Cross-fire on an Asus M4A785TD-V EVO

N

NeedaComputer

Member
It has two PCI-e x 16 physical slots. One is full bandwidth, the other one is said to be only "4x" bandwidth. I'm not sure what that means.
A friend of mine may be giving me two ATI 4850's.
Should I try to "Cross-Fire" them? Should I try to "Cross-Fire" the onboard HD4200 graphics? I'm not sure how to pursue this possible good fortune. Thanks in advance for the advice!
 
not sure of the compatibility with the onboard and 4850's onboard sli/crossfire don't seem to be to that compatible with too many cards, besides it would be pointless.

X4 means your second slot is restricted to x4 speed which means it will have a bandwidth restriction and it will run at a quarter of the speed of the x16 slot which means you will prob notice a performance drop.
 
If he just gives them to you, what not C/F them. Even if Hybrid would work with a 4850, running them in crossfire even if the second slot is X4 would be better then Hybrid.
 
Hybrid crossfire won't work. I have a very similar board myself, and used to have a 4850 in it. At one point I was considering picking up another for crossfire and was doing research on the very same question. If the slots are PCI-E 2.0, which they likely are, that means that they actually are equivalent to 8 lanes of PCI-E 1.0. I saw a benchmark performed with a 5850 in this quest for knowledge in which they restricted the bandwidth of the card down from 16x, to 8x, to 4x and 1x just for laughs. They only found a huge disparity in performance when they reached 1x, while the 4x performance only dropped by about 5-10% if my memory serves correctly. Keep in mind, this is a 5850, which has much higher bandwidth than a 4850. Especially in crossfire, I would say your difference would be minimal if any, go for it!

EDIT: Found the benchmarks, they used a 5870, not a 5850, and the card maintained 95% of its performance on only 4 lanes of PCI-E 2.0. http://www.techpowerup.com/reviews/AMD/HD_5870_PCI-Express_Scaling/25.html
 
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Wow CardboardSword, thanks for all the great info! I haven't checked back here in a while. I just got the cards yesterday, static everywhere and they were just stacked on top of each other, no static bags or anything. I'll have to give them a try, I hope they work.
 
I ended up purchasing an XFX Radeon HD 4650 1GB for the S-Video capabilities, (I'm running a late '90's Sony Trinitron Television with s-video that my wife and I like for watching vids.) About a week after I bought it, my friend came through with the two free 4850's with 512mb each. I purchased a Crossfire cable and crossed my fingers.
Well, here are the results using free 3DMark05:

4650 = 11500
4850 x 1 = 19887
4850 x 2 = 22465

I thought the crossfire would yield a better improvement than that, but I'm still very happy for only having to purchase a cross fire cable. Now I need to figure out what to do with the 4650. Thanks for all the help!
 
That's only one benchmark. Crossfire will yield different results with different benchmarks and games. Some scale well, others not so much. 4850 in CF was a fairly desirable set up not too long ago, so I assume the drivers would be good for it and the scaling respectable.
 
Sometimes 05 and 06 don't scale two GPU's well. 3dmark vantage would be the better comparison, but you need a key just to run it.

With that said, results are not linear when it comes to benchmark scores like 3dmark 05 and 06. Going from 20 to 22k does not mean only 10% improvement in gameplay.

Also, the x4 slot WILL still bottleneck the second 4850, but you still should get pretty decent improvement. Most people don't realize that the x4 slots on these boards are NOT PCIe 2.0 slots, they are 1.0 slots....so x4 1.0 translates into x2 2.0, so it's actually only 1/8 the bandwidth of a x16 2.0 slot. And then, another thing most people don't know, is that x4 slot runs through the south bridge not the north bridge chipset. This creates a even more expensive bottleneck. So even though it is a second PCIe slot, and you CAN Xfire on it, it's really not what that slot is meant for.

Now, with all that said, OP- What games do you play and what resolution monitor are you running?
 
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Most demanding game is Civ5, then the Orange Box and Company of Heroes. I can get around 280 - 300 fps running TF2 full blown on 1440x900. I'm quite happy with this free setup. Thanks for all the great info.
I assumed that the second slot would not yield huge results, but for free, I can't complain.
 
I do run Furmark (multi-gpu). That has a bench mark utility. I don't know that I'll be tearing down to do a single vs xfire anytime soon. But once I do, maybe I'll resurrect this thread from the dead. Thanks again everyone for the insights and advice!
 
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