Are later spec PCIexpress video cards backwards compatible?

[TrevorS]

I'm in the process of putting together a new machine and my preferred motherboard is PCIExpress 2.0 whereas I see there are video boards out there spec'd at 2.1 and 3.0. My expectation is that a spec mismatch would result in the video board performing at less than full capability, but I wouldn't expect it to not work at all -- just fallback to the support capability of the motherboard spec.

Can anyone tell me the reality?

Thanks !

[Gizmo]

To the best of my knowledge, that is correct, with the possible exception of power requirements. IIRC, PCIe 3.0 increased the amount of power available from the slot. However, since video cards pretty routinely exceed the power available, they almost universally come with an auxiliary power connector, which makes that issue largely a "don't care".

[TrevorS]

Excellent! I read a post where a guy was saying his new video card won't work because its spec is ahead of his board, and so he was going to set it aside for a future mobo upgrade. Didn't make much sense to me, so I thought I'd ask.

[Gizmo]

In looking at the spec more carefully, it IS possible that a 3.0 card might not be backwards compatible to a previous PCIe slot.

This is because the 3.0 spec changed the encoding scheme from 8b/10b (which has a 20% signaling overhead) to 128b/130b (which has a 1.54% signaling overhead). Moreover in addition to the new encoding scheme, the data are scrambled in a very specific way requiring a descrambler at the other end. This allows the 3.0 card to achieve a throughput equivalent of 10 Gb/s per lane (as compared to PCIe 2.0), while only signaling at an actual rate of 8 Gb/s.

Because of this, the card may elect not to support slower speeds. IMO, this is rather stupid, since it's pretty well documented that current video cards don't even hit the full bandwidth of 16x slots at PCIe 2.0 speeds (much less 3.0) but it DOES make the silicon and drivers less complex.

[TrevorS]

Interesting, thanks for looking at that! My guess was the marketeers would require backwards compatibility in order to ensure sales. Without it, they'd just be begging for a boatload of returns. I actually looked for a later PCI-e spec motherboard for my just completed AMD build, but found nothing affordable more recent than 2.0. It appears AMD 3.0's just started showing up at the upper end in the last few months. Guess that's better than being bundled into the Steamroller release !

Still, as you pointed out, unless you demand bleeding edge performance, seems like 2.0 is just fine.