Shared PCI Express lanes: PCI-e x1 board plugged into x8 slot that is shared with x16 slot

Posted on 2013-11-13
Last Modified: 2013-11-20
I have a Gigabyte motherboard GA-Z68X-UD3H-B3.  It has two PCI Express x1 slots, one PCI Express x16 and a PCI Express x8 slot.  The manual for the motherboard says "When the PCIEX8 slot is populated with a PCI Express graphics card, the PCIEX16 slot will operate at up to x8 mode.  I have a video card in the x16 slot. Unfortunately the x1 slots are on either side of the video card and thus I can't put the audio card on one side because it blocks the fan of the video card so I put it on the other side.  I then have this Media Dashboard front panel device that has USB 3.0 ports but they connect to the MBD via a PCI Express x1 slot.  But I can't plug this connector into the x1 slot on fan side of the video card because the fan blades hit it.  Thus I'm left with plugging it into the x8.  

So, here's my questions:
1.) If I plug this x1 connector, basically USB 3.0 front panel ports, into the x8, which the x16 slot with the video card drop down to x8
2.) Will I just loose a lane on the x16 slot?  I guess you just don't loose one lane though?
3.) Will nothing happen because the x1 will have no effect on the x16 slot since it's not another video bd.
4.) If performance is affected in anyway, will it only be affected when something is plugged into the USB 3 front panel port.
Question by:JoeBoyd
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Expert Comment

ID: 39646226
As far as I know, #3 is the best answer.

Will nothing happen because the x1 will have no effect on the x16 slot since it's not another video bd.

There should be something like 32-40 PCI-E lanes available to the PCI-E bus (depends on your motherboard), so they shouldn't interfere with each other at all.

If you are really doing some heavy video, AND loading up the x1 card, you might get some issues. But it will be more of a "controller is #$%^& busy dealing with the video card" than a "lane" problem.
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Accepted Solution

_ earned 500 total points
ID: 39646528
OK, here we go, I found this. Looks like it has 24 lanes:

On the all important videocard front there are two options - integrated graphics via the board's HDMI/Display Port/DVI-D/Analog video outputs, or PCI Express based discrete graphics cards. In terms of videocards, the GA-Z68X-UD3H-B3 motherboard will handle one PCI Express 2.0 x16 videocard in x16 mode or a pair of nVidia Geforce / ATI Radeon videocards in SLI or Crossfire, in x8/x8 mode. Intel's Sandy Bridge CPU supplies x16 lanes of PCI Express 2.0 bandwidth for the graphics cards, the Intel Z68 chipset a further eight PCIe lanes for system devices.

So even though it looks like the x8 slot uses the same lanes as the x16 slot, I'm still going with it shouldn't affect the video or the usb, unless you are doing some heavy video.
Off the top of my head, each lane on a PCI-E 2.0 can do 1GHz of throughput. Should be enough for normal circumstances.

Author Closing Comment

ID: 39662570
Thank you both of you for your answers.  Coral47 provided me with a little more detail and the link.  And it was enough info to help me decide to plug in the connector from the Media Dashboard to the PCIe-8 connector
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Expert Comment

ID: 39663419
Thank you much.   : )

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