Question PCIE 3.0 x8 into a PCIE 4.0 x4

[TryingToVirtualizeStuff]

Hello,

I'm currently building an x570 system which will use the following 3 PCIe cards:

Nvidia A4000 (PCIE 4.0 x16)
Aja Kona 5 (PCIE 3.0 x8)
Aja Kona 5 (PCIE 3.0 x8)

The motherboard I will be using is the following (Asus ProArt X570-Creator WiFi): https://www.asus.com/Motherboards-Components/Motherboards/ProArt/ProArt-X570-CREATOR-WIFI/

I don't see a problem with the first 2 cards, but I'm wondering if the 3rd card will work as well?

Given that PCIE 4.0 is twice the bandwidth of PCIE 3.0, from a bandwidth perspective everything should work.

The motherboard itself lists the following about its PCIE slots:

Expansion Slots
Slot 1	Slot not available on this board
Slot 2	PCI Express 4.0 x16
Slot 3	Slot not available on this board
Slot 4	Slot not available on this board
Slot 5	PCI Express 4.0 x8 in a x16 size
Slot 6	Slot not available on this board
Slot 7	PCI Express 4.0 x4 in a x16 size

Will I be able to get all 3 PCIE cards to work, or can I assume that the 2nd PCIE 3.0 x8 can't fit in a PCIE 4.0 x4 (in a x16 size)?

Thank you for all your help,
Nelson

 

[coercitiv]

To me it sounds like a potential problem, but it all depends on the actual PCIe bandwidth required by the second Aja Kona 5 to function properly. Physically adding the card won't be a problem, but if it works, it will do so in PCIe 3.0 x4 mode.

 

[TryingToVirtualizeStuff]

To me it sounds like a potential problem, but it all depends on the actual PCIe bandwidth required by the second Aja Kona 5 to function properly. Physically adding the card won't be a problem, but if it works, it will do so in PCIe 3.0 x4 mode.

Hello coercitiv,

Thank you for the input.

Are you sure when you say: "Physically adding the card won't be a problem, but if it works, it will do so in PCIe 3.0 x4 mode."

...you really mean:

"Physically adding the card won't be a problem, but if it works, it will do so in PCIe 4.0 x4 mode."

If so, is PCIE 4.0 x4 mode the same as PCIE 3.0 x8?

Thank you,
Nelson

 

[IntelUser2000]

"Physically adding the card won't be a problem, but if it works, it will do so in PCIe 4.0 x4 mode."

If so, is PCIE 4.0 x4 mode the same as PCIE 3.0 x8?

Thank you,
Nelson

No, coercitiv is right. It'll work in PCIe 3.0 x4 mode. Why 3.0? Because the device itself only supports PCIe 3.0, and the lane width is a physical parameter, and has one copper connection per lane. If the motherboard is giving the device access to only 4 lanes, the device will have 4 lanes of whatever PCIe itself supports. Hence, PCIe 3.0 x4.

The signaling is different between the two. You cannot make a PCIe 3.0 device turn into 4.0. So if that card was a PCIe 1.0 card, then the device will run at PCIe 1.0 x4.

It's just like if you have a USB device that supports the 12Mbps 1.0 spec. You can stick that to USB 3.2 that supports 20Gbps, but it's still going to run at 12Mbps. You need both the device and the port support USB 3.2 to get the full bandwidth.

 

[biostud]

PCIe 4.0 x4 has same bandwidth as PCIe 3.0 x8, but it is not the same. You would need a board that has x8 on the third slot as well. From their homepage:
AJA's KONA for PC Supported System Configurations

*4-lane PCIe slots can support the following AJA cards: KONA 1, KONA LHi, KONA LHe Plus

**8-lane or 16-lane Computer PCIe slots can support the following AJA cards: KONA 5, KONA 4, KONA HDMI

You would need this motherboard:

Pro WS X570-ACE｜Motherboards｜ASUS Global

 

[TryingToVirtualizeStuff]

No, coercitiv is right. It'll work in PCIe 3.0 x4 mode. Why 3.0? Because the device itself only supports PCIe 3.0, and the lane width is a physical parameter, and has one copper connection per lane. If the motherboard is giving the device access to only 4 lanes, the device will have 4 lanes of whatever PCIe itself supports. Hence, PCIe 3.0 x4.

The signaling is different between the two. You cannot make a PCIe 3.0 device turn into 4.0. So if that card was a PCIe 1.0 card, then the device will run at PCIe 1.0 x4.

It's just like if you have a USB device that supports the 12Mbps 1.0 spec. You can stick that to USB 3.2 that supports 20Gbps, but it's still going to run at 12Mbps. You need both the device and the port support USB 3.2 to get the full bandwidth.

Hello IntelUser2000,

A huge thanks for the explanation (as it now makes sense).

I was wondering if it'd be possible to use a riser card to split a single PCIE 4.0 x8 into two PCIE 3.0 x8?

For instance, the following SuperMicro riser card (not sure if it fits into an standard motherboard PCIE slot):

Card: RSC-F2B-88G4
Website: https://www.supermicro.com/en/support/resources/riser

...outputs two PCIE 4.0 x8 or PCIE 3.0 x8 signals.

Would something like this work?

