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Hey guys,

I am building my first rig, 6 GPUs Gugabyte R9 170, with a Gigabyte motherboard and 2 PSUs Evga Nova Gold, each PSU by 750 watts.
The PSUs are connected between them with an add2psu module.
Now i am wondering how to connect the cards.

So far I have connected all the stuff this way:

1. 1st PSU connnected to MB
2. 2nd PSU connected to 1st PSU via add2psu
3. 1st PSU connected to 3 of the cards with common main cards cables
4. 2nd PSU connected to the other 3 of the cards with common main cards cables
5. 1st PSU connected as well to 3 of the cards with the molex of the riser cables.
6. The 3 cards connected to the second PSU are not connected with their molexes to anything, just with the main cables to the second PSU.
7. If this also is important, one of the cards is directly put into the MB, the other 5 are connected via risers.

Is this correct and if not what should I consider as a change?

Any advice welcome and thanks in advance  :)

You are right. However you can connect ALL the molex to the main PSU. Never connect the molex to 2nd PSU (if they are not cut).

I have almost the same problem, running 6pcs 270 cards with 2PSU's
https://bitcointalk.org/index.php?topic=457287.msg5043351#msg5043351

Problem is that my 270's only have one 6pin-connector on the pcb, this allows 75W. The 270 needs about 150W each so the other 75W needed has to come from motherboard/powered riser. That means in my case 6 x 75W = 450W from molex on the risers, and most PSU's only have the molex/sata connectors on 1 rail, and very few PUS's allow 450W on 1 rail. Therefore i need to split the 450W load on 2 PSU, but even 225W is not possible on my PSU (molex on 1 rail, allowing 18Amps, 12V*18Amp = 216W, wich means im 9W short...?)

I belive i need to buy a new PSU with a singel 12V rail, like the Corsair CX750, 62AMP on 12V = 744W on 1 rail.

The solution is to buy a single rail PSU.

You can also undervolt.

What about only 3 cards of 6 being connected woth powered molex, is it OK or I need to connect all 6? AFAIK 3 GPUs can sometimes run without powered molex, so is it OK?

Yes i belive this is my solution? I will buy a CX750 today.

6 x R9 270
# 1 - Corsair CX750 (1x rail 12V = 744W)
# 2 - Fractal R2 750W (2x Rail 12V * 30amp = 360W * 2 = 720W)

# 1 - Corsair CX750
* Motherboard 150W
* 2 GPU = 4 * 75 = 300W
* (2x + 6pin 2molex)
* Fans 10W
(total about 62% of 750W)

# 2 - Fractal R2 750W
* 4 GPU = 8 * 75W = 600W
* (4x 6pin + 4molex)
(total about 80% of 750W)

If you have enough molex connectors from the PSU, then connect all 6. It is OK to connect only 3.

You have to connect ALL the riser molex to the main PSU.

# 1 - Corsair CX750
* Motherboard 100W
* 1 GPU = 1 * 75 = 75W
*
  6 PCIE slot
  (4+2)*75W = 450W

* Fans 10W
(total = 100+75+450W +10W = 635W 85% of 750W)

# 2 - Fractal R2 750W
* 5 GPU = 5 * 75W = 375W
* (5x 6pin + 0molex)
(total about 50% of 750W)

Like this?

https://i.imgur.com/PfCVNKm.jpg

The only problem i have then is regarding PSU #2, the Fractal R2. It has 2 rails and limited connectors, im thinking like this:

Rail #1 - 30amp*12V=360W
1x pci-e (1*75W)
6x molex -> 3x pci-e 6pin (3*75W=225W)
= 300W for 4pcs 6pin

Rail #2 - 30amp*12V=360W
1x pci-e (1*75W) -> 2 x 6pin (will make this converter myself)
OR
1x EA12V -> 6pin?

Your picture is right indication of the set up.

For fractal R2, you can use the 4*6pin from Rail1 and one from the EA12V->6pin.  I do not know the distribution of power between rails. I just assume Rail 1 is the 4*(6+2)pin+peripheral, and Rail 2 is the MB connector +CPU12V.

