FAQ: Everything you need to know about SLi and CrossfireX

The End All Be All FAQ about SLI and CrossfireX
SLi, CrossfireX, what does it all mean to you? Is it just another buzzword in the cloud known as the internet? Just another random thing the enthusiasts are messing with to make their PC's even better?

Let's start with a basic definition. CrossfireX and SLi are both multi card solutions. This means that there are two or more cards working in tandem to help process information and relay it to your computer monitor at an even quicker speed than with only a single card. CrossfireX and SLi both rely on multiple cards, and 'bridges', soft flexible pieces of plastic and metal that connect two cards together and transfer information between the two cards easily.

What is a multi card solution?
The short answer is a graphics processing unit (GPU) that has more than one card doing work, at the same time.
Let's start with a basic definition. CrossfireX and SLi are both multi card solutions. This means that there are two or more cards working in tandem to help process information and relay it to your computer monitor at an even quicker speed than with only a single card. CrossfireX and SLi both rely on multiple cards, and 'bridges', soft flexible pieces of plastic and metal that connect two card together and transfer information between the two cards easily.

SLi, the acronym means 'Scalable Link Interface'. It's just another term cooked up by the engineers and marketing people over at nVidia. It's a multi card solution that relies on two of the same cards to be running parallel to each other. More on this later though.
CrossfireX, is a term created by the engineers and marketing peeps over at ATi/AMD. It's the other multi card solution that you hear about normally, and it doesn't stand for anything. It's the second version of Crossfire released by ATi; more on this later though.

Okay, so now that we have a basic definition, let's get into some more complicated information, what a SLi interface looks like, what a Crossfire/CrossfireX interface looks like, connectors, what you can do with SLi/CrossfireX, performance increases, driver support... And even more.
First, you want to know if your motherboard is SLi/CrossfireX capable. With everything before Intel Core i7, the motherboard is either SLi capable or CrossfireX capable.

With the i7 platform, most X58 motherboards support BOTH CrossfireX and SLi.

With an Intel manufactured board, something like an X38, P43, P45, X48, or something of the like, you are most likely going to have a CrossfireX board.

With an AMD manufactured board, such as the 740G, 770, 780G, 790GX, 790FX, or something of the like, you are most likely going to have a CrossfireX board.

With a nVidia chipset, like the 590, 650i, 680, 690i, or something of the like, you are most likely going to have an SLi board.

Now that we have that out of the way, you've got to make sure that your motherboard actually supports one of the other. Don't just go by what I say, make sure to check up that your motherboard actually does support one or the other, do some research! Also, don't forget to make sure that your motherboard actually has two PCI-Express slots.

Note about PCI-Express 2.0: PCI-Express 2.0 standard slots are available on most new motherboards. These 2.0 standard slots support the same x16 lanes than a PCI-Express 1.0 or 1.1 slot supports, but it provides more power to the card than a 1.1 or 1.0 can. PCI-Express 2.0 video cards are backwards compatible with 1.1 and 1.0 slots, so if you have an older board, don't worry about the card being compatible. The thing you would have to watch out for is power requirements. Make sure the power supply is up to snuff.

Okay, so now that you've determined that you have a board that supports one or the other, you want to know what each board can support. Here's a basic breakdown:
Nvidia SLi:
•Must match GPU's
•Must use SLi connector
•Tri and Quad SLi available with newer motherboards
•Must use full x16 slot
•Some motherboards only support x16x4 mode (more on this later)
•Card must be of 6 series or higher
•Card MUST be PCI-Express.
•Card Compatibility list:

