[SOLVED] PCIe 3.0 riser in 4.0 motherboard slot

[jdroberts7726]

My partner and I are having a debate about PCIe speeds on a riser he wants to get to vertically mount his GPU (RX 6800 XT).

Background:
Vertical mount PCIe riser is PCIe 3.0.

His side:
He believes that, because it's a physical connection, and that there are no pin quantity differences between PCIe 3.0 and 4.0, he can plug the riser into a PCIe 4.0 slot on his motherboard and still get PCIe 4.0 speeds. He believes that it's dependent on what is processing the information (CPU or chipset) and whether or not the cable can handle the higher frequencies.

My side:
I believe that, because the riser was built with the PCIe 3.0 standard, that he will not be able to get those speeds. I believe that it isn't dependent on what's processing it, because PCIe 3.0 slots were built to handle only half the bandwidth of PCIe 4.0 slots, so the riser will bottleneck his GPU.

What do y'all think?

 

[Nemesia]

Hi Jdroberts7726.

A PCIe 3.0 riser will be PCIe 3.0

You need a PCIe 4.0 riser for that.

 

[Rogue Leader]

He (your partner) is wrong.

I've literally done exactly that, and the GPU (RX 5700XT) ran at PCie 3.0

As for performance, running the 5700XT at PCIe 3.0 had 0 performance impact. For something newer and faster, it still would have minimal impact. This has been proven over and over with prior gen GPUs.

 

[jdroberts7726]

He (your partner) is wrong.

I've literally done exactly that, and the GPU (RX 5700XT) ran at PCie 3.0

As for performance, running the 5700XT at PCIe 3.0 had 0 performance impact. For something newer and faster, it still would have minimal impact. This has been proven over and over with prior gen GPUs.

Would you be able to explain that? I've not been able to find much information about PCIe standard limitations, just that there are limitations and what they are (speeds, bandwidth, etc.). He (my partner) believes that what matters most is if the cable between the riser and the PCIe 4.0 port can handle the frequency/throughput of PCIe 4.0, regardless if it's marketed as PCIe 3.0 or not

 

[Rogue Leader]

Would you be able to explain that? I've not been able to find much information about PCIe standard limitations, just that there are limitations and what they are (speeds, bandwidth, etc.). He (my partner) believes that what matters most is if the cable between the riser and the PCIe 4.0 port can handle the frequency/throughput of PCIe 4.0, regardless if it's marketed as PCIe 3.0 or not

In theory he is right, IF (and thats a big IF) the riser was made well enough then it could work in PCIe 4.0, in fact AMD mentions this themselves, but they also say that while it could work, if its not certified at PCIe 4.0 it could cause stability issues, they highly recommend to limit the slot to 3.0. Your partner is assuming that somewhere out there is an overbuilt riser cable, when I assure you, these are the types of things made in a backwater shop, not a high end facility. If the cable were overbuilt enough to handle it, it would be sold as PCIe 4.0, and at a premium. This is easily evidenced by the fact you can get a 3.0 riser cable for 20 bucks, a 4.0 one is almost 80.

https://www.amd.com/en/support/kb/faq/pa-270

To elaborate more technically, PCIe 4.0 is rated at 85 ohms resistance, and has thicker shielding, PCIe 3.0 is 100 ohms. Thats where your possible interference and instability comes from.