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Rampage VI Apex Quad GPU Not Detected x8 Slots

F1FTW3D
Level 7
Hi all,

I'm new here as I just picked up a new ROG Rampage VI Apex motherboard and am having a GPU detection issue. Hopefully you guys have the secret sauce to making it work! I did my best to research other posts, but couldn't find a solution other than RMA.

Issue

-GPUs not recognized if in slots 2 or 4 (physical x16, electrical x8)
-BIOS Shows "No VGA Detected" in either slot 2 or 4
-Computer recognizes/boots/works fine with a GPU in slot 1, 3 or both

Best Guess

-Is it possible this has something to do with CPU lanes? The 7740x only has 16

Troubleshooting (based on what I found in other posts)

-Flashed motherboard to latest BIOS (1004)
-Confirmed all GPUs work when in slot 1, 3, or both
-EZ-Plug molex power connected
-All PCI-x switches are "On"
-Cards physically reseated numerous times
-Both 8 pin power connectors confirmed plugged in on each card

Rig

Mobo: Asus ROG Rampage VI Apex
GPU: 4x XFX Radeon Vega 56
CPU: Intel i7-7740x (4-core, 16 PCI-e Lanes)
RAM: Corsair Vengeance LPX 2 x 8GB DDR4 2666 (installed in banks C&D - closer to main mobo power connector)
M.2 Drive: Samsung 960 EVO 250 GB (installed on PCH Dimm.2 - closer to main mobo power connector)
PSU: EVGA 1600 P2
Optical: LG WH16NS40 SATA Blu Ray Writer

Thank you in advance!!!

-F1FTW3D
3,365 Views
8 REPLIES 8

jab383
Level 13
Yep, you nailed it. The CPU ran out of lanes. Check page 1-9 in the User Guide. The 16-lane 7740X is limited to 2-way SLI.

Rats. I understand the SLI/Crossfire point, but if I just need the cards to work independently, is a new CPU still the only option? I thought I've read about cards working even if only at 4x.

Also what's weird is even if I have just 1 card in, if it's in an x8 slot, it's not recognized. Is the mobo assuming if a card is in an x8 slot, it's using over 16 lanes?

Menthol
Level 14
You need a 7900 or above chip with 44 pcie lanes to support 4 Nvidia cards, if you install the single GPU in pcie slot #1 or top slot it should be X16 unless you have an addon card in any other slot

All boards have limitations when using a 4 core Kabylake X CPU, so yes. you will find this information in the manual

hi i have the exactly same problem but with an 7800X eventhough that has 28 PCIE Lanes.

first i thought that my cpu has a problem an checked it with another Board, the MSI Raider X299 and there all PCIE Slots worked with my 7800X, so i thought it must be a failure of my Apex Board and changed it to another one.

But then to my suprise - the same thing again PCIEX8_2 and PCIEX8_4 are dead ! - so if not my new Apex is broken again it must be wanted by Asus to behave in that Way.

But in my opinion it is quite a downer an not easy to unterstand why a High End Board like the Asus Apex is not able to share the remaining free Pcie Lanes with all the PCIE Slots on the Board, where even one of the Cheapest X299 Boards like the Msi Raider (that cost half the Price) is able to do this.

If i had known this before i would have choosen another Board because its really annoying that with an 28 Lane CPU and only one Graphics Card i am not able to choose all remaining PCIE Slots for my PCIE Sound Card an have to use PCIEX4 or PCIEX16/X8_3 to get it working...

i am defnitily hot happy with that especially in mind that other really cheap Boards do not have this limitation!

F1FTW3D
Level 7
Agreed! Hopefully Asus will address this with a BIOS update.

In the meantime, I'm holding tight with 2 GPUs, as quad air-cooled GPUs presents its own set of problems.

Thanks all!

F1FTW3D wrote:
Agreed! Hopefully Asus will address this with a BIOS update.

In the meantime, I'm holding tight with 2 GPUs, as quad air-cooled GPUs presents its own set of problems.

Thanks all!


You will need so address this with a CPU update to 7900X minimum.

Like Praz said, this is all in the manuals. This will also give you issues with DIMM.2 bandwidth.



“Two things are infinite: the universe and human stupidity, I'm not sure about the former” ~ Albert Einstein

Praz wrote:
Hello

Both the included and online manuals are very clear that PCIEX8_2 and PCIEX8_4 slots are non-functional when using a 28 lane processor. This is by design at the hardware level not a BIOS limitation.




first of all this behaviour should be cleary marked in the specs, not only in the manuals, because i bet not many people read the whole manual before buying hardware.

i did - but i did not expect that the x in the manual means that 2 PCIE Slots are completely dead - i suspectect it means - that its useable as long as there are free pcie lanes available and that they are shared within the remaining free pcie slots - as i know it from all the motherboards from all different brands i head before - and as it is on nearly all other x299 Boards out there.

it should be up to the users choice wether he wants to use a certain pcie slot and therefore have reduced connections bandwith or not - and not just switch 2 pcie slots completely of by hardware design and give the user no choice!

furthermore with an 28 lane cpu and 1 graphics card there are still enough lanes to have all other pcie slots with X8 or at least X4 connection Bandwith and still enough Dimm2 Bandwith, but as i said - i also have 2 dead pcie slots with my 7800X and no choice.

Praz
Level 13
Hello

Both the included and online manuals are very clear that PCIEX8_2 and PCIEX8_4 slots are non-functional when using a 28 lane processor. This is by design at the hardware level not a BIOS limitation.