Quote from: digitalindustry on May 27, 2013, 05:44:08 PM

Quote from: mistfpga on May 25, 2013, 01:43:33 AM

Quote from: ISAWHIM on May 14, 2013, 04:30:30 AM

The "ground" is all the same, for everything. You are getting ground from the PCIe pins, and also from the 6-8pins above. (Thus the two extra ground wires in the 8-pin, which are just the two from the 6-pin, shared.)

That is the "common" = "-12,-5,-3,-1.5" all in one. Also the "Frame" and anything-else metal in the computer.

That is fine if you use a single psu. ground is ground is ground. makes no odds, and the slots are spec'd high enough on most mbs to happily allow 75 watts grounded into the socket without issues.

However, my situation is a little different:
I use a 250 watt seasonic to power the motherboard and pci-e slots. the psu has 22amps on the 12v.
I use a 750 watt seasonic psu to power the cards.
In my 4 slot motherboards (4 x 16)
Whith these motherboards I use a slightly higher spec psu (again a seasonic,14a and 15a over two rails)
I am using underclocked semperons
A very small ubuntu install (copied from bamt)

Now no matter what psu I use for the motherboard, slot 4 never gets enough juice if all 4 slots are full. (hashrate is normally rocksold 492.1) add a 4th card in slot 2 (0-3) this now hashes full speed and the 4th card jumps between 390 and 441. So I need a powered riser. But I need to connect the grounds up on the riser, I cannot risk the 750 watt psu grounding into the 250.

Do the 12v and 3v have different grounds? (there are 3 ground on the front notch) is each ground for something else? I guess it wont take too long to workout, an inline multimeter should do the trick. I only have two old cards though, still hopefully it wont blow the card.

Does this sound reasonable? do you know anyone who has a schematic for a card with ground connected on the riser. in the pic on cablesaurus it looks like B4 and or B7 leaving out A4. although B4 and A4 makes slightly more sense, but this is just guesswork. am I trying to fix an nonproblem (letting the card ground from teh 750 into the 250 - every fibre of my being says no)

cheers

steve

i'm really gald i came across this - i'm in almost excatly the same situation - but i have two large PSU Maxrevo 1500w so i can just use one for the mobo and say 5 cards if i get them up - with un-powered on an z77a-gd65 only 4 cards would ever work. and i'm not even sure powered risers will help , but they are the best chance and it should be fun to try {burning smell}

but its an important point for multi PSU - so if anyone knows that would be great - i was thinking about mixing actually the cables and being a little tricky .

what about this :

1. Running PSU one that is jumped to say 3 cards , PLUS the 4 or 6 pin on the Motherboard ?

2. Then with PSU 2 plug the 24 pin in to the motherboard - and two or 3 more cards.

each PSU is connected to a common ground then.

I was thinking of this too, use the cpu connector on the psu powering the gpu 6 pin connectors, but I think that is its own circuit. my other idea was to use the 20 pins of the 250 seasonic to power the atx and use the cpu 4pin, then use the 4pin extra atx (the bit that actually powers the pci-e) off the big psu (I have not checked what the other pins are and if this is plausible)...

However, I plugged a 6950 in to a 1x riser with the back drilled off, this was not attached to the motherboard. I then confirmed that B1, B2, B3, A2 and A3 are all on the same circuit (using a voltmeter set to ohms) then verified all the ground are connected. I then striped these wires and soldered onto a 20awg. I took only the three grounds in front of the slot (B4, B7 and A4) and soldered these to a 20awg wire. put a molex connector on the end (female plastic, male pins).

Now, no matter which psu I attach it to, it only grounds through that psu, as far as i can tell. (I am now using a 430 antec neo as the mb psu because it is more likley to be able to handle the overcurrent/wattage) The 430 antec (powering 24pin atx, 3 pci slots and 4pin cpu) did not pull more any more watts than if the 4th card was not connected. Also the volts stayed ~240v - so it would appear to work. And if I did add it to a molex on the 430 antec psu the psu would auto shutdown after 3-4 mins when the fans started to make proper noise. All hashrates were rock solid though.

NOTE: the antec is only rated for 384 watts on the 12v it auto shutsdown around 405 watts. (it was hitting this)

I would not advocate anyone try this, I still have a lot more experimentation to do, but I need to wait until I can get access to better testing equipment and mike (who actually knows real things about proper electronics - I just guess Wink

) and to be completely honest I am not that sure I am interpreting the results correctly. I will find out tomorrow...