MP 1,1-5,1 Dual 8 pin GPU wiring

[cuppuccino]

Hey folks,

so I decided to get a RX 5700 XT for my MacPro 5.1. Unfortunately the card I want has 2x 8-pin slots. I already own a dual-6-pin-to-8-pin adapter. Theoretically this should deliver enough power for the card in silent bios (75W from the PCI plus 2x 75W from the mini 6-pins equals 225W).

But what about the second 8-pin on the card, can I leave it unplugged? Or would you suggest to swap the dual-6-pin-to-8-pin and use 2 discrete 6-pin-to-8-pin adapters so both slots are plugged? I read that it might be a bad idea as the card might think it can draw 150W from the 8-pin although the 6-pin only provides 75W.

EDIT: I just read that people use a Y cable (female 8-pin-to-dual-6+2-pin) to split the 8-pin into both GPU slots and also 'balance' the energy consumption that way. Would this be a feasible solution?

Can anyone help me sort this out?

Thanks!

 

[tpivette89]

If you want to power a dual 8-pin card, the pixlas mod is pretty much your only solution. Alot of people shy away from this mod, but its really not hard to do, and it eliminates any doubts or worry about power delivery to even the most demanding of graphics cards.

 

[cuppuccino]

I read that a lot of people use the RX 5700 XT without Pixlas with no issues. I really just wanted to clarify the wiring. If it doesn't work the Pixlas mod will be my last resort.

 

[tpivette89]

If the GPU has more than a single 6 or single 8 pin hookup, you will either need the pixlas mod, or a lot of hacked SATA to 6/8 pin adapters to power it in addition to the dual mini 6-pin power connectors. That is messy and usually risky.

Most 5700 XTs have either a dual 8-pin or 6 + 8 pin connectors. A pixlas mod is your safest/easiest/cleanest bet when powering this card.

With anything more an RX 580 (single 8-pin connector), the Pixlas mod should be your FIRST resort.

 

[KeesMacPro]

If it doesn't work the Pixlas mod will be my last resort.

It's not a matter of "to work or not to work".
Fact is that when the MP 4,1/5,1 was developed, there were no GPUs with such high power draw.
High currents passing through the backplane is just asking for serious trouble.
This is apart from the (already expired) expected life time of the capacitors...

The Pixlas avoids this issue completely by linking the PSU directly to the GPU.
Theoretically you could connect the 5700XT any way you want (with as a side effect a regular shutdown during high graphics load), but it will shorten the life of your cMP substantially i.e. risking a cooked backplane at any time...

 

[h9826790]

75W from the PCI

This is a false assumption. No engineer will programme the card to draw 75W from the PCIe slot if there are two 8pins aux power sources, and the card isn't that power hunger.

what about the second 8-pin on the card, can I leave it unplugged?

NEVER do this unless the manual clearly says you can do so. Modern GPU usually has pretty good protection. For older card, this action may cause permanent damage.

I just read that people use a Y cable (female 8-pin-to-dual-6+2-pin) to split the 8-pin into both GPU slots and also 'balance' the energy consumption that way. Would this be a feasible solution?

Most likely will be OK if your card has "silent mode" that won't draw too much power. But of course, no guarantee.

 

[riggieri]

You can run a dual Mini 6 pin to single 8pin connector and then a single 8 pin to dual 8 pin. That balances power over both 6 pin connectors.
At your own risk but I power a Vega 56 like this and that is more power hungry than a 5700XT

 

[timothybrooks]

Has anyone done the Pixlas Mod on a Mac Pro 3, 1? All the videos and blogs I find are for the 4, 1 or 5, 1. The wiring harness that is behind the optical drives is very different on the 3, 1. Anyone have any thoughts? I need 2 8 pins for my video card. Thanks.

 

[timothybrooks]

It's not a matter of "to work or not to work".
Fact is that when the MP 4,1/5,1 was developed, there were no GPUs with such high power draw.
High currents passing through the backplane is just asking for serious trouble.
This is apart from the (already expired) expected life time of the capacitors...

The Pixlas avoids this issue completely by linking the PSU directly to the GPU.
Theoretically you could connect the 5700XT any way you want (with as a side effect a regular shutdown during high graphics load), but it will shorten the life of your cMP substantially i.e. risking a cooked backplane at any time...

Hello, Any experience or reference on this mod to a Mac Pro 3, 1? The wiring harness is different from the 4, 1 or 5, 1. Photo attached. There may be a space issue for the attachments behind the optical drives but I think I can get by with that. Just looking for any comments on the wiring scheme for the 3, 1. Thank you!!!

 

[KeesMacPro]

Hi,

I never did the Pixlas mod on a MP 3,1 , but i'm sure it can be done.
AFAIK basically there are 2 power rails : 12volts and 5volts.
My guess is (since the highest power draw is the 12V rail) that connector P1 and P2 deliver the 12 V (the thickest cables).
With a multimeter you could find out the 12V and ground lines and solder them to the pixlas cable similar to the MP4,1/5,1 mod.
The prefab cable for pixlas has yellow and black lines: yellow=12v and black=ground.
To measure the lines you could strip them a bit without cutting the copper , measure them, mark them and solder the GPU cables to it or use the clips as described in the thread here:

Pixlas 4.1 Mac Pro mod

I'm creating a thread for my 4.1-->5.1 mod to keep everything in one place. I will document my upgrade to X5690, PCIe SSD (Apple SSUBX) and PSU mod to allow for two modern gfx cards using only the internal PSU. I have gotten my Mac Pro now (2x 2.26GHz) 4.1 from 2009. It had one Nvidia 120GT...

forums.macrumors.com

 

[timothybrooks]

Thank you. I literally just found the attached diagram for the 3, 1. I'm just waiting for the cable.

