I cannot get the following setup working on the BB Marshall ( and I suspect with the P8P67 WS Revolution as well ) :
When using only the two extended cards, is the rig stable? What about three cards? Have you tried using those cheap pcie1x-pcie16x powered extenders?
Are your pcie16x-pcie16x risers externally powered?
If they aren't, you may be drawing more juice with those four GPUs than the board is able to supply.
If the rig works fine with less cards but fails at four, make sure you use powered extenders.
Retuening to PSUs, do you have another PSU you could try?
The AX1200 is great but stuff happens. A few failed capacitors may result in elevated ripple levels.
The only problem is, you'd need a local electronics guy with an oscilloscope to be able to say for certain whether or not the PSU is to blame.
Much easier to just use another PSU.
The rig does not work even with just one 5870 connected with an extender. I just get black screen at Linuxcoin.
If that same 5870 is used but in the slot it works fine.
If 4*5870s are used directly to the mobo it works perfect.
If 3*5870 are direct on the mobo and just one 5870 uses an extender Linuxcoin fails to start.
Basically whenever I use just one extender on the BB Marshall it makes the driver somehow crash so none of the other cards also work.
Tried with Linux, Debian, Windows etc. BAMT and same problem everywhere. I got multiple AX1200s and BB Marshalls so it really is strange.
BIOS works fine all the time with extenders. Open source radeon driver works 100% as well and all the cards are detected in lspci and aticonfig --list-adapters.
Xserver does not work and I get black screen all the time even with just one card and one extender and even with SSH any X application does not launch at all.
My personal belief is that there is some extra latency and the aticonfig / fglrx driver module does not properly initialize when using an extender but I am not 100% sure.
I am pretty sure it is not a power problem because even powered extenders do not work. It is either a software ( fglrx issue / bug ) or a mobo issue I think.
I can post detailed dmesg and other logs if needed.
Thank you for trying to help !