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LEOMODE

macrumors 6502a
Original poster
Jun 14, 2009
558
56
Southern California
Got RTX4090 Founder's Edition and 6900XT. Since 4090 is taking up both power slots, 6900XT has no connector. I heard from someone here that you can install an external PSU. Wondering how I can achieve it, especially needing a recommendation for the parts that has a switch that you can turn it on or off so I can turn it off when on Mac OS, and turn it on when on Windows. Thanks.
 
Last edited:

joevt

Contributor
Jun 21, 2012
6,761
4,100
One method is an entire PCIe expansion chassis but it might be overkill for your purpose.
https://www.netstor.com.tw/product_sublist.aspx?SerID=6&Title=TurboBox

The Netstor NA255A turns on automatically when you turn on the host computer. It's made for double wide GPUs but I suppose it can fit wider GPUs if they only need the first two slot openings to line up for display connectors.
https://www.netstor.com.tw/product_info.aspx?PID=PID_170324287237937

They have a gen 4 (NA255A-G4) version for triple wide GPUs. The gen 4 bandwidth will be useful for your gen 3 host only for gen 4 cards that are x8 or less.
https://www.netstor.com.tw/product_info.aspx?PID=PID_230116037118808
 

LEOMODE

macrumors 6502a
Original poster
Jun 14, 2009
558
56
Southern California
One method is an entire PCIe expansion chassis but it might be overkill for your purpose.
https://www.netstor.com.tw/product_sublist.aspx?SerID=6&Title=TurboBox

The Netstor NA255A turns on automatically when you turn on the host computer. It's made for double wide GPUs but I suppose it can fit wider GPUs if they only need the first two slot openings to line up for display connectors.
https://www.netstor.com.tw/product_info.aspx?PID=PID_170324287237937

They have a gen 4 (NA255A-G4) version for triple wide GPUs. The gen 4 bandwidth will be useful for your gen 3 host only for gen 4 cards that are x8 or less.
https://www.netstor.com.tw/product_info.aspx?PID=PID_230116037118808

Thanks but instead of buying an external PCIE expansion, wouldn't we able to just achieve by just buying an external PSU and just connect the power to RTX4090, but connect RTX4090 PCIE to Mac Pro's PCIEx16 slot?
 

Matty_TypeR

macrumors 6502a
Oct 1, 2016
638
548
UK
I run a 4090 in my mac pro 7.1 with win 11. i use an external corsair SF-750 psu with corsairs own 4090 flat cable which i pass through a PCi slot on the rear of the mac. i use a PSU switch as per picture below to switch on the psu to power the 4090 when booting into win 11.



Has been working flawless for 6 months and the 4090 kick's in win 11. but when in Mac OSX i dont switch the 4090 PSU on. had no issues at all in OSX as the 6900xt does OSX.

In win 11 disable the 6900xt in device manager, no drivers loaded for AMD just the Nvidia drivers for the 4090. boots straight into win 11 and again had no issues.
 

LEOMODE

macrumors 6502a
Original poster
Jun 14, 2009
558
56
Southern California
I run a 4090 in my mac pro 7.1 with win 11. i use an external corsair SF-750 psu with corsairs own 4090 flat cable which i pass through a PCi slot on the rear of the mac. i use a PSU switch as per picture below to switch on the psu to power the 4090 when booting into win 11.



Has been working flawless for 6 months and the 4090 kick's in win 11. but when in Mac OSX i dont switch the 4090 PSU on. had no issues at all in OSX as the 6900xt does OSX.

In win 11 disable the 6900xt in device manager, no drivers loaded for AMD just the Nvidia drivers for the 4090. boots straight into win 11 and again had no issues.
Thanks for your reply! Two questions: Don't you need to connect your display cable back to 4090 from 6900xt in Boot Camp mode? That was cumbersome for me. Just FYI, you can still connect 6900XT in Boot Camp, but use 4090 in different apps, so even though the display is connected to 6900xt, you're still using 4090 for that app.

