SLI Noob Question

[Clarpet]

Okay, so I want to build a new system with SLI as an option. I understand how to pair cards, etc. But what I don't get is this:

SLI requires PCI Express.

Most MoBos have 1x PCIe x16 and 1x PCI x16 2.0, but not 2 slots of either.

If I want to run SLI with 2 PCIe x16 can I run one in the regular x16 slot and the other in the 2.0 slot?

And yes, I looked throught the stickies, but this specific question isn't addressed.

Thanks for any help!

[jason4207]

You seem a bit misguided on the spec. You have PCIe 1.1 mobos and PCIe 2.0 mobos. They don't mix them. PCIe 2.0 has twice the bandwidth of PCIe 1.1.

Mobos have 1 or more PCIe slots of varying lengths. Typically you see x1 slots (very short) and x16 slots (the long ones with the little locking tab). Older boards had 1 or more x16 PCIe 1.1 slots and 1 or more x1 PCIe 1.1 slots. Newer boards have 1 or more x16 PCIe 2.0 slots and 1 or more x1 PCIe 2.0 slots.

So, if you see a modern board w/ 2 long slots you can rest assured that they are both x16 PCIe 2.0 slots. However, some boards will reduce down to x8 PCIe 2.0 bandwidth when 2 cards are inserted. This is OK since that is still plenty for modern cards.

To run SLI on the Intel 775 platform you'll need an nVidia chipset board as the Intel 775 boards only support cross-fire (ATI). I would avoid the nVidia boards, though.

The new Intel 1156 or 1366 boards are all capable of supporting SLI if you see 2 x16 slots, but just make sure it explicitly says so somewhere as not all boards have the license. What's nice about these boards is they support SLI and cross-fire, and they have much better scaling than older setups.

[Clarpet]

You seem a bit misguided on the spec. You have PCIe 1.1 mobos and PCIe 2.0 mobos. They don't mix them. PCIe 2.0 has twice the bandwidth of PCIe 1.1.

Mobos have 1 or more PCIe slots of varying lengths. Typically you see x1 slots (very short) and x16 slots (the long ones with the little locking tab). Older boards had 1 or more x16 PCIe 1.1 slots and 1 or more x1 PCIe 1.1 slots. Newer boards have 1 or more x16 PCIe 2.0 slots and 1 or more x1 PCIe 2.0 slots.

So, if you see a modern board w/ 2 long slots you can rest assured that they are both x16 PCIe 2.0 slots. However, some boards will reduce down to x8 PCIe 2.0 bandwidth when 2 cards are inserted. This is OK since that is still plenty for modern cards.

To run SLI on the Intel 775 platform you'll need an nVidia chipset board as the Intel 775 boards only support cross-fire (ATI). I would avoid the nVidia boards, though.

The new Intel 1156 or 1366 boards are all capable of supporting SLI if you see 2 x16 slots, but just make sure it explicitly says so somewhere as not all boards have the license. What's nice about these boards is they support SLI and cross-fire, and they have much better scaling than older setups.

While I appreciate the answer the are a few things *wrong* with your answer that I already know through some basic reading on other hardware sites:

1) You don't have to use an "official" SLI board w/an Nvidia chipset.
2) Some boards support both SLI & Crossfire but aren't "certified"; i.e. the chipset will allow it and the manufacturer supports in but they don't have the "gold seal" approval of Nvidia or ATI
3) There ARE boards that have a slot labeled as PCIe x16 *and* another slot labeled as PCIe x16 2.0. Many say that that the PCIe x16 slot (as in NOT the one with the 2.0 label) run at x4 and the 2.0 slot runs at x16. They also say they support SLI/CF.


So here's the real question: why the hell are there 18 thousand different types of PCIe slots?

No, actually the real question is: What slots do I need to run SLI? If I *need* 2 PCIe X16 2.0 slots, then those boards are available, but that theory doesn't match up with the fact that boards that DO NOT have 2, 2.0 slots *do* say they support SLI/CF.

I'm confused.


***edit**: I think the point of confusion here is NOT the difference between PCIe 1.1 & 2.0. The confusion is *why* they separately label the board as having a PCIe x16 slot ***and** a PCIe x16 2.0 slot? What is the difference? Do I really need to care, or is there no difference?

Here's an example, from a very popular Gigabyte board:

http://www.newegg.com/Product/Product.aspx?Item=N82E16813128405

Expansion Slots
PCI Express 2.0 x16 1
PCI Express x16 1 x PCI Express x16 slot, running at x4 (PCIEX4)
PCI Slots 2

[jason4207]

While I appreciate the answer the are a few things *wrong* with your answer that I already know through some basic reading on other hardware sites:

1) You don't have to use an "official" SLI board w/an Nvidia chipset.

You do if you want to run SLI on the 775 socket and not use some kind of hacked driver/BIOS. Technically any board that can support Cross-fire can physically support SLI, but there are flags in the drivers that prevent it. After 775 nVidia got out of the chipset business w/ 1156/1366. Now they just hand out licenses to grant SLI support on those platforms

2) Some boards support both SLI & Crossfire but aren't "certified"; i.e. the chipset will allow it and the manufacturer supports in but they don't have the "gold seal" approval of Nvidia or ATI

If there are 2 x16 PCIe slots then it is physically possible. But w/o the license you'll have to use a hacked driver and/or mobo BIOS which might be illegal.

