Please help me understand the basics of PCIe PLX

[EnergyEclipse]

So I did further research into they of multiplexing and PLX. 

 

Courtesy of some very informative articles by that of anandtech and TQA Daily I have found my self understand the fundamentals of this kind of technology. Along the way I found myself referring to Wikipedia a lot, to whom have a very good description of defining multiplexing thank you to there reference of its usage scenarios in telecommunications, and there helpful GIFs.mupon this topic I believe I can be further assisted by my father should I require, as I believe he would be more knowledgable on a topic to which is worked with for most of his life.

 

For the understanding of PLX in general I can refer to this quote from a TQA Daily arcticle,

 

"A mainboard has a set limit of PCIe lanes, as I have talked about before, consumer boards currently have 16 to 20 PCIe lanes available from the processor, but what if you needed more?

 

The feature is PLX, and what is does is amazing when you want a third GPU, or more hardware than your processor would normally support. Your mainboard would typically only be able to connect devices with connections up to 16 to 20 PCIe lanes, unless your using a X99-3 Processor, or similar. This means that consumer mainboards would only be able to connect 2 GPU cards from Nvidia in SLi at any given time, but wait a minute, aren't there mainboards that claim higher on the same consumer platforms?

 

The answer is yes, there are other mainboards that can for some reason connect more than 2 graphics cards under SLi, and they use PLX to

be able to achieve this, by multiplexing the PCIe lanes. This can double the available number of PCIe lanes available to the mainboard, while using a CPU that has fewer than needed PCIe connections built within."

 

He goes on to better explain multiplexing and the motherboards supporting PLX and forth, and I have reference now to the anandtech article which looks into the more depthful technical side of things.

 

I will continue to research further into this topic, and again that you to everyone who have been awesome in helping me come to grips as a complete noob on this kind of topic originally.

 

Have a great day, and goodnight for now!

-EnergyEclipse

 

 

 

 

[Colin Donoghue]

I thought It did 8x 4x 4x

[Glenwing]

37 minutes ago, EnergyEclipse said:

Hi there! So I understand this topic features some similar questions to the ones I asked in a previously thread today, but I wish to keep thing separated so I can easily access the information quickly and distinguishabley for future reference if that is to make sense. 

 

I am mostly just wishing for help digesting this information, as to me this was like when I learnt computers could do so much more than just let me make funny drawings in MS Paint but kind of on a massively more complex but awesome scale.

 

I was recently coming to grips with the idea of using a technolgy called PLX in order to find a way of achieving 3 way 980ti SLI on a i7 4790. My understanding of why I cannot do it because of CPU limitations is because my processor has only 16x PCIe lanes to transfer data and so forth, however those 16x can be divided to 8x/8x for two cards in dual way SLI. but the cannot be divided down again to 4x/4x/4x for three way SLI as 4x is not sufficient, correct? Only Crossfire can do that with 4x a card.

 

I watched BuildZoid's video on his attempt at explaing it, although no offence to him he didn't make much sense with how often he changed what meant want and he admitted he was pretty tired during the video, so he wasn't very much help.

 

I read an article on anandtech about this, and it did some very cool work on explaing PLX, but it did start to get a tad bit confusing at the point were it tried to explain how PLX kind of like how it's assembles the data to be funnelled the information through the limitation of only having 16x on a hardware based level but then having it then somehow assembled on the other side, a bit like the 'PLX Co' on BuildZoid's video, however that is the point in which I am struggling to understand. What exactly is it PLX does and how do I get it to work in order to achieve 3 way SLI?

 

Thank you massively in advance for your replies! 

-EnergyEclipse

 

 

PLX is a company, they make (among other things) PCI Express switches, often generically called "PLX chips" by people in the tech community.

 

Since the CPU has only 16 lanes, switches are used to allow lanes to have some flexibility in how they are configured. When you have an x16 slot, and a slot below it wired for x8, that is 24 lanes worth of wiring, but only 16 total are provided by the CPU. To wire up the slots, 8 of those lanes are routed through a simple PCI Express switch called a quick switch, which basically acts like a railroad track switch and disconnects 8 of the lanes from the top slot and connects them to the second slot instead, if a device is detected there, moving it from x16/x0 to x8/x8.

