Slot lanes 16x or 8x Using a PCIe M.2 expansion card

Discussion in 'Motherboards' started by Nosferatu_UK, Oct 23, 2019.

  1. Nosferatu_UK

    Nosferatu_UK New Member

    Messages:
    5
    Howdo all,

    I was just wondering:
    I have an Asus Prime x370 Pro running Ryzen 7 1800x

    I have seen in my manual for the mobo that if 2 x PCIe /VGA cards are used then the PCIe slot 1_16x will go from 16x to 8x and the second PCIe slot 2 or 3 will also be 8x

    If i have one 2080Ti (which i do) in slot 1 at x16 lanes, but if i add a PCIe to M.2 expansion , will this reduce the GFX card lanes to x8? As the mobo only accepts 1 x M.2 slot,so thats my main runner for Win 10 OS, but i have another 500GB M.2 on the expansion card and in slot 3 PCIe. But i swear theres a decrease in gfx performance.
     
  2. Shlouski

    Shlouski VIP Member

    Messages:
    1,369
    So far from what I've seen the m.2 slots won't take lanes from a pci x16 slot, on the motherboards I have the most the m.2 slot takes is lanes from the sata ports and disables them. I looked at your motherboards specs and didn't see any note about something changing once the m.2 is populated , I very much doubt they would take lanes from the pci x16 slot as an m.2 only requires x4 lanes.
     
  3. Nosferatu_UK

    Nosferatu_UK New Member

    Messages:
    5
    Many thanks for clearing this up :)

    My onboard M.2 is populated but the mobo only comes with 1x M.2 slot, but i have a PCIe expansion card to have a second M.2 as storage, thats why i worried about the gfx card lanes switching to 8x instead of 16x. But as you said if the M.2 only use 4x, im sure it will be fine :D
    Thank you!
     
  4. Shlouski

    Shlouski VIP Member

    Messages:
    1,369
    Im sorry about my previous post, I should have read your question better, but I became distracted.

    Which slot have you installed the expansion card into?

    Personally I would install the expansion card in the lowest pci x16 slot.

    If you have installed it in the second x16 slot I'm not sure what speeds these x16 slots will run at.
     
    Last edited: Oct 23, 2019
  5. OmniDyne

    OmniDyne Active Member

    Messages:
    621
    The first two PCI-e slots have 16 lanes available, so if you use both slots bandwidth is reduced to 8 lanes on each slot. The second PCI-e slot is always x8. Your M.2 slot has 4 lanes reserved. Using the M.2 slot does not take bandwidth from the PCI-e X16 or x8 slots.
     
    Last edited: Oct 23, 2019
  6. Nosferatu_UK

    Nosferatu_UK New Member

    Messages:
    5
    The expansion card is in Slot 3, lowest one to the bottom of the motherboard.
     
  7. OmniDyne

    OmniDyne Active Member

    Messages:
    621
    PCIe Slot 3 shares bandwidth with the chipset, not the CPU, so no, PCIe slot 3 does not take bandwidth from the PCIe x16 slot or the second PCIe slot.

    The second PCIe x16 slot only utilizes 8 lanes. Utilizing the second slot takes bandwidth from the first x16 slot. The first and second PCIe x16 slots are connected directly to the CPU. The third PCIe slot has 8 lanes but utilizes the chipset and not the CPU, so using this slot does not affect the first two PCIe x16 slots.
     
    Last edited: Oct 23, 2019
    Shlouski likes this.
  8. Nosferatu_UK

    Nosferatu_UK New Member

    Messages:
    5
    Many thanks :D thats awesome to hear!
     
  9. Shlouski

    Shlouski VIP Member

    Messages:
    1,369
    Yeah I realised the second slot was only x8 and i would likely reduce them both to x8 if another x16 card was installed, but i wasnt sure what would happen with an x4 card.

    Yeah, I knew that one was separate which is why I suggested putting the card in the lowest x16 slot.

    Thanks for the input dude :)
     
    OmniDyne likes this.
  10. beers

    beers Moderator Staff Member

    Messages:
    8,456
    You already had this addressed but the 4x lanes to M.2 are dedicated.

    The two main x16 slots share lanes so with one card it's 16 and with two it divides out to 8x/8x
     
  11. OmniDyne

    OmniDyne Active Member

    Messages:
    621
    Ah I see what you're saying. My reading comprehension ain't so good as of late ha.
     

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