What are the hardware characteristics make a GPU Crossfirable/SLI'able with another?

[Torn Mind]

I know that Crossfire requires two GPUs in the same tier(same hundreths number, i.e x8xx) and generation, but what hardware properties do the GPUs must have in order to be able to Crossfire with each other?

I know about load balancing and how it wouldn't make sense to pair a slow card and a fast card, but how are the cards prevented from working together? Is it through software or is the hardware designed to be incompatible through some means, such as transistor count or something?

 

[Fx1]

stick with 2 identical cards.

 

[Torn Mind]

stick with 2 identical cards.

I know about the requirements.
I was looking for more of a description of how the hardware works and what exactly prevents Crossfire/SLI. Perhaps I should have posted in Highly Technical...

 

[Kenmitch]

I think it's a faster card has to wait for the slower card kinda thing which kinda defeats the purpose of going crossfire/sli in the 1st place. Using identical cards is the best way to go for the most part....Think memory sticks is one way to look at it. Fewer issues if you use matched pairs.

 

[blackened23]

I know that Crossfire requires two GPUs in the same tier(same hundreths number, i.e x8xx) and generation, but what hardware properties do the GPUs must have in order to be able to Crossfire with each other?

I know about load balancing and how it wouldn't make sense to pair a slow card and a fast card, but how are the cards prevented from working together? Is it through software or is the hardware designed to be incompatible through some means, such as transistor count or something?

The main issues:

1) compatible motherboard
2) adequate case airflow (big one)
3) blower cooler vs open air cooler on the GPU. The latter requires much more effort and care to work.
4) identical cards. You want cards to have the same memory, clockspeeds, etc.

Really the biggest issue is keeping them adequately cooled. Blower cooled GPU's are usually better, although with effort you can make open air cooled cards work as well as long as they are double slot. Your case must have excellent case air flow for either type, more so for open air aftermarket GPUs.

Do not ever use triple slot coolers for SLI. They are always a no-go unless you water cool (ala Asus DC II).

 

[KentState]

SLI/CF is pretty much video card RAID and all the limitations of a software solution is present. The driver has to be the same which leaves little room for variation.