That said, on second thought, I think the 3 PCIE 4.0 x8 motherboard should work for me, as the GPU I'm using is an Nvidia A4000 (which should be fine in a PCIE 4.0 x8 slot, as I seriously doubt there'd be a major difference between sticking that card into a PCIE 4.0 x16 vs a PCIE 4.0 x8).

Thank you for all your help,
Nelson

 

[TryingToVirtualizeStuff]

PCIe 4.0 x4 has same bandwidth as PCIe 3.0 x8, but it is not the same. You would need a board that has x8 on the third slot as well. From their homepage:
AJA's KONA for PC Supported System Configurations

*4-lane PCIe slots can support the following AJA cards: KONA 1, KONA LHi, KONA LHe Plus

**8-lane or 16-lane Computer PCIe slots can support the following AJA cards: KONA 5, KONA 4, KONA HDMI

You would need this motherboard:

Pro WS X570-ACE｜Motherboards｜ASUS Global

Hello Biostud,

A big thank you for the motherboard recommendation, as I didn't think about looking for 3 PCIE x8's.

Regards,
Nelson

 

[Stuka87]

It should be noted that some motherboards split out the PCIE bus. While each dedicated slot will negotiate its speed independently, if one x16 is split into two x8's (or a 16 that is split into on x8, and two x4's), both x8's will be required to run at the same speed. eg: if one card is 4.0, and one is 3.0, both will be forced to run at 3.0 speeds.

 

[KentState]

Hello IntelUser2000,

A huge thanks for the explanation (as it now makes sense).

I was wondering if it'd be possible to use a riser card to split a single PCIE 4.0 x8 into two PCIE 3.0 x8?

For instance, the following SuperMicro riser card (not sure if it fits into an standard motherboard PCIE slot):

Card: RSC-F2B-88G4
Website: https://www.supermicro.com/en/support/resources/riser

...outputs two PCIE 4.0 x8 or PCIE 3.0 x8 signals.

Would something like this work?

That said, on second thought, I think the 3 PCIE 4.0 x8 motherboard should work for me, as the GPU I'm using is an Nvidia A4000 (which should be fine in a PCIE 4.0 x8 slot, as I seriously doubt there'd be a major difference between sticking that card into a PCIE 4.0 x16 vs a PCIE 4.0 x8).

Thank you for all your help,
Nelson

Those are bifurcation cards and something your MB would need to support. Most likely, you would need a Threadripper board for that. The Proart only states bifurcation for CPU and storage RAID so it's not clear if the PCIE slots do. Would have to dig into the manual.

Your board has will support two x8 devices on the upper PCIE slots to the CPU and the lower is x16 using the chipset. Meaning, your A400 will be at half bandwidth and then one of your other cards will be sharing bandwidth with all your devices like USB, Wifi and storage.

If this is more than a hobby, look at Threadripper and the 3960x. It will cost more, but give you more dedicated PCIE lanes and the extra threads will help with video processing. I personally have both a 5950x/x570 and 3970x/sTRX4 builds and the PCIE limitations are very noticeable once you go beyond a single GPU and NVMe drive for serious work as well as the quad channel memory.

 

[thecoolnessrune]

Hello IntelUser2000,

A huge thanks for the explanation (as it now makes sense).

I was wondering if it'd be possible to use a riser card to split a single PCIE 4.0 x8 into two PCIE 3.0 x8?

For instance, the following SuperMicro riser card (not sure if it fits into an standard motherboard PCIE slot):

Card: RSC-F2B-88G4
Website: https://www.supermicro.com/en/support/resources/riser

...outputs two PCIE 4.0 x8 or PCIE 3.0 x8 signals.

Would something like this work?

That said, on second thought, I think the 3 PCIE 4.0 x8 motherboard should work for me, as the GPU I'm using is an Nvidia A4000 (which should be fine in a PCIE 4.0 x8 slot, as I seriously doubt there'd be a major difference between sticking that card into a PCIE 4.0 x16 vs a PCIE 4.0 x8).

Thank you for all your help,
Nelson

As an aside to things already mentioned, what you referenced was a passive PCIe Riser. It's taking a slot and breaking it out with Bi-furcation. Other models do it by connecting into a proprietary slot that has multiple PCIe slots worth of connectivity wired into it. They work passively, requiring the motherboard to support it (Bi-furcation), or for the board and riser to be wired in a compatible, but proprietary way to allow multiple slots of connectivity to be presented in one card edge.

The alternative, and the spirit of what you're thinking of is a PCIe Switch Chip with accompanying re-timer circuitry. This device acts as a PCIe end point itself, allowing other devices to be connected downstream and then transmitted upstream over a single link. The technology is very expensive for PCIe 4.0 right now, and is really only visible in the enterprise segment. In the old days of PCIe 2.0 and Core 2 days, NVIDIA used this technology for their nForce 200 + 780i chipset designs. A PCIe Switch on the board allowed triple x16 PCIE slots by inputting one PCIe 2.0 x16 signal, and outputting 2 PCIe 2.0 x16 downstream ports. The switch muxed everything together. Again, it's effective, but was power hungry and expensive even back then. PCIe 4.0's tight signaling makes these switches cost way more now (over $100 per unit more than the previous gen PCIe 3.0 switches). https://www.anandtech.com/show/2400/2

 

[PingSpike]

Recent gigabyte and MSI boards often have bifurcation options in the bios these days, at least the AMD ones I've looked into. It's a cabling mess though versus just finding a board with a second 8x slot, but those boards I've grown much less common.