Also, where do you get that PSU switch as per the picture? Is it this one?

It'll be great if you can show the photo of how it's ran (Mac Pro opened, GPU inserted, cables connected, external PSU). That'll be super helpful!
 
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Matty_TypeR

macrumors 6502a
Oct 1, 2016
638
548
UK
the PSU switch can be purchased in many places, the display output isnt a problem for my monitor, it has Thunder port USB-C input, display port input and HDMi input, so i use display port for 6900xt and usb-C for 4090. 2 cables never have to swap any out.
 

LEOMODE

macrumors 6502a
Original poster
Jun 14, 2009
558
56
Southern California
the PSU switch can be purchased in many places, the display output isnt a problem for my monitor, it has Thunder port USB-C input, display port input and HDMi input, so i use display port for 6900xt and usb-C for 4090. 2 cables never have to swap any out.
Ah i see for the display. You basically just connect both GPU's to 2 separate cables onto your monitor. I have 3 LG monitors which supports 144hz via DisplayPort only and that monitor only has one DisplayPort so in order to achieve 95hz on Mac OS, I had to use DisplayPort. But I should definitely think about connecting both like you said (maybe DisplayPort to HDMI so I can just connect to Mac OS using HDMI to support 95hz, as it won't go beyond 120hz anyway for my Mac OS).
 

Matty_TypeR

macrumors 6502a
Oct 1, 2016
638
548
UK
You can purchase a display port switch, so you connect a Display port cable to both GFX cards, the switch has 2 display port inputs. then the output of the switch another display port cable to monitor. then use the switch to select GFX card 1 or 2 another solution.

 

LEOMODE

macrumors 6502a
Original poster
Jun 14, 2009
558
56
Southern California
You can purchase a display port switch, so you connect a Display port cable to both GFX cards, the switch has 2 display port inputs. then the output of the switch another display port cable to monitor. then use the switch to select GFX card 1 or 2 another solution.

Great information! Unfortunate that I can't connect to triple monitors all at once :(
 

LEOMODE

macrumors 6502a
Original poster
Jun 14, 2009
558
56
Southern California

LEOMODE

macrumors 6502a
Original poster
Jun 14, 2009
558
56
Southern California
I run a 4090 in my mac pro 7.1 with win 11. i use an external corsair SF-750 psu with corsairs own 4090 flat cable which i pass through a PCi slot on the rear of the mac. i use a PSU switch as per picture below to switch on the psu to power the 4090 when booting into win 11.



Has been working flawless for 6 months and the 4090 kick's in win 11. but when in Mac OSX i dont switch the 4090 PSU on. had no issues at all in OSX as the 6900xt does OSX.

In win 11 disable the 6900xt in device manager, no drivers loaded for AMD just the Nvidia drivers for the 4090. boots straight into win 11 and again had no issues.

This guy isnt recommending in facebook as he says it can break Mac Pro’s motherboard. Any reason why it would do that? Why would it when you turn it off when you are not using it and simply powering it on when you are using it on Boot Camp? I bought Corsair 850w psu.
 

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Matty_TypeR

macrumors 6502a
Oct 1, 2016
638
548
UK
Well i have had no issues with external PSU and why would it destroy the mac logic board? there is no way it can really. if it has no power when in Mac OSX it cant boot. also you are protecting the Mac 7.1 PSU as its power comes from external PSU. as i said no issues here from the set up and its been running sweet for over 6 months, nearly 8 months but of course the choice is yours.
 

LEOMODE

macrumors 6502a
Original poster
Jun 14, 2009
558
56
Southern California
Well i have had no issues with external PSU and why would it destroy the mac logic board? there is no way it can really. if it has no power when in Mac OSX it cant boot. also you are protecting the Mac 7.1 PSU as its power comes from external PSU. as i said no issues here from the set up and its been running sweet for over 6 months, nearly 8 months but of course the choice is yours.

