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Video card bandwidth in PCIE3 slot of X570 Taichi? |
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shmerl 
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 Topic: Video card bandwidth in PCIE3 slot of X570 Taichi?Posted: 14 Aug 2019 at 7:02am |
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X570 Taichi manual says:
Does it mean that single video card will operate in PCIe x16 mode only in PCIE1 slot? What will happen if I put it in PCIE3 slot? It would a better fit in my case. It won't operate at x16? My CPU is Ryzen 9 3900X. |
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Ray62
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Posted: 14 Aug 2019 at 2:06pm |
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Yes, only the PCIE1 has wired 16 lanes as in every AM4 board. See manual:
PCIe slots: PCIE1 (PCIe 4.0 x16 slot) is used for PCI Express x16 lane width graphics cards. PCIE2 (PCIe 4.0 x1 slot) is used for PCI Express x1 lane width cards. PCIE3 (PCIe 4.0 x16 slot) is used for PCI Express x8 lane width graphics cards. PCIE4 (PCIe 4.0 x1 slot) is used for PCI Express x1 lane width cards. PCIE5 (PCIe 4.0 x16 slot) is used for PCI Express x4 lane width graphics cards |
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shmerl
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Posted: 14 Aug 2019 at 2:09pm |
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Why are then PCIE3 and PCIE5 slots referenced as (PCIe 4.0 x16 slot) if they can't ever operate in x16 mode? That's a bit confusing.
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Ray62
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Posted: 14 Aug 2019 at 3:35pm |
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They must be mechanically compatible for GPUs, even if they only use 8 or 4 lanes. But that is a very old thing for mainstream motherboards.
If you want more fully PCI Express x16 lanes slots, you have to use a Threadripper or Intel socket 2066 board. |
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gizmic
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Posted: 14 Aug 2019 at 3:53pm |
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thats like saying why usb 1 doesnt do usb 3.0
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ksquared
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Posted: 14 Aug 2019 at 9:50pm |
Yes, I'd agree about being confusing. They are physical x16 slots, but won't operate as x16 slots. To me, even more confusing is that once you use both PCIE1 and PCIE3, the PCIE1 slot then drops from the x16 and becomes an x8 mode. But I guess most (or all) video cards can still operate fine in the x8 mode? So then, is it if you use PCIE1 and the PCIE5 slot, the PCIE1 slot will still drop from the x16 and become an x8 mode? That is the way I'd read the manual, but if you add up the potential 8+8+4 you have 20 lanes, so is it possible the PCIE1 slot would remain x16 and the PCIE5 slot as x4? |
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Ray62
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Posted: 15 Aug 2019 at 5:31am |
PCIE1 and PCIE3 are connected to the CPU and use 16 lanes, either in 16-0 or 8-8 mode. The answer to your first question is no. PCIE5 and M2_3 are connected with 4 shared lanes to the chipset, so only one can be used at the same time. So, if M2_3 is not in use, the answer to your second question is yes. |
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shmerl
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Posted: 15 Aug 2019 at 9:38am |
If both are connected to the CPU, what exactly prevents PCIE3 from operating in x16 with only one device in it? It shouldn't be hard to detect that only one device is in use, and not to split the bandwidth. |
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Ray62
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Posted: 15 Aug 2019 at 3:56pm |
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I agree, 0-16 mode sounds possible, but not mentioned in the manual.
ASUS X570 Strix Series for example offers the same. 16-0, 8-8, 0-8 mode is given for X370/X470/X570 boards since the beginning of the Ryzen platform. I don't know if there are any exceptions. Ask Asrock support, if the PCIE3 slot has 16 or only 8 lanes wired. |
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Under test: MSI X570_MEG_ACE | AMD Ryzen 9 3900X | Scythe Mugen5 | 2x16GB F4-3200C14D-32GTZ@3466 | ASUS GeForce GTX 1080 Ti FE |
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gizmic
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Posted: 15 Aug 2019 at 5:37pm |
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flip the board around you'll see the pins are different on pcie1 and pcie3. if it only reaches half way its only wired for x8
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Matisse: 3 x PCI Express 4.0 x16 slots (PCIE1/PCIE3/PCIE5: single at x16 (PCIE1); dual at x8 (PCIE1) / x8 (PCIE3); triple at x8 (PCIE1)/ x8 (PCIE3) / x4 (PCIE5)