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PCIe Lane limit ???

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November 14, 2006 6:00:03 PM

I was wondering if there is a limit to the number of PCIe lanes that can be on one motherboard. I've done some research and found nothing to indicate if there is a limit. Current mobo's have 2 16x slots, 1 8x slot and 1 1x slot along with 2 PCI slots. I was wondering if the mobo makers could put 4, 5 or 6 16x slots instead. Is it technically possible ? I know it's possible to put a 1x card in a 4x, 8x or 16x slot and it automatically scales back to 1x, so why not just have 6 or more 16x slots avaliable on a motherboard and be done with it. I've also read that the PCIe standard 2.0 is due out some time in 3Q 2007 and will practically double the PCIe bandwidth. I've also read that PCIe 32x is a standard avaliable now, but no one makes use of it. Can anyone shed some light on limits here?
Thanks peeps...

More about : pcie lane limit

November 14, 2006 6:44:13 PM

Wikipedia states that the theres a 32 lane limit..

http://en.wikipedia.org/wiki/Pci_express

However I can't access the PCI-e technical docs at pcisig.com wihtout being a member so I can't verify it.
November 14, 2006 6:50:52 PM

Thats 32 lanes per slot. Current mobo's have 2 16x, 1 8x and 1 1x, that's 41 lanes total.......
I've done some research and can't find any limiting values. All I've found are articles stating that it's future scalable.
To what degree ?
Can we have a mobo with 8 16x slots ?
Anyone know ?
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November 14, 2006 7:09:16 PM

You can have as many lanes as you like, but each lane adds more hardware to the northbridge.

That is to say, 1000 lanes would require a massive (in terms of transistor count) northbridge, and as such would be prohibitively expensive.
November 14, 2006 7:24:22 PM

as Gakalist mentioned, that's a 32 lanes per slot limit, but of course, each extra lane is going to require more pins; with the current design style of the slots, that would mean a x32 PCI-e slot would be twice as long as a x16 slot, making perhaps only the GeForce 8800GTX the only consumer PCI-e card out there that would be long enough to accomidate all the pins. (of course, making pins smaller, perhaps in an array similar to seen on AGP cards, would reduce the slot size)

As for a total, I doubt there's an actual technical maximum; it's based upon however much PCI-e controller hardware is built into the Northbridge, as darkstar782 said. And the more transistors you cram in, the more expensive it gets; that's why, for instance, SLi chipsets that support only two x8 slots for SLi are cheaper than chipsets that can actually support both slots at 16 lanes per.

That, and the more lanes you have, the more traces you'd have running across the motherboard, which might not fit it all without adding more layers to the motherboard, and hence dramatically increasing the cost of that.

But yes, to answer your original question, it would be quite possible to design a chipset, and accompanying motherboard, that would have nothing but PCI-e x16 (or even PCI-e x32) slots. That'd be mighty expensive, and limited to super-workstation duties, though, as us "normal" folk don't have use for more 2, possibly 3, PCI-e slots with that much bandwidth; 2 for a dual-card solution, and a third for a GPU working for physics. (but that requires less bandwidth, only a x4 slot if not less)

Yes, anybody reading this can drool over their dreams of octo-SLi, but I don't think the ATX standard provides room for 8 slots. :p 
November 14, 2006 10:52:19 PM

Well, learn something new everyday. :) 
Good topic.
November 14, 2006 11:15:10 PM

It's like the number of cylinders in an engine in a car. There is no limit, it's all about how many you can fit. Some motherboards have more than others and you have to pay more for them like a car with a V8 is more expensive than a car with a V6.
May 14, 2012 7:29:22 AM

OK maybe I am missing something. just cause you add more lanes and even a chip that can handle that many lanes doesn't mean that 4 Gbps times 8 slots of data can be processed at the same time.
there has to be a bottle neck somewhere.
Processor power, GPU power, something has to be the limiting factor.
a c 156 V Motherboard
May 14, 2012 7:58:46 AM

I cannot speak for the AMD CPU's, but the limit for modern Intel systems is determined by the CPU. An LGA1155 or 1156 CPU only has 16 PCIe lanes. The only way to add more is to add a bridge chip such as the nVidia NF200.

An LGA2011 CPU has 40 lanes.
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