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Asus P8Z68-V Pro Gen3 ??? re PCIe 3.0/2.0 x16_2 slot

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February 20, 2013 9:24:50 PM

Simple question - The manual clearly describes the consequences of using the 3rd x16 slot in x4 mode - but I've missed any explanation of what happens when using the 2nd slot in x4 mode. I'd like to have a clear understanding BEFORE committing to an external enclosure & RAID card for port replication, at x4 mode using this slot.

This board has 3 PCI x16 slots. Currently as x16: _1 = hd6870; _2 = open; _3 = another card at x1 (Auto mode - which only drops the PCI x1 _2 slot - which is already blocked by the dual slot graphics card).

Would using the x16 _3 at x4 mode lower the x16 _1 slot (graphics processor) to x8 mode? - as would clearly be true if the x16_2 slot is used to cross-fire a second graphics card.

As an x16 slot is backward compatible with a PCIe card at x4 mode, I have little or no doubt it would work. But I do not understand what the consequences would be, and I'm not fond of surprises. Especially when such cause things to stop working - e.g. eSata connectors, USB 3 headers, etc.

Any guidance will be gratefully appreciated. Thanks.

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February 20, 2013 11:30:19 PM

Using the second PCIe x16 slot will knock down the primary slot to x8 even if you are only using it at x1 or x4 speeds, but it won't cause anything like ESATA or USB to stop working. In fact, with your video card, you likely won't see any performance decrease even with the primary slot running at x8 speeds. This is an older(ish) article, but it shows the difference between running cards similar in performance to your in both x8 and x16 modes (just ignore all the SLI/Crossfire stuff): The Difference Between PCIe x8 And x16. So at most, you might see a ~2 FPS drop while gaming.
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February 20, 2013 11:48:29 PM

Thanks. Running the 3rd x16 slot at anything other than x1, results in a loss of eSata, 2 PCIe x1 slots, and the onboard USB3_34 header. Thus - my question. I remember the rather surprising comparisons - a few years ago - showing the very small difference between x8 and x16 modes. Like you say - not a huge deal. Thanks again for the quick reply.
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February 28, 2013 1:16:54 AM

Best answer selected by defg0003.
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February 28, 2013 1:17:36 AM

PS-Matt said:
Using the second PCIe x16 slot will knock down the primary slot to x8 even if you are only using it at x1 or x4 speeds, but it won't cause anything like ESATA or USB to stop working. In fact, with your video card, you likely won't see any performance decrease even with the primary slot running at x8 speeds. This is an older(ish) article, but it shows the difference between running cards similar in performance to your in both x8 and x16 modes (just ignore all the SLI/Crossfire stuff): The Difference Between PCIe x8 And x16. So at most, you might see a ~2 FPS drop while gaming.



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