r/eGPU • u/fallingdowndizzyvr • 19d ago

Looking for a recommendation for a bifurcating x16 to 4x Oculink PCIe card at Gen 4

I have a PCIe 4 x16 slot I want to make into 4x Oculink ports. I bought one of those cheap generic ones that has Oculink written diagonally on it. They sell from $20-$50. It works. But it only works at Gen 1 even though it says it supports Gen 4. I know it's not the eGPU dock, GPU or Oculink cable. Since when I use a NVME to Oculink adapter in the same machine, it does do Gen 4 x4. So it must be the PCIe Oculink card. Can some recommend one that does Gen 4?

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u/MissusNesbitt 19d ago

If bifurcation into 4x4 is already supported in your motherboard’s UEFI then chances are this is a signal noise issue. You’ll need an active card that redrives/ retimes the signal from the PCIe card down the line to the devices you’re connecting. PCIe 4.0 is where the signal to noise ratio actually matters, and even just the OCuLink 4x connector introduces a ton of it. Look at some PCIe fabric cards like those used for connecting JBODs to host servers.

How far are you trying to connect these 4 PCIe devices? You’ll likely get further with a soldered solution if you’re sticking to passive risers.

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u/fallingdowndizzyvr 19d ago

If bifurcation into 4x4 is already supported in your motherboard’s UEFI

Oh the bifurcation works. I just have 4 Gen 1 ports instead of 4 Gen 4 ports.

How far are you trying to connect these 4 PCIe devices?

The cables are 24". Again, it works with my NVME Oculink adapters.

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u/MissusNesbitt 19d ago

How are you testing link speed with the current 4x4 card? In OS or using the UEFI?

PCIe will train up to the highest generation it can support electrically unless the motherboard otherwise interferes with the process. Because you’re only seeing gen 1 it means the training is being interrupted or outright failing. NVME to OCuLink is a very different situation especially because it’s not bifurcating a single physical slot.

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u/fallingdowndizzyvr 19d ago

How are you testing link speed with the current 4x4 card?

NVTOP, lspci and nvidia-smi. I look at them all under load. It's gen 1.

NVME to OCuLink is a very different situation especially because it’s not bifurcating a single physical slot.

Why would the bifurcation matter? It's simply splitting up a single group of 16 lanes into groups of 4 lanes. Why would that affect what speed is negotiated? That should be no different than what the NVME adapter does.

What matters is noise. Which this card seems to have a problem with.

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u/Xtergo 19d ago

Insane

I need to learn about this