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use both PCIE x16 gen4 and gen3

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shinsuryu View Drop Down
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    Posted: 10 Apr 2021 at 3:47am
Hi,

I've got a B550 Phantom Gaming 4. The description (https://www.asrock.com/mb/AMD/B550%20Phantom%20Gaming%204/) says it has 1 PCIe 4.0 x16 and 1 PCIe 3.0 x16 (PCIE1 and PCIE3 on the picture in the user manual).

Although, in the bios (Advanced/AMD PBS), the "PCIe/GFX Lanes Configuration" seems to only allow 1 x16 at a time. And "PCIe x16 Bud Interface" only seems to allow changing generation for the PCIE1 port.

How can I make sure that the PCIE3 port is correctly using x16 gen3 while PCIE1 is using x16 gen4 ?

Many thanks in advance for your help,
Kind regards
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ASRock_TSD Quote  Post ReplyReply Direct Link To This Post Posted: 13 Apr 2021 at 8:08pm
Dear shinsuryu,
Thank you for the posting.

To check the link speed of PCIe slot, you can use some monitor utility like GPU-Z to check.

If you still have other question, please feel free to post.

Thank you,

Best wishes,
ASRock TSD
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shinsuryu View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote shinsuryu Quote  Post ReplyReply Direct Link To This Post Posted: 24 Apr 2021 at 5:31pm
Hi,

Thanks for the suggestion regarding GPU-Z.

I've switched my GPUs PCIe slots and still can't make them both use x16.

Using GPU-Z, I found the following:
- GPU-1 (PCIE1 port): supports x16 4.0, using x4 3.0
- GPU-2 (PCIE3 port): supports x16 4.0, using x16 4.0

What bios settings should I use to allow GPU-1 to use x16 3.0 ?

Many thanks in advance for your help
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ket View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ket Quote  Post ReplyReply Direct Link To This Post Posted: 24 Apr 2021 at 8:25pm
Theres a bit of confusion with the wording as its telling you two different things at the same time. What is meant is that the board has 2x full length PCI-E 16x slots however due to how bandwidth is divided between other devices and ports the top PCI-E slot will run at a maximum of 16x PCI-E bandwidth while the lower full length slot will run at a maximum of 4x PCI-E 4.0 bandwidth. In short, everything is running as it should be

Remember that in essence 4x PCI-E 4.0 bandwidth is 8x PCI-E 3.0 bandwidth and 16x PCI-E 2.0 bandwidth. You have plenty of bandwidth going around for your GPUs basically and you aren't being limited in any way. Only the most powerful GPUs of today (RTX3080/90, RX6800XT/6900XT, etc) will see some limitation once you drop down to PCI-E 4.0 4x bandwidth but even then the hit isn't that big and its only a factor at CPU limited resolutions such as 1080p.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote shinsuryu Quote  Post ReplyReply Direct Link To This Post Posted: 24 Apr 2021 at 10:04pm
Well, I've got 2 RTX 3060 and I specifically bought this mother-board to have one on x16 4.0 and the other on x16 3.0, as expected given the board description.

However, I would be happy with both running on x8 3.0. But not x4 3.0.

I don't find how I could change settings to raise the 2nd GPU to x8 3.0.
Would it even be possible in the bios?

How does the motherboard auto set higher bandwidth on PCIE1 and not on PCIE3 (which, given the mb description should allow x16 3.0...) ?
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ket View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ket Quote  Post ReplyReply Direct Link To This Post Posted: 25 Apr 2021 at 12:33am

The B550 PG uses PCI-E 4.0 for the first full length PCI-E slot the rest of the system falls back to PCI-E 3.0 spec, thats just how B550 is. If you want to change bandwidth allocated you need to see if you have an option in the UEFI that allows that, sometimes the option is called "PCI-E Bifurcation" or just "PCI-E Lane Configuration", I'm not sure if asrock uses either of these terms or something else. What is your full system specification?
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