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What are PCI-E Lanes? - Page 2

post #11 of 19
Thread Starter 
atomicfission92: Not sure what bridge you're referring to?

ENTERPRISE1701: Thank you. Now that I understand the bandwith part a bit better, I don't understand the bandwith allocation the ASUS's motherboard I'm looking at is doing.

The board has 4 16x PCI-E slots, but can only deliver the following at best - 16x, 0x, 4x, 0x respectively.

I don't get it - what do the 0x ones stand for?
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post #12 of 19
0x means im gessing when not being used becasue its impossible to run on 0x bandwidth .

Now 4 x bandwith will occure if you have 4 cards running as each card will be given 4x bandwidth to play with as its 16x split between 4 cards.

However ive never heard of this however the logic would be that.

Also the 16,0,,4,0 maybe some kind of layout.
post #13 of 19
Thread Starter 
I'm pasting a paragraph from the review regarding this issue:

Quote:
Obviously the major differentiating feature of the P5W64 WS motherboard versus other motherboards based on Intel chipsets currently, is its additional 24 lane, 3-port PCI Express switch that Asus used to bring out additional PCIe connectivity for the chipset. The IDT 89HA0324PS PCI Express switch utilizes a single X8 connection to the 975X Northbridge and then in turn fans-out a pair X8 PCIe connections downstream to the expansion slots. So, in total, this chip takes the already existing 22 lanes within the 975X to a full 30 available for graphics and PCIe expansion.

The four full-length slots on this motherboard can be configured in either an X8,X8,X4,X8 configuration or X16,X0,X4,X0 (slots 1-4 consecutively). So in short, in addition to a standard dual graphics ATI CrossFire setup in the first two slots, you also get an additional X8 enabled full-length slot available for higher function cards like video cards, RAID controllers etc. On a side note, Asus retains two of the available PCI Express lanes for the dual PCI Express Gigabit Ethernet controller that is on this board, thus only the remaining 28 lanes are configured in expansion slots.
As they say, this board allow 30-2=28 lanes for the PCI-E. So x8,x8,x4,x8 is understandable since it equals 28 lanes. However, 16x,0x,4x,0x equals 20 - so where are the other 8 lanes?

Further more, in this review it is said that the 975X chipset usually allows only up to 22 lanes. That would mean that no 975X chipset board is actually able to deal with FULL double 16x SLI(in that case 32 lanes would be necessary).
If this is so, how come I never heard of this being a very significant disadvantage of the 975x chipset?
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post #14 of 19
Quote:
Originally Posted by GGuyZ
I'm pasting a paragraph from the review regarding this issue:



As they say, this board allow 30-2=28 lanes for the PCI-E. So x8,x8,x4,x8 is understandable since it equals 28 lanes. However, 16x,0x,4x,0x equals 20 - so where are the other 8 lanes?

Further more, in this review it is said that the 975X chipset usually allows only up to 22 lanes. That would mean that no 975X chipset board is actually able to deal with FULL double 16x SLI(in that case 32 lanes would be necessary).
If this is so, how come I never heard of this being a very significant disadvantage of the 975x chipset?
Nice find.

However for the 975 chipset that isnt a large disadvantage. Performance differences between say 8x and 16x is not noticeable at all. As there is no pci-e card that fully utilizes the 4x bandwidth then you wont see any improvement on running on a higher bandwidth.
post #15 of 19
Thread Starter 
thanks

I see your point.

Any ideas on how spreading the lanes in this way: 16,0x,4x,0x makes sense? since it doesn't add up to 28 lanes like it's supposed to. I assume this might be a typo?
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post #16 of 19
Well it looks to be a typo to me also..However someone here may know.
post #17 of 19
Another note you should watch out for, is that the 975X has 24 lanes (As you wrote), which means it can have 16x on one lane, and 8x on the second lane. It can only use quadratic numbers (1,2,4,8,16,32 etc.) in lanes. You can (AFAIK) only go up to 16x per PCI-E slot. You can expect if some mobo has more than 16 lanes, it can only be dual PCI-E compatible.
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post #18 of 19
In regards to the x0 slots you are talking about, those are the smaller pci-e slots found on many motherboards in betweenthe longer pci-e. Those i believe are for older SLI compatibility.
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post #19 of 19
you mean the PCI-E 1X. Doubt that would be the case due to the fact they share the left over lanes that arnt used..Thus it still has a bandwidth.
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