PCIe Bifurcation Splitters?

Hey everyone! I have an ASRock X399 Taichi that (allegedly) supports PCIe bifurcation.

I am currently using 3 out of 4 slots, two dual slot GPUs and a single slot 10Gb NIC.

I am possibly looking to add more devices in the future, more than one slot would allow.

Are there any non-PLX PCIe splitters out there to do the job? Especially something that would allow me to use two adjacent slots with maybe half-height cards? Do you know of any such thing?

Edit: Maybe something like this: https://www.aerocooler.com/rc27332x16c15-2u-2-slot-pcie-x16-flexible-riser-card-w-15cm-ribbon/ but in a ribbon cable or tower computer friendly?

Interesting idea. It should work, right? Isn’t that what the low cost NVMe carrier boards do, routing 4 lanes to each m.2 without using a switch chip.

There are much less expensive risers on eBay, might be worth a gamble of you really need more slots.

Also could use simple extension cables to make use of slots that are covered by a large card.

My x399 has two full x16, two full x8 wired to the CPU, along with three x4 via m.2 slots.

I think it has two or three more slots that are not so fully wired but have the full size x16 connector. My 10g card is connected to one of the promontory slots, and is still wicked fast, faster than any network storage I have available.

That is exactly what those boards do! They just do it in a single slot package since the nvme size allows it.

The problem is that most ebay risers are 90 degrees risers, meaning that they face down once you plug them in and I don’t have the space for that. They might also be proprietary since many server OEMs use proprietary slots(?) and risers.

My X399 Taichi has 2x 16 and 2x 8 slots and 1 or 2 PCIe 2.0 x1(?) slots wired through the chipset. Unfortunately I don’t have an onboard 10Gb NIC but that is OK since I am using SFP+ gear anyway.

I did email a company that produces some x4 splitters to see if they are interested in making something that could be of use for X399 and normal tower computer cases.

It is a shame really that all companies that produce X399 did not add more PCIe slots, since you can bifurcate them manually, something that lower tier boards do by default (e.g. one x16 slot or 2x x8 slots on 115x Intel boards).

I feel a vertical GPU bracket could be modified to hold a larger riser easily enough. Many have room for 3 or even 4 slot GPUs to mount, so, two single slot cards should fit ok. You still need a chassis that supports such a thing. Use the x16 extender that comes with the kit to connect the splitter riser to the motherboard.

From just following the traces on that circuit board, it looks like the first x8 lanes go to the bottom slot and the next x8 lanes go to the top slot. A cheaper version is available without power injection.

PCIe 2 and 3 are electrically identical if I recall correctly, so it shouldn’t matter that it’s listed as gen 2 device.

My x399 Designare is similar, 2x16, 2x8 and 1x4 pcie 2.0 attached to the chipset. I confirmed, all three m.2 slots are pcie 3.0 and x4 bandwidth to the cpu. That checks out math-wise, 32 + 16 + 12 = 60, and the remaining 4 lanes connect the cpu to the chipset.

Taking the second x8 slot and splitting it x4 x4 would be plenty of bandwidth for a network controller, and something like an ssd, capture card or thunderbolt 3 adapter.

This is the bracket available for my Lian Li case… room for four single slot cards.

Now you got me thinkin about expansion options, since I’m pondering moving my threadripper into a server role.

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@wendell Any idea if something like this could be easily/cheaply produced?

Having so many PCIe lanes and so many x4 devices but only 4 slots is infuriating!

I suppose not many people have this problem but having half-height NICs and HBAs take up whole x8 slots for themselves seems very wasteful to me.

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Soon! ™


i remember i used one on a old fm2+ i ordered from a china company i can no longer find… most of them take a x16 slot and break it down is now usually in external boxes like how ltt did theirs a while ago

time stamped


Yeah but something like this is bulky and requires case modding whereas you could adapt one motherboard PCIe slot and 2-4 “case slots” to 2-4 half height slots with the right PCB or some sort of riser cable (which would provide even more flexibility).

I know Wendell and EposVox were also complaining about the lack of more than 4 slots on HEDT platforms so they might hopefully be onto something!

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well you could get a bunch of the tri maybe quad bifurcation cards and ribbon cable them in the case?
like using

finding cables (male pcie 90 degree) maybe a issue but that would be a working method

or cable the bifurcation card and run cards directly off the bifurcation pcb


this looks more for btc mining then your idea but might wokr for your idea

Unfortunately this would occupy the whole slot where it is plugged in.
I have an X399 Taichi board in a Corsair 900D case. The motherboard has 4 PCIe slots to the CPU and 1 to the chipset (pretty useless for anything but USB controllers and soundcards).

Each motherboard slot corresponds to two on the chassis, apart from the bottom one.
If you have two 2-slot GPUs in slots 1 and 3, you are left with a two slot gap at slot 2 and in my case a 4 slot gap at the bottom slot (the 900D can accomodate up to 10 slots).

Slots 1 and 3 are x16 and slots 2 and 4 are x8 electrical. That means that with bifurcation you can split slots 1 and 3 to x8/x8 or x4/x4/x4/x4. Slots 2 and 4 can be split to x4/x4.

If you plug something like this in slot 2, you take up a slot and can only use the one underneath. There is no point in doing that.

If you plug it in slot 4, you can only double it so effectively you get 5 in total.

If you could produce this at a 90 degree angle and use half height cards, then you could use it at both slots 2 and 4 and end up with a total of 6. If you produced a flexible solution that can plug in slots 1 or 3, then you could multiply them 4 times. If they are flexible enough, you could use them with the bottom 4 unused chassis slots and could get up to the full 10 slots theoretically. It would also be usable in any computer case without mods.

90% or more of X399 motherboards use this layout, probably many of the X299 use something similar as well. Most 10Gb NICs and HBAs come with half height adaptors for server use.

MIning solutions are no good unfortunately. Devices like 10Gb network cards and HBAs need their PCIe 3.0 x4 bandwidth and the mining solutions only usually provide PCIe 2.0 or 1.1 x1.

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this is why 90 degree male pcie cables need to exist to be flat ribbons to fully use the system grumble grumble there has to be some one who makes these cables

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Oh I see, you meant to use the ribbon to connect the adapter to the motherboard!

I just immediately though of connecting the adapter straight on the motherboard and then use 90 degree ribbon cables. My bad!

I see what you mean but you would need a really short ribbon cable as they are usually very bulky and hard to twist.

But a PCB that has both the female and male parts in-line and the proper spacing for a half height card to plug in and then be screwed to the case would be ideal.

Side view:

these exist and could be paired with the bifurcation card ( maybe ) or just pull the card that is blocking a slot away

im dumb they make bifurcation cards like what i described

All the msi boards starting with b450 have bifurcation setting in the bios, but I don’t know if it’s only for 4x4x4x4x split or if you can do other splits

Does the supermicro bifurcation riser work on all motherboards though? Or is it like the Dell and HP proprietary ones?

They also only seem to produce the 1 to 3 one and I don’t know how that splits the lanes. I also have not seen them available anywhere but did not really look that much.

What about power? Would a kind of janky solution like that be able to provide 25W or more to each card?

Edit: Where did you find this?

Edit 2: The overhang on the left seems too long to fit in a case.

You can circumvent the power requirements by using a powered riser on each of the slots
But try and keep it short

from my reading on hardfourm when i first looked at it if the mobo supports it then it should work

keeping away from super micros YUGE custom connections

this one has a shorter cable and smaller pcb

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Very interesting!

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