PCIe Gen2 x16 into PCIe Gen3 x8

[alexusa]

I was wondering if placing a PCIe Gen2 x16 card in a PCIe Gen3 x8 slot would work? I have a PCIe Gen3 motherboard which can take up to 4 PCIe Gen3 x16 cards or 8 PCIe Gen3 x8 cards. Since the bandwidth is doubled for PCIe Gen3 compared to Gen2 (1GB/s compared to 500MB/s), would I be able to connect 8 Gen2 x16 cards into this board? Thanks in advance.

Alex

 

[ShintaiDK]

I assume you got X79 or Ivy Bridge CPU? Else you dont have PCIe 3.0.

Also an electrical PCIe 3.0 x8 would downgrade to PCIe 2.0 x8. Not x16.

 

[Don Karnage]

Need an Ivy Bridge processor or Sandy Bridge E to use PCIe 3.0

 

[alexusa]

It's a dual socket server motherboard containing the Patsburgh (C600 -A) chipset and two E5-2600 series processors. I believe Ivy Bridge for server boards does not come out until 2013.

Shintai, what do you mean by "an electrical PCIe 3.0 x8 would downgrade to PCIe 2.0 x8. Not x16"? If it needs the same amount of bandwidth why wouldnt it be supported?

Thanks in advance.

 

[alexusa]

Need an Ivy Bridge processor or Sandy Bridge E to use PCIe 3.0

Don, I have a Sandy Bridge EP board

 

[Don Karnage]

Don, I have a Sandy Bridge EP board

Oh then you won't have an issue at all then

 

[alexusa]

Thanks for the quick reply! So you're saying that a Gen2 x16 card will operate at full bandwidth if plugged into a Gen3 x8 slot?

 

[ShintaiDK]

Thanks for the quick reply! So you're saying that a Gen2 x16 card will operate at full bandwidth if plugged into a Gen3 x8 slot?

No, it will run at PCIe 2.0 x8. Its an x8 slot, not x16.

A PCIe 2.0 x16 card in your case would need a PCIe 3.0 x16 slot to operate at full speed. Even when it gets downgraded to 2.0.

 

[alexusa]

Shintai, why? The amount of bandwidth required for a Gen2 x16 is equal to the amount required for Gen3 x8

 

[OVerLoRDI]

Are these physical 8x length plugs or are they full length 16x plugs with circuity for 8x bandwidth?

 

[OVerLoRDI]

Actually what I am asking is irrelevant. If the slots are good for pci-e 3.0 8x, that means they have 8 lanes of pci-e connected to them. Plugging a pci-e 2.0 card, the card will run at 8x pci-e 2.0. Despite the bandwidth being doubled from 3.0 to 2.0, you still restricted by the number of lanes.

 

[ShintaiDK]

Shintai, why? The amount of bandwidth required for a Gen2 x16 is equal to the amount required for Gen3 x8

It just doesnt work that way.

Your slot is electrical x8 and thats the key issue. Else you would have x16 3.0 speed as well.

Im not sure how you even imagined it would end up as x16 2.0 from an x8 3.0 slot.

 

[alexusa]

OverLord, This is a riser containing two physical x16 slots. The riser spec says that you can either use a Gen3 x16 card in one of the slots or 2 Gen3 x8 cards in both slots (x8 electrical, x16 mechanical). Now since the bandwidth for Gen3 is double that of Gen2, I'd imagine you should be able to use two Gen2 x16 cards in those Gen3 x8 slots. Physically they fit because both the slots and the cards are mechanically x16.

 

[ShintaiDK]

OverLord, This is a riser containing two physical x16 slots. The riser spec says that you can either use a Gen3 x16 card in one of the slots or 2 Gen3 x8 cards in both slots (x8 electrical, x16 mechanical). Now since the bandwidth for Gen3 is double that of Gen2, I'd imagine you should be able to use two Gen2 x16 cards in those Gen3 x8 slots. Physically they fit because both the slots and the cards are mechanically x16.

Its still limited by the lanes. Nomatter of the bandwidth. Its completely irrelevant if its PCIe 1.0, 2.0 or 3.0.

I assume you ecpect to run 2 cards at x16 speeds.

The riser is simply either splitting the x16 into 2 x8 lanes if you use 2 cards. Or one x16 lane if 1 card.

 

[greenhawk]

Now since the bandwidth for Gen3 is double that of Gen2, I'd imagine you should be able to use two Gen2 x16 cards in those Gen3 x8 slots. Physically they fit because both the slots and the cards are mechanically x16.

what you are missing is that the Gen2 cards do not know how to talk "Gen3" so can not take advantage of the faster running pci-e x8 lanes to get effectivly the same bandwidth of a 16 lane Gen2 card. No matter what you do, the electrical wire count will be set by the manufacture and you can not change it.

To get a full 16 lane PCI-E slot, you would need it designed that way (pci-e plexing(?) chip). If that was possible, it would have been mentioned in the product spec of the riser card.