PCI Express x16, but 1 lane?

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DonP
Posts: 433
Joined: Mon Jun 09, 2003 5:47 pm
Location: London, UK

PCI Express x16, but 1 lane?

Post by DonP » Thu Aug 19, 2010 3:32 pm

Hi everyone,
Well, after six years of good service my underclocked, undervolted XP2500+ has given up the ghost - specifically the mobo's gone - everything else is fine.
No matter - I've already got a Point of View MB-D510-MATX on order to replace it.

The manual (not available at the web site, but someone was kind enough to scan it) states that there is, among other slots on the board, a PCI-E 16x.. but the manual says that it only supports 1x lane.
I understand the concept of lanes, but what does this mean in practice? That I can only use PCI-E 1x cards, I guess...?
Does anyone know any better?
TIA.

cmthomson
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Location: Pleasanton, CA

Post by cmthomson » Thu Aug 19, 2010 4:46 pm

PCIEx16 uses a very big connector (about 5 inches long, lots of pins). PCIEx1 uses a small connector (less than 1 inch, hardly any pins).

All (or I guess I should say nearly all) PCIE devices are configurable at boot time to use some or all of the lanes their chips support.

Although I've never come across the situation you describe, it does make some sense. The board has a big x16 connector that you can plug a graphics card into, but will only use one data lane instead of the more typical 16 or 8 lanes most motherboards would use. There is a good chance that the graphics card will automatically adapt, and run with compromised performance.
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faugusztin
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Post by faugusztin » Fri Aug 20, 2010 1:26 am

It is PCI-E x16 physically (look like a PCI-E x16 slot), but electrically it is only PCI-E x1 (so you have only the first few pins connected, the rest of the connector is just a dummy part.

Most times you get a PCI-E x16@x4 connectors in lowend boards with limited PCI-E lanes (like H55 boards for example).

AFAIK NVIDIA cards run even on PCI-E x1, ATI requires PCI-E x4.

DonP
Posts: 433
Joined: Mon Jun 09, 2003 5:47 pm
Location: London, UK

Post by DonP » Sun Aug 22, 2010 10:17 am

Thanks for the responses - sorry I didn't get back sooner but I ended up buying the board (not much choice really - I needed three slots and something like an Atom) and have been offline for a bit.

I did read about the lanes and that it's a negotiable thing but it made me then think why not have motherboards with all slots being x16 slots (apart from manufacturing costs) and just specify a limit in number of lanes.. so a board with four x16 slots and a maximum of, say, 16 lanes, could support either one x16 card or four x4 cards, or one x8, one x4 and two x2s, rather than being restricted in your card choice based on the slot types.

And you are both right about about the cards adapting - I even found a x16 to x1 adaptor - and a blog posting about how at least the card they tried did work with a degraded performance.

Now my niggle with this whole idea is how do I know whether a card will accept less lanes than its optimum expected number of lanes.. I guess it's down to reading manuals of the card that interests me.
I'm actually after a 4 or 6 port SATA card or a dual or quad gigabit card.

Thanks again guys.

DonP.

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