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A few questiosn about PCIe slots


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#1 RB_Kandy

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Posted 27 June 2012 - 11:21 PM

Before I even get into this, I am assuming that "PCIe x16" is another way of writing "PCIe 1.1" Let me know if I am wrong.

It is my understanding that all PCIe is backward compatible, or at least that's what I gather from the information overload I get on Wiki about it.
However, I decided to look at a motherboard with PCIe slots, and, PCIe x16 is small, but PCIe 2.0, and 3.0 is big.
So I'm guessing backward compatible in this case means a graphics card meant to be plugged into PCIe x16 can be plugged into a PCIe 2 and 3, but a graphics card meant to be plugged into a PCIe 2 or 3 will not fit into a PCIe x16.
So the PCIe slot on the MoBo should always be equal or higher than the device your plugging in, and never the reverse.
Have I got that right?

OK, the next question is, how to interpret how many PCIe slots I am getting in a motherboard.
Take this motherboard for example.
It says:
PCI Express 3.0 x16: 2 (single @ x16 or dual @ x8)

PCI Express 2.0 x16: 1 @ x4

PCI Express x1: 2

PCI Slots: 2

I don't understand the single and the dual, and I don't understand @x16, @x8, @x4 thing.

Edited by RB_Kandy, 28 June 2012 - 02:22 AM.


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#2 coxchris

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Posted 28 June 2012 - 12:27 AM

ok

PCI-e 3.0 x16 is the newest PCI-e bus x16 is the data path or bits per second

So they are backward compatible to PCI-E 2.0 x16 and so forth

3.0 is the speed on the PCI bus and x16 is how much data can be pass to the CPU on that bus (single GPU)


the next part of the question is if you want to SLI/Crossfire

SLI/Crossfire is a way to take two graphics card and make it into one. Assuming you have power to run two GPU

SLI/Crossfire is a connector that you have to purchase if you want to have multiple GPU

if you do SLI/crossfile you have to split up the data path on the PCI bus to 8 bps (dual x8)

If you have both PCI 3.0 slots used than your running in a config that allows only 8 bits on that bus to CPU
PCIe 2.0 x16 is the standard use of day it runs @4 bit per second

PCIe x1 is a special PCI slot for sound cards, adapter cards, etc

PCI slots is old technology that motherboard still use today

I hope I made it easy for you to understand. If you have any questions please post back

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#3 RB_Kandy

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Posted 28 June 2012 - 02:20 AM

OK, so where it says "PCI Express 3.0 x16: 2 (single @ x16 or dual @ x8)" what it means is there are two physical slots on the MoBo, if you use just one, it can transmit data at 16 bits per second. But if you SLI two Graphics cards, each slot will only transport 8 bits per second.
I think I understand. It's like the motherboard has 1 PCIe 3.0 (which allows for 16 bits) but they split it into two slots, and so if you use both PCIe 3.0 slots the maximum transfer rate is still 16bits per second. So it's one PCIe 3.0 split into two physical ports; rather than being two completely independent PCIe 3.0 ports that could both run at 16 bit.
Sorta like having one hose that pumps 16 gallons per second, and splitting into two hoses that can pump at 8 gallons each.

So if I understand things correctly, this next motherboardclaims:

A. 4 x PCIe 3.0/2.0 x16 (dual x16 or quad x8)
B. 2 x PCIe 3.0/2.0 x16
C. 1 x PCIe 3.0/2.0 x16 (x8 mode)

And it means that
A. is four PCIe 3.0's specifically made for crossfire/SLI and can run 1 graphic card at 16bits, 2 graphics cards at 16bits, 3 graphics cards at 8bits, 4 graphics cards at 8bits

B. is two PCIe 3.0's and does not allow for any sort of duel channel crossfire/SLI and run at 16bits each.

C. is one PCIe 3.0 that can run at 8bits only, cannot be used in duel channel Crossfire/SLI with the other PCIe's.

In "A" it says duel and quad, does this mean you cannot Crossfire/SLI 3 cards; only 1,2, or 4? Or is it merely implying that running more than 2 cards would reduce it to 8bits, whether it's 3 cards or 4 cards?

Edited by RB_Kandy, 28 June 2012 - 02:23 AM.


#4 coxchris

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Posted 28 June 2012 - 09:10 AM

the x16 x8 x4 are data paths for example you have a freeway that how much data travels to the CPU

This motherboard was deign for a very high end desktop or a sever

A. yes in a sense It was design for quad GPU or dual GPU

Dual runs at x 16bit per second
Quad runs at x8bits per second

b. Is backward compatibly you can stick in Pci-e 3.0/2.0 at 16 bit per second

c. is you have more gpu and if you fill that slot it will run at x8 bit only

In A configurations you can SLI and Crossfire x 16bits if you have two it will run at 8bits per second.

The motherboard was design to allow more than 2 cards if you stick 2 GPU in SLI/Crossfire it will run x16. Motherboards was design to share the data path that why you have x16 x8 x4

http://www.newegg.com/Product/Product.aspx?Item=N82E16813157280 my motherboard

I can only run SLI x4 bit per secould or dual gpus

Right now I have a single GPU that runs at x16 bits per second to the cpu

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