PCI-E Speed Question

[gamerteck]

Hi,

I'm looking to get a new mobo but I'm unsure on how PCI-E, in crossfire mode, will be effected by the PCI-E slot speeds.

I believe that I want both cards, in crossfire, to run at x8 if the first PCI-E slot rates at x16? I'm assuming this since the mobo has a PCI-E slot that is x16 so when I put in a second card it will split the speeds between both, which result in x8 speeds for both?

The mobo specs say - 2 x PCI Express 2.0 x16 (green @ x16 mode, orange @ x4 mode).
I have no idea how the orange@ x4 will affect the crossfire speeds. If I add a second card does it mean they will both run at x4? I know for ram that if you get a high and low speed ram the high speed ram will operate at the speeds of the slower one. Is this the same for PCI-E?

Thanks
~G

 

[lopri]

What board are we talking about?

Oh, welcome to AnandTech Forums.

 

[gamerteck]

Thanks, glad to be here, very helpful and informative forum.

Here is the mobo i'm talking about - ASRock - A780GXH/128M

 

[lopri]

Hmm.. It doesn't look like that board will support x8/x8 configuration. It looks like the top slot is fixed at x16.

 

[gamerteck]

How can I be sure that a mobo will operate at x8/x8. Will it say, for example, 2 x PCI Express 2.0 x16 (green @ x16 mode, orange @ x8 mode)?

 

[cbn]

Originally posted by: gamerteck
How can I be sure that a mobo will operate at x8/x8. Will it say, for example, 2 x PCI Express 2.0 x16 (green @ x16 mode, orange @ x8 mode)?

I'll bet that mobo runs the video cards at x16 and x4 (like the old P35 mobos did)

PCI-E 2.0 @ x4 might be a problem now for some good video cards and this bottleneck will only get worse with each succeeding generation of GPUs.

 

[Sylvanas]

According to the link, one slot is locked at 16x (green slot) and the other is 4x (orange) this usually means that the 4x slot is electrically only capable of 4x mode.

 

[AstroGuardian]

Yes the second slot will work x4 mode. First hand info. I just checked with a colleague from the hardware department.