are happy with frame rates around 60.