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Is a 790FX chipset required for 3870X2 CF?

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February 19, 2008 8:56:13 AM

As 3870X2's in CF would be treated in windows as 2 GPU's, does that correspond to what the chipset 'sees', so could you run dual 3870X2's on a 770/790X chipset, or are you going to need 790FX for CF?
Doesn't seem to be solid information on the AMD website....
Any ideas?
February 19, 2008 9:50:14 AM

You need a 790x to have 2 pcie x16 slots for crossfire. And to my knowledge windows sees the x2 only as a single card. I could be mistaken though.
a b U Graphics card
February 19, 2008 11:13:19 AM

Slobogob said:
You need a 790x to have 2 pcie x16 slots for crossfire. And to my knowledge windows sees the x2 only as a single card. I could be mistaken though.


Slightly mistaken...here are the chipsets that will support CFX in x16 mode:

790x
790fx
X38
580x (ATI Radeon Express 3200)
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February 19, 2008 11:34:03 AM

Thanks :) 
So basically for AM2+ I need either 790 or 790FX for 3870X2's in CF.
Can't work out if the FX is worth the extra though...
February 19, 2008 11:36:13 AM

jitpublisher said:
Slightly mistaken...here are the chipsets that will support CFX in x16 mode:

790x
790fx
X38
580x (ATI Radeon Express 3200)


Actually i'm not. While the chipsets you listed are crossfire compatible, they are not all 770 or 790 chipsets as the OP asked.
The 580x has, if i recall correctly, only two mechanical x16 slots but if used, both only use x8 lanes each (Same as the 790x). That's beside the point though.

To clarify it further:

The 770 is for single cards.
The 790X is for two cards in crossfire.
The 790FX is for xfire IF the board has the needed number of PCIe x16 slots. Some of the FX boards have only two.
February 19, 2008 11:42:25 AM

LukeBird said:
Thanks :) 
So basically for AM2+ I need either 790 or 790FX for 3870X2's in CF.
Can't work out if the FX is worth the extra though...

I'm not too sure about that either. The problem is, the 790X has two mechanical X16 slots but they are connected with x8 only.
The 790FX has a total of 32 lanes available. So if you get a board with 790x that has four x16 slots you can operate one card at x16, two cards at x16 or four cards at x8. Three cards should be possible too, but i'm not sure how the lanes are split then.

My take on this is, that if you really want to use two 3870 x2s in crossfire, i would take a 790fx board. If there are cards that can use the extra bandwidth provided by PCIe x16, then it's them. In addition, if you spend that amount of money on GPUs you don't want to hamper their performance by saving 50$ on the mainboard. That's my opinion though.
a b U Graphics card
February 19, 2008 12:31:00 PM

Slobogob said:
Actually i'm not. While the chipsets you listed are crossfire compatible, they are not all 770 or 790 chipsets as the OP asked.
The 580x has, if i recall correctly, only two mechanical x16 slots but if used, both only use x8 lanes each (Same as the 790x). That's beside the point though.

To clarify it further:

The 770 is for single cards.
The 790X is for two cards in crossfire.
The 790FX is for xfire IF the board has the needed number of PCIe x16 slots. Some of the FX boards have only two.



The OP asked "is" a 790fx required....although probably hands down the best, it is not the only one.
All the chipsets listed will support (with a little more clarification) 2 cards CFX in full x16.
That, however does not mean that a particular board will have 1, 2 or 3 or more PCIe lanes, but that is the specifications of the chipset's capabilities.
Yes, the 580x, or rd580, or as more commonly known, the Radeon 3200 will indeed support 2 cards in crossfire at full x16 on both lanes, it was the first chipset to do so.
You may thinking of the RD570, which would support 1 lane at x16 and the other at only x8 in crossfire.

