What is this "P55 Chipset PCI-E 2.0 16x limitation" ?

[Justin]

Hi all,

I've been thinking about getting a  GA-P55A-UD3 motherboard to replace my ageing AM2 mobo for quite some time now, but I've read that there's some kind of "PCI-E 2.0 16x limit"? Specifically, a user said that if one graphics card is installed and SATA 3 and USB 3 devices are in use, then the graphics card is forced to run at 8x. Why is this? Why can't both run at 16x?

I've ordered a Crucial C300 hard drive to replace my ageing IDE drive as well, but since it uses SATA 3 and my radeon 5770 is currently on a PCI-E x16 2.0 lane, will both of them have to run at 8x? It can't be right, because SATA and PCI-E is different if I'm not mistaken?

Thanks in advance

[Dark Mantis]

I have just checked the manual for that board and as far as I can see what you were told was wrong because the PCIe 16x bus is seperate to the PCIe bus that supports the SATA3 etc. So they do not share bandwidth.

[Justin]

Hmmm, maybe I'm not understanding this properly, but why does it still say something like that on the product page? So will I still get to use my radeon 5770 at full speed on the PCI-E x16 2.0 lane, whilst the SSD is also running at its fastest?

bottom note - http://www.gigabyte.com/products/product-page.aspx?pid=3439#sp

* When SATA3 / USB 3.0 (Marvell 9128 /NEC USB 3.0 Controllers) work at turbo mode, 1st PCIE x16 will work at x8 speed. " (My Crucial C300 has the marvel controller)

[Dark Mantis]

Yes I see what you mean. I was basing my answer on the block diagram from the manual and on thta it shows the lanes as totally seperate. The PCIe graphics runs directly to the CPU and the Marvell controlled SATA3 lane runs to the P55 chipset. Therefore I can't see how one would interfere with the other. See here: http://download.gigabyte.eu/FileList/Manual/mb_manual_ga-p55a-ud3_v2.0_e.pdf
Maybe someone else will have a different viewpoint.

[Peteruk]

The block diagram simply doesn’t show it but there is a BIOS option to force it to run off the P55 by disabling “Turbo SATA3 / USB3.0”. 

[Dark Mantis]

Ah, that explains it then. I did read the note at the bottom but it didn't seem to make sense. How did you find that out.

[Peteruk]

Ah, that explains it then. I did read the note at the bottom but it didn't seem to make sense. How did you find that out.

Because theirs an option in the BIOS to make it make sense.

[Justin]

Alright, I think I got it, so I will be able to use my GPU at a x16 2.0 lane, along with my SATA3 SSD at non-turbo mode? Is this correct?

[Dark Mantis]

That's how I understand it from the description peteruk gave and obviously he checked!

[Justin]

What is "turbo" SATA3/USB3 anyway? If I disable it in the BIOS, what becomes of it? will it still run at the 6gb bandwidth?

[Dark Mantis]

you will not be able to use the full SATA3 6Gb fully anyway even my SSD doesn't even get close to pushing it and theres next to no USB3 devices yet anyway.

[Justin]

right, thanks a lot for the clarification.

[Peteruk]

Being that the PCI-e is 2.0 @8x speed would be the same as a PCI-e 1.1 slot @16x which when  gaming with a high end card like a HD 5870 does not decrease performance much if at all.

Even though you went for a Crucial C300 SSD which can break 3Gbps SATA speeds you should just use the Intel 3Gbps SATA ports.

[Dark Mantis]

Being that the PCI-e is 2.0 @8x speed would be the same as a PCI-e 1.1 slot @16x which when  gaming with a high end card like a HD 5870 does not decrease performance much if at all.

Even though you went for a Crucial C300 SSD which can break 3Gbps SATA speeds you should just use the Intel 3Gbps SATA ports.

Good reply, although I would have to differ regarding the SSD speed. The theory is correct but in practice even the SATA3 spec'd SSDs like the Crucial never get up to the full throughput of 6Gbs.

[Peteruk]

Good reply, although I would have to differ regarding the SSD speed. The theory is correct but in practice even the SATA3 spec'd SSDs like the Crucial never get up to the full throughput of 6Gbs.

No not up to 6Gbs but passed 3Gbps.