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LEOMODE

macrumors 6502a
Original poster
Jun 14, 2009
558
56
Southern California
FULL DISCLOSURE, TRY THIS AT YOUR OWN RISK. THIS IS NOT AN OFFICIAL METHOD NOR ENDORSED BY APPLE, NVIDIA, AMD, OR ANY OTHER COMPANIES. I AM NOT RESPONSIBLE FOR YOUR ACTIONS AND THE CONSEQUENCES OF THIS POST.

This can work for 5,1 too I assume.

Introduction
Mac Pro 7,1 has enough juice to power up 2 GPUs or even 3 GPUs depending on the GPU's power draw. Starting RTX4090, it required 4 power cables made by Belkin AUX Power Cable Kit for Mac Pro, which will take up both spaces in Mac Pro supposedly to accommodate 2 non-MPX compliant GPUs (e.g. Nvidia cards, AMD non-Apple cards).

I have 3 GPUs. Apple's AMD W5700X which doesn't require the Belkin AUX Power Cable Kit for Mac Pro kit, AMD 6900XT which requires 2 power cables to be connected to Mac Pro, and RTX 4090 which requires 4 power cables to be connected to Mac Pro. In order to run both AMD 6900XT and RTX 4090, the only way possible is to run an external PSU to power up RTX 4090 and use Mac Pro's 2 cables to connect to AMD 6900XT.

I created an initial post asking how to connect. Matty_TypeR provided helpful instructions on how to do this, but because his machine is already installed and cannot be taken out to take photos of the machine setup, I thought it will be helpful for everyone to show my step-by-step instructions here.

Required Items
Your mileage might vary, but below are the parts that worked for me, so feel free to try out different parts if necessary.
* Belkin AUX Power Cable Kit for Mac Pro (to connect AMD 6900XT)
* 24-Pin ATX Red LED Power On/Off Switch (to start up the external PSU, while the PSU itself has power, it actually doesn't turn on the PSU, so you not only need to turn on the PSU switch but also turn this Power Switch in order to fully turn on the PSU)
* Corsair 750 Watt PSU (external PSU, since the PSU is only using the GPU it will meet the GPU's power limit)
* Corsair-only compatible 12+4 pin to Dual 8 pin cable (to connect the RTX 4090 to the external PSU)

Step-by-step instruction
1. Connect PSU to the wall outlet (do not turn it on yet)
2. Connect the 24-Pin ATX Red LED Power On/Off Switch to the PSU (do not turn it on yet)
3. Install AMD 6900XT and RTX 4090 in any PCIEx16 slots
4. Connect AMD 6900XT to Mac Pro PCIE slot and connect Belkin AUX Power Cable Kit for Mac Pro
5. Connect RTX 4090 to Mac Pro PCIE slot and connect Corsair only compatible 12+4 pin to Dual 8 pin cable
6. When you close the Mac Pro case, make the cables go around the case so it can go through the PSU
7. Connect Corsair-only compatible 12+4 pin to Dual 8 pin cable to the PSU
8. Turn on the PSU and 24-Pin ATX Red LED Power On/Off Switch
9. Boot up Boot Camp. You can turn off Step 8 if you're booting up on Mac OS
10. Make sure to turn on Step 8 before you even turn on Mac Pro when you're going into Boot Camp mode, as although the RTX 4090 will be on, it will not detect in Windows and not work. It must be booted up first before you turn on Mac Pro.

Q&A
* I hooked up the PSU and turned the PSU on, but why isn't the PSU fan running?
As long as the 24-Pin ATX Red LED Power On/Off Switch is on, the PSU is on.

* I turned on the PSU but Windows is not detecting RTX 4090.
As mentioned in Step 10 under Step-by-step instructions, if Windows is already turned on, it will not detect RTX 4090. So make sure to restart Windows if you turned the PSU & on/off switch on.

* Why do you need another on/off switch when you already have an on/off switch on the PSU?
Quoted from tsialex: 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, and the additional switch does just that.

Will be updated more when questions are asked in replies.
 
Last edited:

flat4

Contributor
Jul 14, 2009
272
77
Need a video that would be awesome for the people who own one and they rest of us that like to watch
 
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