3x 7950 rig PCI problems

[Kinetic915]

Hello all,

I just built my first 3x 7950 LTC mining rig.  Everything was perfect, but the cards were directly connected to the board, obviously a heating issue.  I ordered 3 PCIe x16 extender cables, and installed them.  Upon booting with the cables I was greeted with a host of problems, the board would not detect the cards and wouldnt post, when I finally did get through to windows I would continutally get the blue crash screen.  I disconnected the cables and am now running 2x 7950s with complete stability.  Question is, is this a normal occurrence with these types of cables?  Did I just get a horrible batch? Any expereince along these lines appreciated!  Thank you!

[lakeluke]

What mb are you using?

[Kinetic915]

Ah sorry

Celeron CPU
CORSAIR Vengeance 8GB
ASRock Z77 Extreme6
CORSAIR 850w
Sapphire 7950 x 3

[Kinetic915]

i just saw on cablesaurus they sell powered extenders? Is that a possible problem?

[nightminer]

Just one of the pci extenders could give you a problem. Did you try running 2 card directly connected and then try running 1 on one of your PCI extenders. If you boot up and get the same problems, change that 1 PCI extender and try another one. Keep eliminating each component.

If that fails repeat the entire process, but this time use a different PCI-e slot on the motherboard.
 

[Kinetic915]

I've been trying that and I seem to have the same problem with every extender.  I'll double check though.  These do not have any supplemental power going to them, could that be an issue here?  So the cables should be generally reliable then? Also other question, can the PCIx1 -> x16 adapters actually be used to incorporate another graphics card?  How stable are these?

Thanks again?

[nightminer]

Wait a minute. I didn't see you only have a 850W power supply.

I killed my 550W power supply running a bare minimum system on one 7950. I had a 620W power supply spare and that solved the problem. My card takes 2 PCI-E connectors at the top, both were connected. My "symptoms" were:

- I could boot and run the system fine
- I could mine for about 5 seconds
- I used the "sensors" tool that comes with linux that monitors voltages and I could see right before it failed that my 12V would drop to 10 V and then the system died

According to this page

http://www.anandtech.com/show/5476/amd-radeon-7950-review/16

under load the card needs 353 Watts. So running the 3 cards alone (no motherboard and no CPU) you already need 1059 W! I'm not an expert with running 3 cards, I have never seen someone suggest such a huge CPU. Would be nice if someone can correct me.

[Kinetic915]

Wow didn't even think it would be drawing that much power :/  When I was researching I found people running that setup at 1800 hash/sec at 650 watts.  But my problems do seem like they could be solved by that!  I will try that thanks again!

[Shaban]

nightmare, I am building a rig also (not trying to thread jack) and I'm putting 3 7950s also, I'm using a 1200w bronze rated PSU... would that be enough?

[Kinetic915]

http://support.asus.com/PowerSupply.aspx?SLanguage=en

heres a simple calculator I found

[paul21]

You can get the cards to run at 925/1250/0.975v at under 200W each. If you're overclocking expect 75+300 watt maximum.

To troubleshoot, try just one card and one extender then swap cards and extenders. If that passes then I would blame the mono or psu for low 12v bus power. Check the levels in the bios.

[btcfanx]

Do you use a "PCI-e short cable"?

http://blog.zorinaq.com/images/pcie-short-schematic.png

[Kinetic915]

Gotcha,  I have no idea how that guy got 3 to run at 1800 at 650 watts.  Is running a board around 1000+ generally a fire hazard/in danger of blowing a breaker?

I am not using the short pcie, full x16

[grottenolm]

Since you have this motherboard:

http://www.asrock.com/mb/Intel/Z77%20Extreme6/?cat=Specifications

You should have those slots:

- 2 x PCI Express 3.0 x16 slots (PCIE2/PCIE4: single at x16 (PCIE2) / x8 (PCIE4) or dual at x8/x8 mode)
- 1 x PCI Express 2.0 x16 slot (PCIE5: x4 mode)
- 1 x PCI Express 2.0 x1 slot

From my perspective (having done PCIe signal quality measurements etc. already) any kind of extender cable are potentially trouble for both signal quality and power supply. Not being detected at boot time sounds a bit more like the former. But fortunately for bitcoin mining you don't need much bandwidth. So here is my suggestion:

Check in BIOS Setup (or rather UEFI Firmware Setup here) if you can set the PCIe mode to the slowest Gen1 speed. This should at least give you the highest chance of success, with no impact at all on mining speeds.

[Kinetic915]

ah gotcha I will try that thanks!  Would this work for LTC mining, which is what I will primarily use this rig for.

[nightminer]

Quick question. What happens if you run just 2 cards with the extenders?

[Kinetic915]

I tried with 2 and could not get the mobo to recognize the cards.  I did not try the Gen 1 trick suggested.  I will try some more troubleshooting today after work
Thanks again!

[Kinetic915]

Bought a 1200 w power supply and also changed to pci gen 1 in bios settings.  Seems to work perfectly! have all cards working thanks everyone

Also any suggestions on getting a 4th card running on this board?

[grottenolm]

Bought a 1200 w power supply and also changed to pci gen 1 in bios settings.  Seems to work perfectly! have all cards working thanks everyone

Also any suggestions on getting a 4th card running on this board?

Great to hear, congrats!

I mean you have 4 PCIe slots, so 4 cards should be fine... what exactly is the problem right now?

[yyrkoon]

Since you have this motherboard:

http://www.asrock.com/mb/Intel/Z77%20Extreme6/?cat=Specifications

You should have those slots:

- 2 x PCI Express 3.0 x16 slots (PCIE2/PCIE4: single at x16 (PCIE2) / x8 (PCIE4) or dual at x8/x8 mode)
- 1 x PCI Express 2.0 x16 slot (PCIE5: x4 mode)
- 1 x PCI Express 2.0 x1 slot

From my perspective (having done PCIe signal quality measurements etc. already) any kind of extender cable are potentially trouble for both signal quality and power supply. Not being detected at boot time sounds a bit more like the former. But fortunately for bitcoin mining you don't need much bandwidth. So here is my suggestion:

Check in BIOS Setup (or rather UEFI Firmware Setup here) if you can set the PCIe mode to the slowest Gen1 speed. This should at least give you the highest chance of success, with no impact at all on mining speeds.

Exactly my thoughts. PCIE is not meant to truly be extended externally. So when putting extender cables on the cards, you're potently adding noise. Signal integrity is very important at even PCIE 3.0 1x speeds.

As for you power usage, I would not take anyone word on how much of a power supply you really need. Buy a KILL-A-WATT meter, plus in one card, power up the system full load, check power consumption, and compare it to idle. Multiply difference by 3x, and add ~20%.

Also so keep in mind most switching power supplies ( Computer/PC type ) operate most efficiently at 25%, 50% or 75% load. So with that in mind, when adding 20% dont be afraid to add a little more if it means matching efficient load statistics. Also for what it is worth, most PSUs are the most efficient at 50% load, but always check OEM specs before making that judgment call. As some do vary.