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Speed of PCIe3.0 slot in crossfire with PCIe2.0

Last response: in Motherboards
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April 6, 2012 7:34:57 PM

Hi,

I am looking for a budget mobo for my upcoming IB build.
I am going to put an AMD HD 6870 in it forthe time being, and crossfire it in future.

So, I need to have a board which can support crossfire decently wihthout bottlenecking my cards much.

I am given to understand that for a mid-range card like a 6870, an x16-x4 configuration is almost at par with a x8-x8 configuration. I know that an x8-x8 configuration is 5-10% faster for the second card, but that amount of performance hit I can take if it saves me some $$.

The one thing I am unclear about is that when crossfire is enabled with a PCi 3.0 lane initially at x16 speed with a PCIe2.0 lane also initially at x16 speed, do both of them get changed to x4, or the PCI3.0 lane remains at x16 with the PCIe2.0 slot degraded to x4???????????

For eg., I was looking at this motherboard -
http://www.gigabyte.in/products/product-page.aspx?pid=4...

SO do both the lanes run at x4 or only the PCI2.0 gets turned into an x4???????//
April 7, 2012 5:09:47 AM

C;mon guyz! No replies?? I really need to know!
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April 7, 2012 5:32:22 AM

I am not quite sure what you want to know but you're never going to get PCIe 3.0 speed with a 6870 card installed since the 6870 is a PCIe 2.0 card. PCIe 3.0 requires that all devices support 3.0 speeds to enable it. Otherwise it will run at the maximum speed of the slowest device on the bus.

Also, that motherboard only supports PCIe 3.0 with an IB CPU installed. This is because the PCIe 3.0 is a part of the IB CPU and it is connected directly to the first PCIe x16 slot on the motherboard. The rest of the slots are PCIe 2.0 according to the webpage.
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April 7, 2012 5:47:25 AM

It says that both the lanes gets transformed into x4 lane.
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April 7, 2012 8:36:49 AM

aviral said:
It says that both the lanes gets transformed into x4 lane.


Thats what I was looking to find out. So is x16-x4 even possible?? Even I think that both the cards should get reduced to the lowest speed for compatibility, like both in x4 for this mobo.
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April 7, 2012 9:28:43 AM

gam0reily said:
Thats what I was looking to find out. So is x16-x4 even possible?? Even I think that both the cards should get reduced to the lowest speed for compatibility, like both in x4 for this mobo.


Sorry to say that it will not support 16x-4x configuration at all when in crossfire mode.It will be working at x4 speed only.
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April 8, 2012 4:30:48 PM

then why the hell are some sites saying "x16-x4 is posible"?
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April 8, 2012 5:08:21 PM

gam0reily said:
then why the hell are some sites saying "x16-x4 is posible"?


The link which you shared on your post is clearing notifying the users that it will shift its mode to x4 when in crossfire mode(Read Multi-Graphics Technology point on your link provided) and when it is not in crossfire mode you will be using x16 mode.

The sites will be saying that x16-x4 setup is possible but you will achieve a x4 bandwidth with crossfire mode.I hope now you understand my point.
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