NVIDIA SLI-Ready GPUs:
NVIDIA GeForce GTX 295**
NVIDIA GeForce GTX 285*
NVIDIA GeForce GTX 280*
NVIDIA GeForce GTX 260*
NVIDIA GeForce GTS 250*
NVIDIA GeForce 9800 GX2**
NVIDIA GeForce 9800 GTX+*
NVIDIA GeForce 9800 GTX*
NVIDIA GeForce 9800 GT
NVIDIA GeForce 9600 GT
NVIDIA GeForce 9600 GSO 512
NVIDIA GeForce 9600 GSO
NVIDIA GeForce 9500 GT
NVIDIA GeForce 8800 Ultra*
NVIDIA GeForce 8800 GTX*
NVIDIA GeForce 8800 GTS 640
NVIDIA GeForce 8800 GTS 320
NVIDIA GeForce 8800 GTS 512
NVIDIA GeForce 8800 GT
NVIDIA GeForce 8600 GTS
NVIDIA GeForce 8600 GT
NVIDIA GeForce 8500 GT
NVIDIA GeForce 7950 GX2**
NVIDIA GeForce 7950 GT
NVIDIA GeForce 7900 GTX
NVIDIA GeForce 7900 GT
NVIDIA GeForce 7900 GS
NVIDIA GeForce 7800 GTX 512
NVIDIA GeForce 7800 GTX
NVIDIA GeForce 7800 GT
NVIDIA GeForce 7600 GT
NVIDIA GeForce 7600 GS
NVIDIA GeForce 7300 GT***
NVIDIA GeForce 7300 GS***
NVIDIA GeForce 7300 LE***
NVIDIA GeForce 7100 GS***
NVIDIA GeForce 6800 Ultra
NVIDIA GeForce 6800 GS
NVIDIA GeForce 6800 GT
NVIDIA GeForce 6800
NVIDIA GeForce 6800 XT
NVIDIA GeForce 6800 LE
NVIDIA GeForce 6600 GT
NVIDIA GeForce 6600*
NVIDIA GeForce 6600 LE***
NVIDIA SLI-Ready Quadro GPUs
NVIDIA SLI-Ready Motherboards:
NVIDIA nForce 790i Ultra SLI
NVIDIA nForce 790i SLI
NVIDIA nForce 780i SLI for INTEL
NVIDIA nForce 780a SLI for AMD
NVIDIA nForce 750i SLI for INTEL
NVIDIA nForce 680i SLI for INTEL
NVIDIA nForce 680i LT SLI for INTEL
NVIDIA nForce 650i SLI for INTEL
NVIDIA nForce 590 SLI for AMD
NVIDIA nForce 570 SLI for AMD
NVIDIA nForce 590 SLI for INTEL
NVIDIA nForce 570 SLI for INTEL
NVIDIA nForce 500 SLI for AMD
NVIDIA nForce Professional
NVIDIA nForce4 SLI X16
NVIDIA nForce4 SLI
NVIDIA nForce4 SLI XE
Intel Desktop Board D5400XS

•
*NVIDIA GPUs that support 3-way SLI
**NVIDIA GPUs that support Quad SLI
***NVIDIA GPUs that do not use the SLI connector.
•

ATi CrossfireX
•Do not have to match GPU's
•Must use CrossfireX connector
•Tri and Quadfire available with newer boards
•Must use full x16 slot
•Some motherboards only support x16x4/x8x8 (more later)
•Card must be an X1550 or higher. X800/X850 are special cards (more later)
•Card MUST be PCI-Express
•Compatibility chart:

Okay, so now that you've determined that you have a necessary card, you want to go about buying a second one.

With NVidia SLi, it is best to try to find two identical cards, but if your original card has been discontinued or is out of stock, find one with an identical GPU.

Note on Clocks and Memory: Nvidia stresses that you should try to find a card with nearly identical specs, but cards with factory overclocks, or different amounts of VRAM (Video RAM), are supported through coolbits. Nvidia graphics drivers will default the card with higher clocks or more memory to the lower performing card, meaning it will decrease the amount of addressable memory if the card has excess.

With ATi CrossfireX, you can use cards that are different, for example, a 4830 with a 4850. The two cards will interface, but the faster graphics card will have to 'wait' for the slower graphics card to process and render the frame before displaying the frame on the screen. Keep in mind that each frame render takes milliseconds, which is why games are measured in FPS (Frames per Second) instead of Frames Rendered, since the amount of frames rendered is more than one per second in most cases.

Note on CrossfireX Compatibility: Remember to check the chart before you buy two cards however, and it is always advisable to buy two of the same cards, however, it is not required. For example, you cannot crossfire a 4670 and a 4870x2. The same can be said that you cannot crossfire a 3870 and a 4870, because they use different GPUs, and are therefore incompatible.

Now that you know what you're getting, let's take a look at what will happen when you get the cards.

If you have an ATi CrossfireX card, you will most likely have a red PCB card (PCB means Printed Circuit Board, or the board where all the transistors, capacitors, VRMs, core and memory is soldered on.) On this card, there should be two CrossfireX connectors at the top of the board, as shown in the picture below.


If you have a Nvidia SLi card, you will most likely have a card with a black, green or blue PCB. It looks similar to a CrossfireX connector, but it is much less wide. It also has less pins on it.


Now, what does a CrossfireX Bridge look like? Normally like this picture.
It's got two connectors and is connected by a flexible ribbon, kind of like an old IDE cable.


What does an SLI connector look like? They come in many flavors, from hard to flexible, and it really depends on which card you order. There are also Tri-SLi bridges (more later)

Now that we know what the bridges look like, lets get into the nitty gritty. What does SLi/CrossfireX do for me? What kind of performance increases can I get? Is there good driver support?

Well, Nvidia says that with a SLi configuration (Two cards) you can get up to two times the performance of just one card! With a Tri-SLi configuration, you can get up to 2.8 times the performance of a single card.

