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How to tell what 3way XF/ SLI performance will be on a motherboard?

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#1
When looking for motherboards most of them have it listed what 2 way will be (usually 8x 8x), but I have a hard time sorting out what 3way performance will be. what's the best way to find this info?
 
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#2
Are you looking for how well 3-way would scale or what the PCI-E lane configuration of each board would be using three cards?
 
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#3
Lane configuration.
 
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#4
It usually states it in the specs along with the 2 way pcie speeds. Whats the mb model?
 

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#5
For what CPU/chipset?
 
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#6
The fastest way, avoiding explaining the presence of surface mount capacitors and switches, would be just to tell us which boards you are looking at.
 
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#7
Well yeah, I could list some motherboards (and I will after this), but I want to know how to read the specs for myself for the future or in case my tastes change. I don't want to just have it pointed out to me which models will and wont work.

Here's a list though
 
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#9
I'll be using this picture as a reference since the SMD caps show clearly.

In this next pic, you can count the surface mount capacitors (in the red boxes) above, or sometimes below (or both as is the case with the first slot in this pic) the PCI-E slots. This tells you the maximum number of lanes the slot is capable of.
http://img402.imageshack.us/img402/2999/201107130320.jpg

Some boards use channel switches that convert one x16 slot into two x8 slots when they sense a second card. Sometimes they are used to further split x8 into two x4 slots. It takes four channel switches to convert x16 to x8/x8.
http://img703.imageshack.us/img703/3226/201107130324.jpg

So for this board. we can see that the top slot has sixteen surface mount capacitors, eight above (some hidden) and eight below it. The second blue slot has eight. The channel switches between the first two blue slots switch the top slot to x8 when the second blue slot is populated. The third blue slot has four caps above it making it x4. This is fixed, it won't change. The fourth blue slot has sixteen caps so it's an x16. The channel switches above it convert it to x8 when the last blue slot is populated.

So let's put this into practice:
With one card, you're obviously going to be running just one x16 slot.
With two cards, you will most likely put them into the first and fourth blue slots which gets you two full x16 slots. You could put the cards in the first two blue slots, getting you x8 and x8.
With three cards, you would put them in either the first, second, and fourth blue slots getting you x8, x8, and x16, or put them in the first, fourth, and fifth blue slots, getting you x16, x8, x8.
With four cards, you would use the first, second, fourth, and fifth, getting you four x8 slots.

The only two boards capable of 3-way SLI/Xfire in your link are the UD7 and the Maximus. The Extreme4 can do 3-way Crossfire, but I wouldn't recommend it.
 
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#10
The only two boards capable of 3-way SLI/Xfire in your link are the UD7 and the Maximus. The Extreme4 can do 3-way Crossfire, but I wouldn't recommend it.
I'm going to take a closer look at your reply later as I'm just visiting here in between doing some deskwork, but why don't you recommend it on the Extreme 4? Is it because it'll end up being 8 8 4?
 
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#12
Because it's x4 running off the PCH.
Ok, so I just looked back at the post you made and it's easy to follow. I understand how to look at a motherboard and tell what it's capable of in terms of multiple GPUs, but what if I don't have a clear hi-res picture to go off of. Is there a way I can just read the description and tell?
 
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#13
Specs should tell the lane configuration, but often they are inaccurate, misleading, non-descriptive, or non-existent. That MSI board I was using as reference is listed on Newegg as PCI Express 2.0 x16: 5 x PCI Express 2.0 x16 (2 x16, 2 x8, 1 x4). Not too descriptive is it? It doesn't tell you which lanes are which. Or that Gigabyte UD7 in your list, 4 (2 @ x16 or 4 @ x8). This doesn't tell us what 3-way would do.

You should also keep in mind how many lanes the chipset or processor can give out. This way if you count more lanes than is possible, you'll know that either A, you counted wrong, B, there's a hidden switch, such as an NF200 (which typically makes sixteen lanes into 32 lanes) under a heatsink, or C, some of the lanes are coming from the PCH.
 
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