The PLX Chip and 2-Way SLI

[borntofly]

Hey Guys,

I have read everything on Anandtech and elsewhere that I could find on the PLX PEX 8747 chip. I have been researching components for a Z77-based gaming build, which will be a 2-Way SLI setup with dual 680s, and now it is down to the motherboard.

Apparently, from what I can determine, the current crop of Z77 boards which do have the PLX chip (such as the ASUS Maximus V Formula) implement it such that we have one PCIe 3.0 slot running all 16 lanes and a second 3.0 slot running 8 lanes.

My question is this: Given that the two graphics cards will communicate with each other via the SLI bridge, how important to framerates is the use of two 16-lane PCIe 3.0 slots (a' la an X79 board)? My supposition is that in the real world having the second graphics card in an 8-lane slot will exhibit no appreciable degradation in performance compared to a dual 16-lane setup, but I have no benchmarking or other data to support this.

I am therefore soliciting your knowledge and opinions.

NOTE: If you guys know of a Z77 motherboard which does implements PLX with two 16-lane slots, please post on that as well, as it would likely render this thread mute.

Thank you so much! You guys are the best.

 

[nature1ders]

From what I understand if you SLI the first PCIE slot drops to 8x and the 2nd is 8x, it don't stay at 16x

 

[borntofly]

Yeah, and according to the tests I read today on HardOCP and Tom's Hardware, there's basically no difference between running 16 / 16 (on an X79) or 8 / 8 (on a Z77). Frame rates are pretty much the same.

Thanks!

 

[Zap]

For 2-way SLI, don't bother with a more expensive board with a PLX chip.

It is required (probably due to Nvidia rather than any technical reasons) for 3-way SLI on socket 1155.

 

[borntofly]

OK, thanks Zap. I was looking at getting the Asus Max V Formula, but I will also check out their non-ROG offerings for a board that has the feature set I want but without having to pay for PLX.

From what I understand, the PLX chip on the MVF won't hurt me though, right? It just maybe won't activate with only the two 680s?

 

[BrightCandle]

In certain compute circumstances it makes a surprisingly large difference, in all the gaming tests I have seen it makes none. That doesn't mean that in some game there wont be a difference but no games tested for this have shown up a problem so far. PCI-E 3.0 8x/8x seems to be good enough for dual SLI/dual crossfire for now with games.

 

[blackened23]

I've read that the PLX chip is disabled on the maximus V in dual SLI configurations.

So you MIGHT be okay. I've mentioned this in your other thread, but the PLX chip should always be avoided unless you're doing tri or quad sli - due to significant latency issues. If you do double SLI with a PLX enabled motherboard it will perform worse than native CPU x8/x8.

So you might be able to disable the PLX on the maximus V; in fact, I remember during the product brief that asus specifically stated the PLX is disabled in single/dual GPU configurations. So here's my question: with that in mind why do you want this motherboard anyway? It is a complete waste of cash if you're not doing some type of tri or quad sli, I don't understand. IMO it would be better to get a P8Z77 deluxe or pro, or something along those lines. You can literally save yourself 100$ or more and get a better motherboard. That board is a waste unless you're doing quad SLI.

The entire purpose of that board having a PLX chip is to enable quad sli - at the expense of inferior dual/single gpu performance (although as stated, PLX should be disabled in that case). That is part of the reason for the premium pricing. If you're not using tri or quad, you're spending money on something you don't need or even want - PLX has significant caveats.

 

[borntofly]

blackened23, you have a valid point. I guess I was looking at the MVF for the feature set provided, like the higher end built-in audio, dual-band WiFi / bluetooth, etc., much of which I would actually use. My case tends to run a little warm so the watercooled VRM sink could also come in handy.

The $280 Formula is actually limited to 2-way SLI, which is what I plan to run (although it will do Tri-Crossfire, probably the reason for the PLX?). Newegg lists the P8Z77-V Deluxe at $290 and the Pro at $240, so I wasn't seeing a huge delta while doingthe price comparisons. I posted on the ROG boards asking the same PLX question, no definitive answer yet.

 

[borntofly]

blackened23: the ROG forum have posts by super moderators stating that the PLX is in fact disabled for one- or two-way SLI for the MV Extreme, but I couldn't find a post for that on the Formula. I am going to keep researching both the P8Z77- and ROG boards...
Thanks, man.

 

[AdamK47]

Fact of the day: My system contains four PLX PEX 8747 chips.

 

[borntofly]

I have just reviewed the full technical documentation on the PLX website for both the PLX 8606 and the PLX 8747 chips. I now feel fairly confident of the following regarding the ASUS Maximus V Formula:

1) That the board will run one graphics card with 16 PCIe 3.0 lanes and will run two cards with x8 / x8 PCIe 3.0 lanes, connecting directly to the CPU's 16 native PCIe 3.0 lanes.

2) The board does NOT contain a PLX PEX 8747, which is used on many Z77 boards to enable three-way SLI or four-way SLI/Crossfire.

3) The board DOES implement a PLX PEX 8608 to utilize it's lane mutliplexing methodology to "double" the CPU's 8 native PCIe 2.0 lanes to ensure that most if not all of the board's PCIe 2.0 devices (e.g., SATA and USB 3.0) will remain operable during simultaneous operations.

I have also reviewed gaming benchmark testing (Tom's Hardware) between PCIe 3.0 vs. 2.0, which revealed only an average of a 5% increase with 3.0 across a variety of graphically intensive games.

Additionally, I have reviewed gaming benchmark testing (HardOCP) between 16 x 16 lane SLI vs 8 x 8 lane SLI, which revealed no real world dicernable differences between the two setups.

I will therefore be purchasing a Maximus V Formula, along with an Intel 3770k, 16 gigs of Corsair Dominator Platinum 2400, a second water cooled GTX680, a second Samsung 840 Pro 512 gig SSD, plus USB 3.0 peripherals (SATA docking station and flash drive). I will apply a nice, healthy 24/7 overclock and I look forward to using the built-in WiFi, Bluethooth, SupremeFX sound, as well as the other ROG exclusive software and hardware innovations.