3) There ARE boards that have a slot labeled as PCIe x16 *and* another slot labeled as PCIe x16 2.0. Many say that that the PCIe x16 slot (as in NOT the one with the 2.0 label) run at x4 and the 2.0 slot runs at x16. They also say they support SLI/CF.

If you see 2.0 anywhere then the entire board is 2.0. Each 2.0 lane has twice the bandwidth of a 1.1 lane. Some boards run x16 when 1 card is inserted and then run x8/x8 if 2 cards are inserted. P35 boards with 2 physical x16 PCIe slots (PCIe 1.1) run x16/x4 electrically all the time, and P45 boards with 2 physical x16 PCIe slots run x8/x8 electrically when 2 cards are installed and x16 when only 1 card is installed. When I say "physical" I mean the length of the slot, and when I say "electrical" I'm referring to the amount of lanes made available to the card by the mobo.

So here's the real question: why the hell are there 18 thousand different types of PCIe slots?

It can be confusing, yes.

No, actually the real question is: What slots do I need to run SLI? If I *need* 2 PCIe X16 2.0 slots, then those boards are available, but that theory doesn't match up with the fact that boards that DO NOT have 2, 2.0 slots *do* say they support SLI/CF.

I'm confused.

If you want 2 PCIe2.0 x16 slots to be at full bandwidth for SLI then you need a 780i or 790i if on 775 or you need an X58 board on 1366. And I guess you could use X38/X48 on 775 if you use hacked software. 1156 socket doesn't have enough PCIe lanes to support x16/x16, and will set to x8/x8 in most cases.

PCIe2.0 x8 is still enough bandwidth for modern cards and tests have shown very little if any loss when running SLI or Cross-fire in PCIe2.0 x8/x8 vs. PCIe2.0 x16/x16. So, running SLI on 1156 is still a very viable solution. And I guess you could run hacked software on P45 as well.


***edit**: I think the point of confusion here is NOT the difference between PCIe 1.1 & 2.0. The confusion is *why* they separately label the board as having a PCIe x16 slot ***and** a PCIe x16 2.0 slot? What is the difference? Do I really need to care, or is there no difference?

Here's an example, from a very popular Gigabyte board:

http://www.newegg.com/Product/Product.aspx?Item=N82E16813128405

I think they are basically saying the board can really only run 1 gfx card well. Yes, it has 2 slots, but the bandwidth in the lower slot is only good for a RAID card or a low-end gfx card (for additional monitor support) or maybe a dedicated Physx card slot.


On the 1156 platform the NB was moved completely to the CPU, and so did most of the PCIe lanes. There are 16 PCIe2.0 lanes going directly to the CPU and you want to use these for gaming whether in a single x16 mode or in a x8/x8 dual gfx card configuration. There are an additional 8 PCIe2.0 lanes available on the P55 chipset, but you can count on these having more latency, and they are not advisable for gaming. Intel has spec'd these 8 lanes in P55 to be in x1 slots or a combination of x2 and x4 slots. Some of these lanes will be used for things like Gb NIC, etc.

That particular board is peculiar in that it doesn't default to x8/x8 when 2 cards are installed. I'm assuming it was a cost-saving decision, and so was the exclusion of an official SLI license. Basically it's using all 16 PCIe lanes to the CPU for the first slot, and then using 4 of the 8 lanes of P55 for the lower slot.

See this 1156 board (http://www.newegg.com/Product/Product.aspx?Item=N82E16813130238) w/ 3 x16 PCIe2.0 slots. They are physically x16, but electrically they vary:

• 3 PCI Express gen2 x16 slots
- If two graphics cards are installed at PCI_E2 & PCI_E4 slots, these two PCIE x16 lanes will auto arrange from x16/ x0 to x8/ x8
- PCI_E5 supports PCIE 2.0 x4 speed only
- When PCI_E5 is in use, all PCIE x1 slots will be unavailable
- supports ATI® Crossfire™ & NVIDIA SLI
• 2 PCI Express gen2 x1 slots
• 2 PCI slots, support 3.3V/ 5V PCI bus Interface

All 16 PCIe lanes to the CPU are for graphics cards and can either be x16/x0 or x8/x8 depending on how many cards are installed. The 3rd x16 slot is only x4 electrically, and if it is in use then all the x1 PCIe slots are disabled. This is to make sure that the NIC, etc still has some lanes left.

[Clarpet]

Ok! Thank you for clarifying that. Its been a while since I built a rig so I missed the bus on PCIe, but I get it now.

I've decided to go with an Athlon Phenom setup and use a 785 or 790 chipset with a Radeon HD 4870 1GB in Crossfire w/the onboard video chipset, with plans to add another 4870 later.

On my BB now but I'll post some of the hardware links later and maybe you could kindly give me your two cents on the setup?

[jason4207]

I'm not very versed on the AMD CPU side of things, but I'm sure someone else will step up to the plate. You might need to post in another sub-forum, though. ;)

My forte includes mainly Intel 775 CPUs and Mobos; DDR2 RAM; nVidia G92 & G200 graphics cards; ATI 4000-series graphics cards.