x16 slots on socket 1156 motherboards

[mfs378]

I want to build a P55-based micro atx system. I need to use 3 pci express cards: video card, a sound card, and a wireless adapter. Right now I'm leaning toward the Gigabyte P55M-UD4 (link).

My question is, does it matter whether I plug the video card into the upper or lower x16 slot? Will the electrical connection be x16 in both cases?

Thanks!

 

[Heidfirst]

that board only has 3 PCI-E slots - 2x x16 form factor + 1x x4 form factor.
I haven't looked into it closely but my money would be on the 2x x16 dropping to x8 electrically when both in use.

 

[mfs378]

As far as I know that is true, but that doesn't quite answer my question. More specifically, my question is: if I plug a single video card into the lower x16 slot, is the electrical connection also x16?

I could imagine that the lower slot is capable of up to x8 electrically, whereas the top one is capable of up to x16 electrically, but I'm not sure if this is true.

 

[Heidfirst]

this is what the Gigabyte product page says:
1 x PCI Express x16 slot, running at x16 (PCIEX16)
1 x PCI Express x16 slot, running at x8 (PCIEX8) (The PCIEX16 and PCIEX8 slots conform to PCI Express 2.0 standard.)
1 x PCI Express x4 slot
1 x PCI slot

http://www.gigabyte-usa.com/Pr...ec.aspx?ProductID=3168

 

[Concillian]

The 1156 design is such that there are 16 lanes available that are directly from the CPU. These will be driven to the x16 slots on any P55 motherboard such that you can use 1 and get x16 or 2 and get x8 / x8. The P55 has 8 lanes available, but they won't be run to the x16 slot. Here's a detailed explanation of why that won't be the case:

CPU = 16 lanes available
Chipset = 8 lanes available

It's safe to assume that the x4 slot comes from the P55 chipset. So now we have:
CPU = 16 lanes available
P55 chipset = 4 lanes available, 4 lanes used for the x4 slot.

So there are two options for the x16 slots:
1) x16(1) = 16 lanes from CPU
and x16(2) = 4 lanes from P55

2) x16(1) = 16 lanes from CPU if only 1 in use , 8 lanes from CPU if both in use
and x16(2) = 0 lanes from CPU if only 1 in use, 8 lanes from CPU if both in use

Option 1 doesn't make sense at all, so any P55 board you see with two x16 slots will have them fed from the CPU as in option 2.

Which means it doesn't matter which slot the video card goes in, since it'll be x8 electrical if you populate the other x16 slot no matter what.

 

[Ben90]

Originally posted by: Concillian
Which means it doesn't matter which slot the video card goes in, since it'll be x8 electrical if you populate the other x16 slot no matter what.

 

[Aquila76]

What if I use just one card but install it into the second PCIe slot? I may need to do this to install a 'jumbo-sized' CPU cooler in a preferred direction. Will it run the card at X16 or X8? The mobo is a MSI P55-GD80, if that helps.