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Is this PCI Express 2.0 or not?

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January 5, 2010 2:06:29 AM

I've got a Gigabyte P55A-UD3 with an i7 860 and ATI 4890. The board has crossfire that can run at 8x/4x. The normal 16x slot is version 2.0, but I can't tell if the 4x slot is PCIe 2.0 or 1.1. It seems weird to have one 2.0 slot and one 1.1 slot, but it's not too clear to me. Tech support was no help at all. Can anyone look at the links and give me opinions?

http://www.newegg.com/Product/Product.aspx?Item=N82E168...
http://www.gigabyte-usa.com/Products/Motherboard/Produc...

More about : pci express

January 5, 2010 1:34:03 PM

The chipset/CPU defines whether you are running PCIe 1.1 or 2.0. And as mentioned above that CPU/chipset are PCIe 2.0 compliant. Now that chipset also only allows 1 x 16 lanes for video. Vendors then use bridges/muxes to split that out for 2 x 16 slots. The total throughput is always limited to 1 x 16 lanes for that cpu/chipset.

While you could insert a second graphics card to Crossfire, I may not do it with only a X4 capable slot. I would say stick with one graphics card on this motherboard.
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a b V Motherboard
January 5, 2010 1:49:13 PM

yea, x8 is ok for crossfire as the penformance impact is minimum, but at x4 there is a big slowdown in penformance
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January 5, 2010 2:11:36 PM

The chipset/CPU defines whether you are running PCIe 1.1 or 2.0. And as mentioned above the X55 CPU chipset is PCIe 2.0 compliant. Now that chipset also only allows 1 x 16 lanes for video. Vendors then use bridges/muxes to split that out for 2 x 16 slots. The total throughput is still always limited to 1 x 16 lanes for that chipset.

While you could insert a second graphics card to Crossfire, I may not do it with only a X4 throughput slot available. I would say stick with one graphics card on this motherboard.

I believe Toms showed the throughput on x16, x8, x4 etc in a benchmark
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January 5, 2010 5:12:18 PM

Quote:
Now that chipset also only allows 1 x 16 lanes for video. Vendors then use bridges/muxes to split that out for 2 x 16 slots. The total throughput is still always limited to 1 x 16 lanes for that chipset.

So, is it 2.0 or only running at PCI Express 1.1? You say that it is 2.0, that it only has 1x16 lanes. PCIe 2.0 4x is equivilant to PCIe 1.1 8x, right?

According to this review, the 4x only limits the 5870 about 4-5%. This doesn't seem like a dealbreaker. Am I reading this correctly?

Could I put my 4890 in the second slot and benchmark it? Or would it run higher than x4 if it was the only graphics card?
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a b V Motherboard
January 6, 2010 1:58:54 AM

that would be the best, bench your 4890 on both slots then compare the results
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January 7, 2010 6:51:23 AM

I tested the XFX 4890 in both the PCI-express 16x and 4x slot. Here are the results. This was with the GPU benchmark in the Crysis demo with all settings to "Very High" at 1440x900. All numbers are the average of three runs. The stock settings were 850/975 and the OC settings were 925/1075.

16x.....= 28.17
4x......= 24.21
16x OC= 30.86
4x OC.= 25.93

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a b V Motherboard
January 7, 2010 5:13:15 PM

Nice test. So it looks like you're losing about 15% performance by moving from the x16 slot to the x4 slot.

To answer your original question: The x4 slot is PCIe 2.0, but it's speed is limited to the speed of PCIe 1.1.

I thought the question was going to be easy to answer, but I ended up having to do a little digging. The first thing I found which is what I wanted to point out originally is that the lanes for those slots run to different places. Take a look at this Intel Product Brief (warning: PDF) or any Intel P55 Chipset Block Diagram:
http://www.intel.com/Assets/PDF/prodbrief/322641.pdf

In Figure 1 on page 2, you'll notice that 16 lanes connect directly to the processor. On some boards this is divided into 2 x16 slots which, when both filled, operate at x8 each. On your board the primary x16 slot uses all x16 lanes that connect directly to the processor. (With your board though, one important note is that: "When SATA3 / USB 3.0 (Marvell 9128 /NEC USB 3.0 Controllers) work at turbo mode, 1st PCIE x16 will work at x8 speed")

All other PCIe slots connect to the P55 chipset's PCH (Platform Controller Hub), as also seen in that diagram. Now here's where things get confusing. The reason I linked to that Product Brief is that when I looked there to determine how the remaining PCIe slots functioned I found something I had read and subsequently forgotten. In the block diagram you'll notice it clearly shows there are 8 PCI Express x1 lanes running into the P55 chipset at 500 MB/s each x1. However, when you follow the * note down to the bottom of the document (page 3) it states that the PCI Express Interface "Offers up to 2.5 GT/s for fast access to peripheral devices and networking with up to 8 PCI Express 2.0 x1 ports, configurable as x2 and x4 depending on motherboard designs."

So while the diagram shows the PCIe lanes operating at a data rate of 500 MB/s - PCIe 2.0 - and the fine print says the lanes are version 2.0, the fine print also states that the lanes operate at a transfer rate of 2.5 GT/s - PCIe 1.1!

At first I thought I had missed something. But after a little googling I determined I was not. The lanes are version 2.0, but their speeds are limited to version 1.1. Why? According to the AT article where I confirmed this answer, "We believe with the DMI link continuing to operate at 1GB/s in each direction, a decent 6Gb/s SAS/SATA RAID card and a few upcoming 6Gb/s drives could easily saturate the link."

Check out this page for their explanation and answer: http://www.anandtech.com/mb/showdoc.aspx?i=3652&p=2
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