Thanks that is helpful. One question, why are using a separate on/off switch if the Corsair psu already has an on/off switch?
 

tsialex

Contributor
Jun 13, 2016
13,144
13,332
Thanks that is helpful. One question, why are using a separate on/off switch if the Corsair psu already has an on/off switch?

All ATX power supplies, differently from AT PSUs, need a power on signal (PS_ON switched to ground) from the motherboard to start (and remove it to stop providing power). The PSU switch on the back is really a power cut-out and not a start the PSU one. When the motherboard switches the PS_ON to ground the PSU starts, the additional switch does just that.
 

LEOMODE

macrumors 6502a
Original poster
Jun 14, 2009
558
56
Southern California
All ATX power supplies, differently from AT PSUs, need a power on signal (PS_ON switched to ground) from the motherboard to start (and remove it to stop providing power). The PSU switch on the back is really a power cut-out and not a start the PSU one. When the motherboard switches the PS_ON to ground the PSU starts, the additional switch does just that.

Got it so if I understood you correctly even if the PSU switch is on it wouldn’t be on so I would need that extra switch to turn it on for the psu to be really on. And where do I connect this extra switch to? To the psu or to Mac Pro? But I can’t find that connector in Mac Pro so I am assuming on the psu right?
 

tsialex

Contributor
Jun 13, 2016
13,144
13,332
Got it so if I understood you correctly even if the PSU switch is on it wouldn’t be on so I would need that extra switch to turn it on for the psu to be really on. And where do I connect this extra switch to? To the psu or to Mac Pro? But I can’t find that connector in Mac Pro so I am assuming on the psu right?

Yes. The green wire (PSU_ON) when showed to ground is what makes the PSU to work.
 
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LEOMODE

macrumors 6502a
Original poster
Jun 14, 2009
558
56
Southern California
Well i have had no issues with external PSU and why would it destroy the mac logic board? there is no way it can really. if it has no power when in Mac OSX it cant boot. also you are protecting the Mac 7.1 PSU as its power comes from external PSU. as i said no issues here from the set up and its been running sweet for over 6 months, nearly 8 months but of course the choice is yours.

So I cannot close the Mac Pro because I need to connect the cables from outside after they are closed. But as you can see in the photos the original connector that came with RTX4090 is too short as it is connected to the GPU power very closely. How did you achieve this? Can you share how they are connected on your build? You also said you just used the cables that came with Corsair so are you talking about the 3rd photo? Would that be enough?
 

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Matty_TypeR

macrumors 6502a
Oct 1, 2016
638
548
UK
If you look at the picture i posted above that cable has the 4090 12 pin connector on it, you don't use the 4 x 8 pin adapter cable that comes with the 4090. look at the picture you will see the 12 pin on one end and 2 X 8 pin on the other. this is a genuine Corsair 4090 power cable purchased from corsair. as the cable is flat it allows you to close the case easy.

your pic shows the 4 x 8 pin nvidia cable adapter. look at the length of the cable in my pic above.
 

LEOMODE

macrumors 6502a
Original poster
Jun 14, 2009
558
56
Southern California
If you look at the picture i posted above that cable has the 4090 12 pin connector on it, you don't use the 4 x 8 pin adapter cable that comes with the 4090. look at the picture you will see the 12 pin on one end and 2 X 8 pin on the other. this is a genuine Corsair 4090 power cable purchased from corsair. as the cable is flat it allows you to close the case easy.

Oh so what you're saying is you had to buy a separate Corsair power adapter, instead of a lot of cables that came with Corsair PSU (in my case I bought the 850w one)?
 

Matty_TypeR

macrumors 6502a
Oct 1, 2016
638
548
UK
thats right a new 4090 power cable from corsair. Only 1 cable comes through PCi slot at back, the corsair power cable.
 

LEOMODE

macrumors 6502a
Original poster
Jun 14, 2009
558
56
Southern California
thats right a new 4090 power cable from corsair.

Got it, would any of the below work you think? I don't think Corsair ones would really matter as long as they fit, but I do see a variety in the end cables.
 

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