Slot for graphics card

[garywood84]

I have a GA-X58-UD3R rev 2.0 motherboard.  I note that the user guide for this board suggests that if you have only one graphics card, it should go in the first PCIe x16 slot.  Can anyone please confirm the reason for this?  Would there be any reason not to put the card in the second PCIe x16 slot?  

I'd prefer to use the second PCIe x16 slot for my graphics card if it's possible, because this would make for a more efficient use of my PCIe slots, since the graphics card obscures the next two slots below where it is installed, and putting it in the second PCIe slot therefore means it will obscure the PCI slot and the lowest PCIe x8 slot, leaving two PCIe slots free above it.  But, if I put the graphics card in the top PCIe x16 slot, it obscures all other PCIe slots bar the very bottom one.

Many thanks,

Gary

[Dark Mantis]

Hi and welcome.

You should be fine to use the second (lower) graphics slot as even the top notch cards wont use more than x8 bandwidth anyway. I think it is merely because it is the closest physical slot to the processor and memory.

[garywood84]

Hi Dark Mantis,

Thanks for your reply.

...as even the top notch cards wont use more than x8 bandwidth anyway.

Just to double-check, here: if I use the second PCIe x16 slot (the lower one), won't it still run at x16 as long as I don't put anything into the very lowest PCIe slot? 

As I understand the labelling system for the slots used in the manual, the top two slots pair together and the bottom two pair, with the lowest slot in each pair being x8 and the top one being x16.  So, if the lower of a pair is filled, the upper slot of that pair becomes x8, but if the lower one of a pair is empty, then the upper one will remain x16 regardless of what's installed in the other pair.  Is that right?

[Dark Mantis]

Yes your understanding is correct.

The PCIE slots usually operate in a ganged mode and so the top PCIEx16_1 and the top PCIEx8_1 will be connected and so if you are to use the PCIEx8_1 slot then PCIEx16_1 will be limited to 8x speed rather than 16x. This goes for the other PCIE slots also. See page 12 of your manual for confirmation.

For optimum performance, if only one PCI Express graphics card is to be installed, be sure to install it in the PCIEX16_1 slot; if you are installing two PCI Express graphics cards, it is recommended that you install them in the PCIEX16_1 and PCIEX16_2 slots.

The PCIEX8_1 and PCIEX8_2 slots share bandwidth with the PCIEX16_1 and PCIEX16_2 slots respectively. When PCIEX8_1 is populated with an expansion card, the PCIEX16_1 slot will operate at up to x8 mode; when PCIEX8_2 is populated with an expansion card, the PCIEX16_2 slot will operate at up to x8 mode.

[garywood84]

Thanks again, Dark Mantis.

For optimum performance, if only one PCI Express graphics card is to be installed, be sure to install it in the PCIEX16_1 slot; if you are installing two PCI Express graphics cards, it is recommended that you install them in the PCIEX16_1 and PCIEX16_2 slots.

This was the sentence in the manual that prompted me to ask this question in the first place, so now I'm a little confused.  Why do I need to use PCIEX16_1 for a single graphics card in preference to PCIEX16_2, if PCIEX8_2 isn't filled?  Wouldn't the performance be the same?

Just to confirm, here's why I want to do this.  By putting the graphics card in the lower, PCIEX16_2 slot, I get more useable PCIE slots, as this table illustrates:

Slot	Configuration 1	Configuration 2 (preferred)
PCIEX16_1	Graphics card	Useable
PCIEX8_1	Blocked by graphics card	Useable
PCIEX16_2	Not usable (would impede cooling of graphics card)	Graphics card
PCI	Useable	Blocked by graphics card
PCIEX8_2	Useable	Not usable (would impede cooling of graphics card)
Total useable PCIE slots	1	2

[Dark Mantis]

I really can't explain why the memo was put into the manual as it shouldn't make any difference at all. Both slots should go to the switch and thewn onto the PCIE bus to the X58 chipset. Apart from the slightly shorter route there is no difference that I can see.

[Tophamster]

Hello Dark Mantis,

I have related question to this shared bandwith between PCI-E slots. Does this mean in general, that the common speed of shared slots is the same as the slower of the cards plugged in?

In my case, it's MB GA-X58A-UD7 and I want to plug 1x speed card (soundcard) in PCIEX16_2 slot and 4x speed card (PCI-E SSD) in PCIEX8_2 slot.
Will SSD card operate at 4x speed, or will be slowed to 1x then ?

Thanks a lot
Tophamster

[Dark Mantis]

Hi Tophamster

That is not how I inderstand it.  Basically there is a limited amount of bandwidth to share between the two or more slots and so if you only have one filled then let's say it will run at 16x. If you then put in another card in a linked slot that bandwidth has to be shared and so both are dropped to 8x. I don't think it would matter if the additional card was a 1x card the available bandwidth would still be 8x for that slot.

[Tophamster]

That makes sense    Thanks!