I'm having issues with my Powercolor RX480 Red Devil GPUs. I can't seem to get more than 2x cards running per system. I have identical systems with Reference RX480s and they run fine with 4x GPUs.
What I've tested so far:
All GPUs work fine on their own plugged directly into the motherboard.
All slots on motherboard work fine with PCIe risers using the Reference RX480s.
Motherboard has 4x PCIe slots (2x full length 16X and 2x short 1X). All 4x work fine on the reference RX480s
I can get 1x Red Devil RX480 card to work in any slot with a USB riser cable. If I plug in two cards it either won't start (the PSU relay just clicks but doesn't turn on. Tried another PSU, does the same) Or if I get it to boot, it won't load the driver for the second card and just detects it as a "Microsoft Basic Display Adapter". On one system I managed to get two to work, but it seems very unstable.
I've split up the 8pin power connectors so each card gets its own power rail on the PSU. It should be more than enough as the PSU supplies 40A per rail x4.
What worries me is that the powered PCIe usb risers are all connected on Rail 1 via the 4pin Molex connectors. But this should still be fine as the majority of the PCIe power drawn happens on the 12V pin, and the 5V that the powered risers convert to 3.3V should not use more than 7W per card ( on the Reference RX480 ) as shown here:http://www.tomshardware.com/reviews/amd-radeon-rx-480-polaris-10,4616-9.html
So I should be safe on the 5V. The AIC1084 chip that does the voltage regulation from 5V to 3.3V is rated to 5A.http://pdf1.alldatasheet.com/datasheet-pdf/view/54861/AIC/AIC1084.html
This leads me to suspect either my powered pcie usb riser cables, the motherboard or the PSU rails (Rail 1 might be overloaded from the powered risers).
I've also gone through the PCIe usb risers, testing everything for shorts or that the pins are connected where they're supposed to be. The risers work fine with the reference RX480s.
MSI Mortar B150
4GB of RAM (tested)
Windows 10 64bit
Antec High Current Pro HCP-1000 Platinum 1000W 80PLUS Platinum.http://www.antec.com/product.php?id=704514&fid=343&lan=us
+3.3V +5V +12V1 +12V2 +12V3 +12V4 -12V +5Vsb
MAX 25A 25A 40A 40A 40A 40A 0.5A 3A
Total ===130W=== ---------- 1000W-------------- 6W 15W
Please help... tearing my hair out in this process of elimination.