Some PCI Express 2.0 clarifications

[Chronicbs]

1. I've read pci-express 2.0 is fully backwards compatible. I'm assuming and just want to clarify that this means you can put pci-e 4x and even pci-e 1x cards with smaller connectors into a pci-e 2.0 16x slot? Not just a full sized pci-e 16x card?

2. Say I have a motherboard with 2 pci-e 2.0 16x slots, that says in specs that both slots have an 8x link in crossfire. Does this also mean if I use the second pci-e 2.0 16x slot for say a pci-e 4x raid card*, the solo video cards pci-e 2.0 16x slot will be cut down to 8x?

Just some questions I've been unable to find the answers to on my own. Thanks in advance for anybodies time.

(*if possible pending my first question.)

 

[krnmastersgt]

1. Yes, it's also rated to run PCI-E x16 at 1.0 and 1.1 speeds (it's all about bandwidth limitations).
2. No, only when you have them connected together to run in Crossfire, running 2 cards at once does not qualify as Crossfire.

 

[ViRGE]

Without knowing what mobo you have, #2 is a bit hard to answer. With some boards, they don't have enough PCIe lanes to fully populate 2 x16 slots, so what they do is have the ability to flip. If you were to install a card in the 2nd x16 slot, half the lanes would be routed to the 2nd slot instead of the first slot, which is how you end up with x8 and x8. If your board is of this configuration, then installing your RAID card would necessitate splitting the lanes and limiting your graphics card to x8.

In other words, you need to tell us what board you have.

 

[Chronicbs]

Foxconn A7DA-S

This is the board I was looking at. I read elsewhere that the foxconn had an 8x/8x link. The Asrock 790gx says its slots are 16x/8x but I would prefer the Foxconn.

My plan is to buy

Phenom 920

Foxconn A7DA-S

Gskill ddr2 1066 4gb (2x2)

4870 X2 2gb

What I'm really wondering is if later I could add this later,

3ware raid card

Without hurting the 1st pci-e 2.0 bandwidth at all.

If it would, then I guess I would need this mb,

Asrock 790gx

So the bandwidth is split properly?



Edit: I just read this at another site reviewing the Foxconn.

"There are two PCI Express x16 slots, which run at x16 in single mode and x8/x8 in CrossFire mode"

So I guess that answers my question. They run at 16x in single mode so I should be able to add the raid card without hurting the bandwidth of my first pci-e slot.

Thanks for the informative replies krnmastersgt and ViRGE.

 

[ViRGE]

Unfortunately you misinterpreted what people are talking about when they mention "Crossfire" mode. Single mode is when only the first slot is in use, "crossfire" mode is when both slots are in use (the vast majority of cases being that the 2nd slot is used for a 2nd ATI card to be linked up in Crossfire). The fact of the matter is that it's exactly as I described earlier: the board has to reroute half the lanes from the first PCIe x16 slot in order for you to use the second. The following is in the motherboard manual:

"The PCI-E2_16X slot will work at PCI Exress x8 mode. When the two slots are installed, they both work at x8 mode."

So installing your RAID card will halve the bandwidth to your video card on this motherboard.

 

[SeanTek]

Similar question here. The specs I am considering are:
GA-EP45-UD3P, Intel P45 chipset
Radeon HD 4850 (silent, not overclocked)
3ware raid card in second PCI Express 2.0 slot

If the bandwidth is halved to the Radeon HD 4850, will it make much of a difference? (If the bandwidth will actually be halved...)

 

[betasub]

LOL. I'm glad you are able to understand Chingrish, Virge. Your explanation has definitely helped me understand the manual.

 

[Chronicbs]

I read in the Asrock AOD790GX/128M manual that the second lane is cut down to 1x when both are used... Looks like I have to spring for the asus then. It clearly says in the Asus M3A78-T manual on a chart that when you use 2 cards, you can use the first and second slot and you will get 16x, 4x; if you use the first and third slot you get 8x/8x bandwidth, and if you use all three you get 8x, 4x, 8x. (total 20x bandwidth) I think the Asus is the only board that will do what I want. (let me use a second pci-e slot without hurting the first.

 

[krnmastersgt]

Only board, that doesn't cost a lot you mean At least a few boards will give you full x16 along all 3 lanes for a hefty premium.