It is better to use two CTX750 if you have not bought the Fractal.

OK, so I connected finally everything, and started the motherboard and the 2 PSUs. The results is that only 2 of the 6 cards start up and while the monitor is plugged into one of them, it doesnt show nothing, just black screen. Anyone  ???

Does the GPU fan spin?
Does the 2nd PSU fan spin?

Yes, exactly, only 2 of the 6 GPUs fan spin. And I exactly plug my monitor into one of them.
And yes, both PSU fan spin.
Also if this helps, the two cards which'w fan spins are the ones connected to the second PSU and with 1x/16x cables to the MB.

If the 2nd PSU fan spins, that means there is power out of it. So it is working.

Can you remove all the 5 cards from the 2nd PSU and add them one by one?

Can you also check if there is screen display when there is only one card connected to the main PSU?

OK, I unplugged all card except to one which is currently connected only to first PSU with 6/8, riser molex is unplugged. This time only this card started (it is different). I plugged the monitor into it, but still, black scree only.

Can you connect the card directly to the slot? We want to make sure the MB is working.

If you see the screen display, then use the riser with molex.

MB is working, however, I suspect that problem may be that I am using an old monitor with VGA to DVI-I adapter...

Edit: Hm, thats interesting, OK, I really gave it a try with the only one card connected directly to the MB without riser and the card doesn't start up!  :o What should that mean?

Do not use the 2nd PSU.

Try other cards.

If none of the cards are working, then the system (MB) is not working. Can you use a multmeter to measure the voltage output from the PSU? The 3.3V, 5V and 12V.

How shall I do this (the measurement)?

OK, I just did the test on the other rig, only 1 card, connected directly to MB slot, and with 6/8 to 1 PSU. Again, not starting. I think the chance both MB or cards not to be working is 0% so obviously should be some misconfiguration of the hardware...  ::)

If you ask this question, I assume you have not used it before. So do not bother.

It is better to work from the beginning.

Disconnect all unnecessary parts. Just connect the MB, keyboard and mouse with memory, GFX card no hard disk. You need to connect the CPU +12V as well.

Power on the system to see if there is on screen display. Try with different monitor.

You can use the Fractal PSU as the main PSU as well.

Try other cards.

Debug update: When the molex of the card is connected to the PSU, the card doesn't run.
Also the card doesnt run when directly connected to MB.
It startups only when connected with riser cable whose molex is not connected to PSU.
Any ideas on that?

When you say startup, does it mean the card has display? If so, card and PSU is working.

Does it happen to all the cards?

You can change the risers to see if it is faulty. If there is display with other risers, then the original one is not working.

Any card and any riser. Card wont start up if directlyconnected to MB. It starts if connected via riser without molex. By "start" I mean fan starts spinning.

I have now purchased and installed a Corsair CX750 as the main PSU, everything connected as desribed above. But the damn 2nd PSU, Fractal R2 keeps shutting down on boot. Its only loaded with 4x 6pin pcie (4*75W=300W) so its not overloaded. Can the problem be that it needs some load on 5V aswell? I remember something like that from back in the days.

"Many PC PSUs (not all) require a base load on the 5V rail to work properly. Connecting an old CD drive or a 12V light bulb (into the 5V rail, see picture) are well proven methods to get this load."
http://reprap.org/wiki/PCPowerSupply

Discussed here https://bitcointalk.org/index.php?topic=50178.0

You can try a different kind of GFX card, if it does not work, then motherboard/memory, cpu might be faulty.

When the system boots, it does not use much power, so it might not be unbalance of the rails.  You can verify that by swapping the PSU. If it does not shut down on boot, then unbalance is the reason.

You also remove some cards from 2nd PSU, and add one after the other.

OMFG i finally got it to work by connecting an old DVD-rom to the molex on the 2nd PSU (Fractal), all to get som load on the 5V-line. Its now hashing with 6 GPU´s for 30min!