I am not sure what you mean "the 790fx is for xfire it the board has the needed number of slots" with "some FX boards only have 2" ??? All you need is 2 slots for Crossfire.
February 19, 2008 12:44:41 PM

SUMMARY: Get a 790FX card so you can run x16 on BOTH cards. If you got the cash for two 3870X2's then you better have the cash for the latest and greatest motherboards. Go 790FX so you don't feel stupid later....
February 19, 2008 1:16:25 PM

jitpublisher said:

Yes, the 580x, or rd580, or as more commonly known, the Radeon 3200 will indeed support 2 cards in crossfire at full x16 on both lanes, it was the first chipset to do so.


I guess that's where things get a little messed up. You are right though the RD580x has 2x 16x lanes. I was thinking of the RD600 (the intel variant, well actually a rd480 spin off). That has only two x8 lanes. They are both branded xpress 3200.

Every time people talk about the 3200 this board comes to mind. I guess because it's a curiosity or so. Never mind.
http://us.dfi.com.tw/Product/xx_product_spec_details_r_us.jsp?PRODUCT_ID=5155&CATEGORY_TYPE=MB&SITE=US


jitpublisher said:

I am not sure what you mean "the 790fx is for xfire it the board has the needed number of slots" with "some FX boards only have 2" ??? All you need is 2 slots for Crossfire.


You got that right. Some FX boards do have more than two mechanical X16 slots to support more than 2 cards in crossfire. The official name is Crossfire X though.

The Gigabyte GA-MA790X-DS4 for example has only two slots, both operation at x16.
The Gigabyte GA-MA790FX-DQ6 has 4 slots to support crossfire x with four slots at x8.

February 19, 2008 1:45:09 PM

Slobogob said:
I'm not too sure about that either. The problem is, the 790X has two mechanical X16 slots but they are connected with x8 only.
The 790FX has a total of 32 lanes available. So if you get a board with 790x that has four x16 slots you can operate one card at x16, two cards at x16 or four cards at x8. Three cards should be possible too, but i'm not sure how the lanes are split then.

My take on this is, that if you really want to use two 3870 x2s in crossfire, i would take a 790fx board. If there are cards that can use the extra bandwidth provided by PCIe x16, then it's them. In addition, if you spend that amount of money on GPUs you don't want to hamper their performance by saving 50$ on the mainboard. That's my opinion though.

That was pretty much my thinking. If i'm going to blow £600 (equivalent of $1200ish) on graphics cards, a little bit more on a better board isn't going to hurt :) 
I'd only kick myself that I hadn't gone better! That and the board may last me until I want to go to AM3
February 19, 2008 1:49:53 PM

wingless said:
SUMMARY: Get a 790FX card so you can run x16 on BOTH cards. If you got the cash for two 3870X2's then you better have the cash for the latest and greatest motherboards. Go 790FX so you don't feel stupid later....

Yep, think that's what I'll do. :) 
It's kind of like the **** information you get from nvidia over which chipsets can support x8 or x16 SLi....
I guess they just want you to buy the most expensive regardless of whether you need it or note....
a c 143 U Graphics card
February 19, 2008 2:01:02 PM

I've got a related question. Will Vista be required for 3870X2 CF? I've read somewhere that Crossfire with more than 2 cards will only be supported in Vista. Technically, 3870X2 CF is 4 cards, but you could also say it's 2 cards because it only uses two slots.
a c 79 U Graphics card
February 19, 2008 2:14:30 PM

yeah, there was a article here at toms about the leaked crossfirex specs and according to that you'll need vista for multi gpu crossfirex (3or4 cores, so 2 3870X2 will need vista)
February 19, 2008 2:39:56 PM

aevm said:
I've got a related question. Will Vista be required for 3870X2 CF? I've read somewhere that Crossfire with more than 2 cards will only be supported in Vista. Technically, 3870X2 CF is 4 cards, but you could also say it's 2 cards because it only uses two slots.


The article Kari is referring too was on TGDaily (by Mr. Demerjian) so i wouldn't use that as a reference. While i don't know how the support is going to be, if i had the intention of buying two 3870 x2 cards, i would call the manufacturer to answer that.
February 19, 2008 4:38:05 PM

Or X48 for the xfire.

I believe that it indeed is in vista as they scale it using AFR and that requires a raised pre-render limit. What I heard anyway. Idk if true.


I think it is vista only though.
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