 

There are more complex PCI Express switches available, which are the ones people are referring to when they talk about "PLX chips". Instead of acting like a railroad track switch which disconnects the lanes from one slot and connects it to another instead, the PLX chip connects to both slots simultaneously, and combines the data more intelligently, so that the devices are not strictly capped at 8 lanes worth of bandwidth. The total amount of data from both devices cannot exceed 8 lanes worth of bandwidth through the switch, but it effectively allows one device to use some of the other device's bandwidth if it isn't using all of it, instead of being capped at x8 speed. For example, with a PLX switch, you would be able to have the top slot operating at x16 (8 lanes directly connected, and the other 8 through the PLX switch) and the second slot also operating at x8 simultaneously (8 lanes through the PLX switch). Since the switch only has 8 lanes total, it cannot provide a full 8 lanes of bandwidth to both devices simultaneously, so the graphics card wouldn't be able to use the full x16 bandwidth and the other device use its full x8 bandwidth at the same time. But if the second device is not using the full 8 lanes worth of bandwidth, then any extra can be allocated to the top slot instead of just being wasted.

[Princess Luna]

10 minutes ago, Colin Donoghue said:

I thought It did 8x 4x 4x

You could potentially run 3 cards in the system simultaneously with that configuration, the catch is that this would only work if they are working independently, good for stuff like mining, but in SLI to force'em behave like a single GPU good for gaming it won't work as every thing has to be identical.

[pas008]

https://www.techpowerup.com/img/12-03-02/pex8747_product_brief_v1.0_20oct10.pdf

[EnergyEclipse]

1 hour ago, pas008 said:

https://www.techpowerup.com/img/12-03-02/pex8747_product_brief_v1.0_20oct10.pdf

I just say but that is quite a confusing but informative no doubt article. Thank's to @Glenwing's clarification on PLX Chips and like there rail track system of 'dividing', things sound clearer now. Thank you very much though for your reply!

 

However @Glenwing the idea of not wasting the bandwidth if one card is not using all and so forth seems like a really good idea, although if apply that metric it is not then actually possible for me to get 3 way at all since SLI needs 8x a card? But then again that is the entire point of PLX as a it is kind of like s repeater. Referencing the kind of railway system makes a lot more people sense, and thank you very for your mammoth help on this matter so far!

 

Have good days guys! Thank you for your help again 

-EnergyEclipse

Edited October 15, 2017 by EnergyEclipse
I got a mind blank for a second then lel XD

[pas008]

3 minutes ago, EnergyEclipse said:

I just say but that is quite a confusing but informative no doubt article. Thank's to @Glenwing's clarification on PLX Chips and like there rail track system of 'dividing', things sound clearer now. Thank you very much though for your reply!

 

However @Glenwing the idea of not wasting the bandwidth if one card is not using all and so forth seems like a really good idea, although if apply that metric it is not then actually possible for me to get 3 way at all since SLI needs 8x a card? DD:  Referencing the kind of railway system makes a lot more people sense, and thank you very for your mammoth help on this matter so far!

 

Have good days guys! Thank you for your help again 

-EnergyEclipse

last 2 mobos had plx chips

I ran 16/8/8 in tri which gave similar results as 16/16 in the games I played at that time

now I run 16/16/ 1x physix but just to power other monitors in truth

[Glenwing]

17 minutes ago, EnergyEclipse said:

I just say but that is quite a confusing but informative no doubt article. Thank's to @Glenwing's clarification on PLX Chips and like there rail track system of 'dividing', things sound clearer now. Thank you very much though for your reply!

 

However @Glenwing the idea of not wasting the bandwidth if one card is not using all and so forth seems like a really good idea, although if apply that metric it is not then actually possible for me to get 3 way at all since SLI needs 8x a card? But then again that is the entire point of PLX as a it is kind of like s repeater. Referencing the kind of railway system makes a lot more people sense, and thank you very for your mammoth help on this matter so far!

SLI will work as long as the card is operating in x8 mode, and a PLX switch will still allow it to do that.

[Colin Donoghue]

3 hours ago, Princess Cadence said:

You could potentially run 3 cards in the system simultaneously with that configuration, the catch is that this would only work if they are working independently, good for stuff like mining, but in SLI to force'em behave like a single GPU good for gaming it won't work as every thing has to be identical.

You only have to use 1x for mining it doesn't use up a lot of bandwidth

[IanCutress]

https://www.anandtech.com/show/6170/four-multigpu-z77-boards-from-280350-plx-pex-8747-featuring-gigabyte-asrock-ecs-and-evga

Page 1 from AnandTech. Has everything you need to know (from 2012) about PCIe switches like the PLX8747.

 

The latest PLX switches are the 9000 series. Ever since Avago (now Broadcom Limited) purchased the PLX company, the price went up 3x then 3x again, so you'll be lucky to see them outside big servers.

 

https://www.anandtech.com/show/9245/avago-announces-plx-pex9700-series-pcie-switches