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Discussion Starter #1
Hi guys.

I'm reading reviews on some Motherboards, and there's one thing I never heard about. They're talking about PCI-E lanes, and claim that nForce chipsets can give 40+ lanes, while the 975X chipet can only produce 24 lanes.

Can anyone explain me what are these lanes and what is their purpose?
 

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I am pretty sure they mean pipelines. Which are the "tubes" if you will that carry the information. The more "tubes" the more information. And its like the internet its a series of "tubes."
 

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Discussion Starter #4
Thank you.


So lanes aren't pipes. They are the tubes in which the data is transferred between two PCI-Es. If that's the case then what would happen when there's only PCI-E slot?

Another question that comes to mind now, is regarding the PCI-E speed spec. When sayin an x16 PCI slot does it mean that it consists of 16 lanes? or am I just mixing things up.
 

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Quote:


Originally Posted by GGuyZ

Thank you.


So lanes aren't pipes. They are the tubes in which the data is transferred between two PCI-Es. If that's the case then what would happen when there's only PCI-E slot?

Another question that comes to mind now, is regarding the PCI-E speed spec. When sayin an x16 PCI slot does it mean that it consists of 16 lanes? or am I just mixing things up.

Pipelines are the "lanes" as they are explaining it, that the information gets travelled through. The more "lanes" the more information ie the more Bandwidth which leads to higher performance.

You still have the pipelines with a single GPU using only 1 pci-e slot.

As for the 16x and 8x im still trying to get that one figured out. But I am pretty sure its the amount of information that can be transferred at one time. Look at what Enterprise posted below.
 

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Quote:


Originally Posted by GGuyZ

Thank you.


So lanes aren't pipes. They are the tubes in which the data is transferred between two PCI-Es. If that's the case then what would happen when there's only PCI-E slot?

Another question that comes to mind now, is regarding the PCI-E speed spec. When sayin an x16 PCI slot does it mean that it consists of 16 lanes? or am I just mixing things up.

No 16x states the bandwidth.

With some motherboards if you are running SLI or CF each PCI-E slot will run on 8x bandwidth ( 16 x split between 2 cards )

However with newer motherboards you can also have the above and keep the higher bandwidth of 16x on each slot.
 

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Originally Posted by ENTERPRISE1701

No 16x states the bandwidth.

With some motherboards if you are running SLI or CF each PCI-E slot will run on 8x bandwidth ( 16 x split between 2 cards )

However with newer motherboards you can also have the above and keep the higher bandwidth of 16x on each slot.

Isn't the number of lanes=the bandwith? Here's what I'm basing this assumption on:

Quote:


Two PCIe devices are connected by a link, and each link is made up of one or more lanes. Each lane consists of two low-voltage, differential signal pairs carrying 2.5 Gbps of traffic in opposite directions. One pair is used for transmitting, and the other pair is used for receiving. To further increase the bandwidth of a link, multiple lanes can be placed in parallel (x1, x2, x4, x8, x12, x16, or x32 lanes) between two PCIe devices to aggregate the bandwidth of each individual lane. In the future, the signaling rate of the link can be increased to provide even more bandwidth.

Asides from that, I think some of you are confusing video card's pipelines with PCI-E Lanes.

Each video cards has it's own number of pipelines which build up to be the bandwith. However, this has nothing to do with the PCI-E lanes(which determine the bandwith as well), since PCI-E slots aren't necessarily meant for video cards.

If I'm wrong on this, please correct me.
 

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Quote:


Originally Posted by GGuyZ

Isn't the number of lanes=the bandwith? Here's what I'm basing this assumption on:

Asides from that, I think some of you are confusing video card's pipelines with PCI-E Lanes.

Each video cards has it's own number of pipelines which build up to be the bandwith. However, this has nothing to do with the PCI-E lanes(which determine the bandwith as well), since PCI-E slots aren't necessarily meant for video cards.

If I'm wrong on this, please correct me.

PCI-E slots are generally only meant for Expansion Video cards as yes.

PCI-E bandwidth and lanes are along the same lines yes.
 

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Discussion Starter #11
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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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.
 

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Discussion Starter #13
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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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.
 

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Discussion Starter #15
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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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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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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