Hi Phil,
This is a subject I can probably answer just about any question you
might have. At work, I am responsible for helping design a system that
uses the PCIe bus extensively. We have to deal with multiple lane widths
(x4 - x16) and Rev 1.x as well as 2.0. I have been through most of the
PCI-SIG specs on a very detailed level.
To answer your question, PCIe 1.x and 2.0 systems are supposed to be
fully compatible with each other. One of the most basic goals put
forward from the PCIe group within the PCI-SIG was that the two standars
be fully inter-operable. As such, all aspects of the system, including
electrical (physical layer), data transport (data link layer) and high
level protocols (transaction layer) were designed so that they can
detect which type of system they are installed on and adjust
accordingly. Thus a PCIe 1.x card will operate in a 2.0 slot, and 2.0
cards will operate in 1.x slots. The protocol involves that the links go
through a "training" sequence upon hard reset to detect which one they
will both operate at. Extended features in the protocol are negotiated
via capabilities that are advertised by the devices. The only difference
with a 2.0 card in a 1.0 slot is that its maximum bandwidth is cut in
half. (1.0 supports 2.5GT/s per lane while 2.0 runs at 5.0 GT/s per lane).
It is also OK to put a smaller lane device in a larger slot, ie an x1,
x4, or x8 card in an x16 slot. However a larger lane card will not
physically fit in a smaller lane slot, although the protocol technically
supports it as the usable lanes is also negotiated. This is why quite
often you might see motherboard that lists a slot as being x4 in an x8
slot, meaning only 4 lanes are wired but physically the slot will
accommodate an x8 card. You might also find x8 lanes in an x16 slot.
This is useful to support PCIe video cards which pretty much come only
in an x16 form factor, but the motherboard chipset does not have enough
lanes to be able to wire all 16.
BTW, the PCI-SIG is pretty far along in solidifying the 3.0 spec which
will double the bandwidth yet again from the 2.0 spec.
Russ Poffenberger
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On 2/26/2010 8:23 AM, Phillip Williams wrote:
> My latest build I'm working on uses an ECS G31T-M mobo which has a pci-e x1
> slot (revision 1.1) and a smaller pci-e1(rev. 1.0a). I'm not sure what
> the smaller slot is for but my real question is, will a pci-e 2.0 card
> work at it's optimum on this board? The ecs site says it takes a x16
> card but when shopping online, cards are listed as pci-e or pci-e 2.0.
> I figure it will be fine but thought it would be wise to check with
> you guys (and gals)
> about compatibility issues.
>
> Thanks,
> -Phil Williams-
>
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