ATi does not specify and exact percentage or multiple value, but we can expect simliar statements to come from their marketing people.

We all know figures change in real life though, and each scenario pans out differently. Say you're using a single GTX260 at 1920x1200 resolution on a 24" monitor. Adding another GTX260 could yield a performance increase of 50% or more, depending on the game. Now, if you were on a 19" screen with a resolution of 1440x900, a single GTX260 would be more than powerful enough to run anything.

If you are one of the lucky few that have a 30" monitor and a resolution of 2650x1600, you almost need a dual card SLi or CrossfireX Solution if you wish to play games at high settings. Currently, the four most recommended options are two GTX260's in SLi, a GTX295 (Dual core card), two 4870 1gb in CrossfireX, or a 4870x2 (dual GPU card).

With the introduction of cards like a 7950GX2 or 3870x2, we saw multiple card solutions on a single PCB. This meant double the performance without having to mess with bridges and specialized motherboards, it was just plug and play if you have a PCI-Express x16 slot. Now we have dual core cards like the GTX295, where there is a single card with a single GPU, but it has two cores in the same GPU. There are still cards like the 4870x2, which offer dual GPU's on a single PCB, and they are still popular.

With card solutions like Tri and Quadfire, or TriSLi and QuadSLi, we are seeing a huge increase in graphics processing power.

Currently, there are a few ways to get Quadfire or QuadSLi, and they mainly revolve around the X58/Intel Core i7 platform. If you have an X58 motherboard, chances are it has four PCI-Express 2.0 slots on it, and both SLi and CrossfireX compatibility. This means you can plug in four cards and use the connectors in a sort of daisy-chain configuration to get Quadfire or QuadSLi. The other way to do QuadSLi would be to get two 9800GX2's, or two GTX295's and SLi link the two together for a QuadSLi configuration. To get a Quadfire configuration, it's basically the same thing. You'd have to get either four cards, or get a pair of 3870x2's or 4870x2's and CrossfireX connector them together.

With the latest 9.3 Catalyst drivers being released, we are seeing quite a big increase in CrossfireX support, and many cards and games are becoming much more stable thanks to it. Nvidia has had some stable SLi drivers out for a few months now, so there is no reason not to go SLi if you're worried about driver support. The same goes for CrossfireX support. ATi has really gotten their drivers together in the past few months, and we're looking at some big increases.

Now finally, there are a few notes I'd like to say at the end.
Firstly, the first generation of Crossfire was called just that, Crossfire. Anything being referred to today is probably CrossfireX, so don't mind it. Crossfire used a strange DVI-looking connector instead of internal connectors to bridge the two cards together. It looked like this:

It was used before the X1950 came out, and featured a 'master/slave' card approach, where one card was designated the main card, and the secondary card was designated the slave card. As you can see, you'd plug the gold plated conenctor into the master, and the DVI male connector into the slave card, then plug a DVI cable into the female connector. This version of Crossfire only supported a single monitor, to my knowledge, and wasn't all that great as an external solution. Plus, it was more expensive to manufacture, which is why ATi switched to an internal design.

Another note:
Some motherboards only support certain configurations of CrossfireX/SLi. x16x16 is the most common, but there are some boards with built in limitations, like the board I have, an AMD 790GX board, which restricts PCI-Express graphics cards to an x8x8 configuration. Since most graphics cards cannot populate a full x16 lanes, this is not normally a hindrance, and most motherboards support x16x16 anyway, so this is mostly a non issue, except for older motherboards. For an x8x8 configuration, the percentage of lost performance is below 10%. For an x16x4 configuration, it can be much higher, up to 30%, but again, it all depends on the GPU and the rest of the system components, like RAM and CPU.

Bottlenecks:
Pairing a E5200 and a 4870x2 is probably not a smart idea, as it is a bottleneck. A bottleneck is when one component is slower than the rest of the components, like in the example I just said. A slower clocked dual core is going to force the 4870x2 to perform at less than it's maximum able performance. The best way to figure out a bottleneck is to ask! Just ask the fellow forumers and they will be much obliged to help you out.

Finally, a TriSLi solution is three cards in SLi. This has only been recently supported, and it's more of an experimental build. Most people will not go with TriSLi, but this is what the connector looks like regardless:

Again, if you have any questions to ask me, go ahead and do so. I know quite a bit about graphics performance, and multi card configurations. If you don't feel comfortable asking me, here are a few websites you might want to check out:
http://www.slizone.com/page/slizone_learn.html - For Nvidia SLi learning.
http://game.amd.com/us-en/crossfirex_about.aspx - For AMD/ATi CrossfireX

Edit: Bolded X58 part and touched up spelling