 

[timothybrooks]

Hello. I completed the Pixlas modification on my MacPro 3, 1. It is a fairly easy process. I want to upgrade my video card. I'd like to have an HDMI output with 4K. Can anyone make a recommendation please?

 

[h9826790]

Hello. I completed the Pixlas modification on my MacPro 3, 1. It is a fairly easy process. I want to upgrade my video card. I'd like to have an HDMI output with 4K. Can anyone make a recommendation please?

Which OS?

 

[timothybrooks]

Catalina. I did the patch upgrade from dosdude.

 

[h9826790]

Catalina. I did the patch upgrade from dosdude.

Then Polaris GPU (e.g. RX580), should able to do that. However, you may need to re-patch (or re-install) Catalina via Dosdude patcher.

I am not very sure how Dosdude patcher works, but it seems that patched the video drivers for the old GPU to work, and sometimes may cause the new GPU doesn't work.

Also, you have to make sure the SSE 4.2 emulation also installed. Otherwise, the Polaris GPU driver won't work on the 3,1.

 

[timothybrooks]

I put a Vega 64 in and it's working great. Thanks for the suggestion. Catalina handled the new card without re-installing the Dosdude patch.

 

[macrumorsnumpty]

Then what happened?

What happens if you have a 6 and 8 pin connector on a graphics card like the RX 5700?
That draws 180. How would one go about powering this card via a mac Pro 5,1?
x2 mini 6 pin (y cable) to x1 8 pin and leave the other x6 pin empty on the card?
x2 mini 6 pins to 6 pin cable and then get x1 6 pin to 8 pin adapter?
EVGA powerlink?
Or is the Pixlas mod the only answer?

 

[h9826790]

Then what happened?

What happens if you have a 6 and 8 pin connector on a graphics card like the RX 5700?
That draws 180. How would one go about powering this card via a mac Pro 5,1?
x2 mini 6 pin (y cable) to x1 8 pin and leave the other x6 pin empty on the card?
x2 mini 6 pins to 6 pin cable and then get x1 6 pin to 8 pin adapter?
EVGA powerlink?
Or is the Pixlas mod the only answer?

It depends on how much the card actually draw. However, technically, any 6+8 pin card can work with just the mini 6pins as long as we balanced the power draw.

Let's say the 6pin draw full 75W and the 8pin draw full 150W. That's 225W in total.

And since we know each mini 6pin can deliver up to about 120W. If we can balance the power draw (e.g. via PowerLink), then each mini 6pin "only" need to deliver up to about 113W, which is just below the hard shutdown limit.

However, since power spike happens, therefore, this technique not always work.

 

[Madr]

It depends on how much the card actually draw. However, technically, any 6+8 pin card can work with just the mini 6pins as long as we balanced the power draw.

Let's say the 6pin draw full 75W and the 8pin draw full 150W. That's 225W in total.

And since we know each mini 6pin can deliver up to about 120W. If we can balance the power draw (e.g. via PowerLink), then each mini 6pin "only" need to deliver up to about 113W, which is just below the hard shutdown limit.

However, since power spike happens, therefore, this technique not always work.

Are you 100% sure the small 6 pin can deliver 120 watt? Is there an official place we can check that? I always thought 75 watt each and haven't used more than that running older cards.

Also is there a place where we can see the correct wiring for 6 to 6 pins cables? I just received some cheap cables from China and the pin out differ from each other. 2 cables are the same. 1 is different. None of the cables i have received have a 1 on 1 pin wiring. Which i think should be logical.

I have searched online but can't seem to find it. And i think it's very important because if the wiring is wrong you can blow up a power rail or a videocard. And not instantly but after some time and maybe more heavy use.

And is there a place in the OS or somewhere else where we can check how much each of the ports are pulling from the power supply?

Because i've seen a lot of topics here where the videocard just died. But it maybe could be avoided if we know for sure it's not the power cables that are the issue. I assume the cables Apple used back in the day were checked but i have ordered a lot of stuff from China that turned around not working. So quality control wise i'm almost sure they don't do a real quality check before shipping items.

Thanks a lot in advance!

 

[h9826790]

Are you 100% sure the small 6 pin can deliver 120 watt?

Yes

Is there an official place we can check that?

No

I always thought 75 watt each and haven't used more than that running older cards.

More than that is fine.

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Also is there a place where we can see the correct wiring for 6 to 6 pins cables? I just received some cheap cables from China and the pin out differ from each other. 2 cables are the same. 1 is different. None of the cables i have received have a 1 on 1 pin wiring. Which i think should be logical.

Pin 1,2,3 are the 12V pins. All others are just ground pins.

And is there a place in the OS or somewhere else where we can check how much each of the ports are pulling from the power supply?

iStat menu is the most popular option.