System constantly reboots with 4x 7950, but not 3x

[kokojie]

I have a 4x 7950 set up, temperatures below 85C, 1200W PSU, mining with cgminer on win7. The system always reboots after mining for a few minutes. But it won't do it if I disable one 7950 and mine with 3x 7950.

What's the problem?

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[Marrs]

Are you using powered risers? If not, and your motherboard isn't a newer board that expressly supports quad SLI/Crossfire, your GPUs may not be getting enough power through the PCI-E slots. The fix would be to use at least one powered riser.

Another possibility is that your PSU has multiple rails, and the one that you have all the GPUs on doesn't provide enough power. If that's the case, put the fourth GPU on the other rail using molex adapters.

[kokojie]

Are you using powered risers? If not, and your motherboard isn't a newer board that expressly supports quad SLI/Crossfire, your GPUs may not be getting enough power through the PCI-E slots. The fix would be to use at least one powered riser.

Another possibility is that your PSU has multiple rails, and the one that you have all the GPUs on doesn't provide enough power. If that's the case, put the fourth GPU on the other rail using molex adapters.

I'm using Risers, but not powered ones, 3 are on riser, 1 is plugged in on the board. So in theory, wouldn't it still reboot if I only mine with the 3 on the riser? but that's not the case, even when I mine with the 3 on the riser, the system does not reboot. But once I mine with 4, even though the other 1 is plugged into the board, the system reboots?

btw, I have no idea whether my PSU has multiple rails, this is the one I have:
http://www.amazon.com/gp/product/B003O8J11Y

this is the 7950 I'm mining with:
http://www.amazon.com/gp/product/B007581QHG

[Marrs]

The problem is that the motherboard itself is unable to draw that much power from the psu through the normal atx connectors to pass to the pcie ports.

Motherboards that support four gpus have an extra port to plug in the psu, so they don't need powered risers for the fourth card.

For a motherboard that doesn't have that, you need powered risers after the third card.

[kokojie]

The problem is that the motherboard itself is unable to draw that much power from the psu through the normal atx connectors to pass to the pcie ports.

Motherboards that support four gpus have an extra port to plug in the psu, so they don't need powered risers for the fourth card.

For a motherboard that doesn't have that, you need powered risers after the third card.

Thanks, what website can I buy powered risers from in the US?

Also, would something like this help, instead of buying powered risers?
http://www.youtube.com/watch?v=ItvBnffj1No&hd=1

[Marrs]

Thanks, what website can I buy powered risers from in the US?

Also, would something like this help, instead of buying powered risers?
http://www.youtube.com/watch?v=ItvBnffj1No&hd=1

http://www.cablesaurus.com/ or ebay.

Yes, that power booster should work, too.

[e521]

kokojie are you mining sha or scrypt?
what's your stable hash/config with 3 cards?
please check this https://bitcointalk.org/index.php?topic=188308.0
thanks

[kokojie]

kokojie are you mining sha or scrypt?
what's your stable hash/config with 3 cards?
please check this https://bitcointalk.org/index.php?topic=188308.0
thanks

I'm mining scrypt, I can stably mine at 620 mh with 3x, I overclocked only a tiny bit. core to 1060, mem clock to 1335. If I mine with 4x, doesn't matter if I overclock or not, it always reboots.

[e521]

kokojie are you mining sha or scrypt?
what's your stable hash/config with 3 cards?
please check this https://bitcointalk.org/index.php?topic=188308.0
thanks

I'm mining scrypt, I can stably mine at 620 mh with 3x, I overclocked only a tiny bit. core to 1060, mem clock to 1335. If I mine with 4x, doesn't matter if I overclock or not, it always reboots.

ok thanks I will try these settings.
I will let you know  if I can put 4 cards on msi mobo without powered risers

[kokojie]

kokojie are you mining sha or scrypt?
what's your stable hash/config with 3 cards?
please check this https://bitcointalk.org/index.php?topic=188308.0
thanks

I'm mining scrypt, I can stably mine at 620 mh with 3x, I overclocked only a tiny bit. core to 1060, mem clock to 1335. If I mine with 4x, doesn't matter if I overclock or not, it always reboots.

ok thanks I will try these settings.
I will let you know  if I can put 4 cards on msi mobo without powered risers

I suggest you don't try it, I burned my motherboard before on a gigabyte mobo, I didn't know the reason, now after research, I know it's because I put 4x gpu on it. Like the good poster said on top, you'll need powered molex to the pcie bus, to have 4x gpu, so unless your mobo provides this function, it can be burned if you try it.

[e521]

my mobo (msi  z77a-g45) doesn't reboot and it recognize 4 cards without shorting but it does only 550kh per card at 900/1250 -I 20