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So I built a new mining rig w/ (4) 7970s, specs are as follows:

i3-3225
Gigabyte G1.Sniper 3 mobo
8GB DDR3
(4) Gigabyte HD7970s (model # GV-R797TO-3GD)
1200W Coolermaster Silent Pro Gold ps

So I had it all running in a case, but it was too hot.  So I took it all out and just had it laying on a rack, and it ran fine.  Problem was, it was still a little too hot.  So I bought (4) PCI-e riser cables (16x to 16x) and tried to use those.  Now, on my mobo (which you can see here (http://www.newegg.com/Product/Product.aspx?Item=N82E16813128552)). I have (4) PCI-e 16x slots.  When you use all four slots, they become 8x slots.  So I set it up so the first slot (very top) had a card plugged directly in, 2nd slot had PCI-e riser to a card, 3rd slot had a card plugged directly in, and 4th slot had PCI-e riser to card.

Now I know all the cards work fine, and I know all the slots work fine because of how I had it setup before.  With it setup this way, all but the 2nd slot work fine.  For some reason, a PCI-e extender cable will work in the 4th slot, but not the 2nd slot.  Now I thought it was just a bad cable, but after trying them all, it still won't work.  Even the cable that works in the 4th slot won't work in the 2nd slot.

Anyone have any ideas?

How about split that rig into 2 rigs - you're trying to do too much with a single machine.

I believe it has to do with an increased resistance from the additional cable length. Basically with 4 cards you are close to the max 12v that can be pulled through the motherboard through the pci slots. When adding those pci-e extenders you go over the limit. So splicing in a molex into the wires that carry 12v on the ribbon cable should work

some people also say increasing a pci latency setting in bios helps but I've never done it

remember to short pci-e presence pins if you are having problems. also some boards shut off pci-e slots if other pci-e slots are populated. i have a msi z77a-g45 and it will run with 6 gpu cards no problem.

Dump the GV-R797TOs. They are running hot anyway. Because their heatsink is thin (not even 2 slots).

which ones are the best ?

Anything after 3 cards needs powered extenders. That will solve your problem.

sorry to bump an old thread, FullLife,

I was wondering what you finally did to get all 4 working with risers?

I didn't know that you could have pci powered extenders until I saw this thread and this:

http://cablesaurus.com/PCIe-x16-Extender-Cable-w-Molex-Connector (http://cablesaurus.com/PCIe-x16-Extender-Cable-w-Molex-Connector)

I think this was the exact problem I am always having. sometimes all 4 cards work, other times the bios only sees 2 or 3, sometimes it doesn't want to load at all.

Now that I think it through, i have 1 slot that only runs at x16 and won't drop to x8.  If I use only one card, with just a riser plugged into this slot it still won't boot properly, but sure enough, if I plug the card directly into that slot it works like a charm.  Then I tried plugging one card into another slot that can drop to x8 with the riser by itself and that works, but then I was having trouble with all the other combinations, and I think the powered riser might be the solution.

edit: hell this might even be the reason my cards aren't able to reach their full potential now that I think of it, but it could be wishful thinking

there is a tread that tell's you how to make your own powered riser by addind a molex. ;D

i just found that thanks!  so i don't want to get my hopes up but it seems like that is exactly what problem I am having.

it's also more possible i will have problems but my dumba** decided to get 12inch risers because i didn't think I'd have enough space for the 19cm(7.5inch) ones which seem to be standard

I fixed the problem by switching the 4th card from a 16x - 16x extender to a 1x - 16x extender.

can you run a card with full potential with a 1x to 16x riser?

Yes, no card is powerful enough to oversaturate 1x. I'm not even totally sure why 16x exists tbqh when all the tests show that 4x handles the most powerful cards of today with no trouble.

For mining 1 pci-e lane is sufficient for each GPU. Bandwidth isn't a problem.

Also, just 1 Gb of slow cheap memory + the cheapest Celeron CPU is all that's required. Unless of course your using the rig for other things.

so with the ribbon risers (maybe powered?) it doesn't matter how many x16 slots you have because I can just put in more cards into the 1x slots?

I am really cheap and my mining mobo only has two x16 slots so with a simple psu upgrade this is great news  :D

I guess now my question is this- How do I know if I need to buy/make any risers powered?

thanks