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.

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

IMHO if they are both connected to the same outlet / extension cord then it shouldn't be any problem but I would like somebody else to chime in here and confirm because I am no expert. Just what I have heard on the forum regarding multiple PSUs and one GPU being powered from 2 diff PSUs.

petala

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