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PCI-E bus/link speed. 8x and 16x?

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September 22, 2008 9:00:05 PM

hey everyone, my pc specs are in the sig. my question/understanding of a mobo with 2 pci-e x16 slots if you use 2 gpu's the bus/link speed will be 8x (i think) now if i take the 2 gpu's out and replace them with 1 gpu, shouldnt the bus/link speed change to 16x. my understanding of "link" speed is how fast the 2 pci-e slots can talk to each other. but if i dont have 2 gpu's and im only using 1 gpu, shouldnt the speed of the single bus slot be running at 16x? reason why i ask is because in gpu-z when i had SLI 9800GT it showed the bus interface as PCI-E x16@8x. but now i have a GTX 260 core 216 (1 gpu) so shouldnt it change to PCI-E x16@16x? and if theres a way to change it, all help is greatly appreciated.

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October 20, 2009 1:28:10 PM

xxcoop42xx said:
hey everyone, my pc specs are in the sig. my question/understanding of a mobo with 2 pci-e x16 slots if you use 2 gpu's the bus/link speed will be 8x (i think) now if i take the 2 gpu's out and replace them with 1 gpu, shouldnt the bus/link speed change to 16x. my understanding of "link" speed is how fast the 2 pci-e slots can talk to each other. but if i dont have 2 gpu's and im only using 1 gpu, shouldnt the speed of the single bus slot be running at 16x? reason why i ask is because in gpu-z when i had SLI 9800GT it showed the bus interface as PCI-E x16@8x. but now i have a GTX 260 core 216 (1 gpu) so shouldnt it change to PCI-E x16@16x? and if theres a way to change it, all help is greatly appreciated.


hi man, i'm having the same problem as you. i have a P6T Deluxe mobo with 3 PCI-e slots @ 16X, 16X, and 1X. i'm using only one video card in the primary PCI-e slot yet i'm only getting 8X not 16X. did you work it out? thanks
a b V Motherboard
October 20, 2009 4:14:15 PM

m0ngy said:
hi man, i'm having the same problem as you. i have a P6T Deluxe mobo with 3 PCI-e slots @ 16X, 16X, and 1X. i'm using only one video card in the primary PCI-e slot yet i'm only getting 8X not 16X. did you work it out? thanks


If you are using the top PCI-E slot then your GPU is running at x16.
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December 29, 2009 4:59:22 PM

i'm using the top pci-e slot but still running at 8x...btw my mobo is dfi inf nf4 sli...and my gc is 8800gts
a b V Motherboard
December 29, 2009 5:06:27 PM

aizuddinaizat said:
i'm using the top pci-e slot but still running at 8x...btw my mobo is dfi inf nf4 sli...and my gc is 8800gts


You need to go into the Bios and make sure you have SLI disabled. If it is enabled it will run both slots at x8. If disabled it will run your top slot at x16
December 29, 2009 11:07:15 PM

OvrClkr said:
You need to go into the Bios and make sure you have SLI disabled. If it is enabled it will run both slots at x8. If disabled it will run your top slot at x16


hey mate, thanks for your input to this puzzling and as yet unsolved mystery.

i'm running an Asus P6T Deluxe v1, and there are no bios settings directly relating to CrossfireX/SLI. the manual tells me these settings are configured in windows, via the nvidia/ati control panels. i found a setting called 'PCIE Spread Spectrum' which can be disabled in order to overclock PCIe. still no joy with this one i'm afraid. this is frustrating the hell outa me.

cpuz and ati control panel both tell me the max supported link width is 16x, yet my 4670 is only connected at 8x, even though it's in the primary (blue) GPU PCIe slot which, theorectically, should ALWAYS run at 16x. there are no bios options to change the speed of the primary GPU PCIe slot, although PCIe slots 2 (white) and 3 (black) can be configured to run at either 8x/8x or 16x/1x.

i've tried all the other power saving features, both in vista ultimate 64 and bios, to no avail.

on the plus side, ati control panel informs me that although the link width is only 8x (which should be impossible), the link bandwidth is 27.9 Mbps, which i understand to be within normal range.

i have everything fully updated and yes, the card is definately in the primary GPU slot. unfortunately, i don't have another card to test the PCIe slot and i bought this PC second-hand, so no warranty; my research suggests some people have fixed this problem by replacing the card, while others have replaced the board.
December 30, 2009 3:31:51 AM

that's true...there is no setting in award phoenix bios to disable SLI setting
a b V Motherboard
December 30, 2009 5:59:33 AM

The P6T Deluxe motherboard supports two operating modes for the three PCI Express x16 slots. You can either choose to run the three slots with x16, x16 and x1 lanes, or you can run them with x16, x8 and x8 lanes.

Only the blue PCI Express x16 slot will always run with full x16 lanes, so this is the recommended slot for users of a single graphics card. The white PCI Express x16 slot can either run with x16 or x8 lanes. The black PCI Express x16 slot supports a maximum of x8 lanes but will only run with x1 lane if the white PCI Express x16 slot is set to run at x16.
December 30, 2009 6:09:44 AM

yeah mate, that's pretty much exactly what i've said in my last post. if the PCIe slot was not functioning properly, you'd expect it wouldn't work at all, not 'half' work. maybe it's the video card, but the link bandwidth seems fine, so i really don't know wtf is going on.
a b V Motherboard
December 30, 2009 6:33:01 AM

ahhhh!!! we are talking about 2 different mobos here...

As far as the DFI, I have no clue since I have never used one nor seen the bios.

As far as the P6T, I helped a buddy build his and we did not encounter any issues with the PCI-E limitation that you are experiencing. I opened CPU-Z and it was showing "Link Width: at x16. Also I am not sure why you are messing with the PCI-E SS setting since you will achieve nothing by overclocking the PCI-E...
December 30, 2009 9:23:34 AM

Ok, got anything useful to add?
!