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I'm trying to understand how it's possible that a passive riser has the following specs:

Output: (3) PCI-E x16

Signal: (1) PCI-E x16 (2) PCI-E x8

This would seemingly be creating 48 PCI lanes out of 32! And yes the vendor is insistent that it's a passive riser.

The riser in question is Supermicro p/n RSC-R2UW-2E8E16. Their riser matrix is here: https://www.supermicro.com/en/support/resources/riser

Thanks!

Vitalydotn
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1 Answers1

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A PCIe x16 slot doesn't necessarily have 16 lanes.

As far as I can tell from their website, "Signal" means how many active lanes there are per connector, while "Output" means the physical connector size.

The picture of your riser shows three mechanical x16 slots. Furthermore, your provided part number is designed to work with this board which provides 32 lanes through its riser connector.

Based on this information, I would therefore conclude that while the riser has three mechanically x16 slots, they are not all electrically x16. One connector has all 16 lanes, and the other two have only 8 lanes each. This gives a total of 32 lanes.

It is perfectly fine for a connector to have fewer active lanes than the size of the socket. All PCIe devices are required by specification to work in a x1 mode, as will as one or more xN modes.

If you plug in a card that is designed for x16 mode, it will still work in the x8 slot, however with fewer lanes enabled. In many cases it will operate in its fallback x1 mode which is the only mode it is required to support.

In many other cases, e.g. lots of graphics cards, it will work in a x8 mode quite happily.

Tom Carpenter
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  • Thanks Tom! I just updated with the part number and list to the vendor's riser matrix. It's a bit sparse on details though :( I'm chasing it with them. Made the requirement clear that it needs to accommodate X16 cards and they came back with this part. – Vitalydotn Apr 10 '20 at 14:10
  • @Vitalydotn you can plug in three x16 cards into that riser. However only one of them will operate in x16 mode. The other two will operate in either x8 mode (if cards support it), or x1 failback mode otherwise. All three will function fine, its just two will run slower. – Tom Carpenter Apr 10 '20 at 14:22
  • Thanks again Tom. I think you're most likely correct! That makes sense, but I wonder how one would tell which slot gets the full X16. Also their matrix as you can probably gleam is very out of date. https://www.supermicro.com/en/products/motherboard/X11DPU-ZE+ is the board they suggested it for, on the left riser slot (X32) – Vitalydotn Apr 10 '20 at 14:32
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    @Vitalydotn looking at the picture of the riser, the top slot (Slot 1) is the x16 one, as it appears to have all 16 pairs going to it (16 on one side will all be Rx, probably 16 on the other side will be Tx). – Tom Carpenter Apr 10 '20 at 14:47
  • Another thing I have seen on risers is using passive switches to allocate lanes depending on which slots are populated. For example, two x16 slots on the riser, and one steals 8 lanes from the other one when a card is installed in that slot. So with one card installed, you get one x16. With two cards installed, you get two x8. – alex.forencich Apr 10 '20 at 17:34
  • @alex.forencich Aye. Quite a few computer motherboards play that trick as well. – Tom Carpenter Apr 10 '20 at 18:44
  • Fascinating. One of your two theories are correct I'm sure. Will update this when (if?) I get an answer from the vendor! – Vitalydotn Apr 11 '20 at 15:04
  • This is confirmed as the correct answer, top slot is X16, other two are X8 electrically. Thank you all! – Vitalydotn Apr 13 '20 at 20:06