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How to understand configuration of pcie x16?

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February 12, 2012 8:08:46 PM

Hello,
I am reading through the ASUS manual for the P8Z68-V_PRO_GEN3. I am looking at page #31 of the 146 page manual. I am looking at the chart entitled: PCIEX16_3 configuration. I am trying to understand the meaning of what appears in the cells of this chart. For your understanding, I am referencing this charts X- axis as its five columns. And it's Y-axis as: Auto Mode, X4 Mode, X1 Mode. Where the columns and rows intersect are the cell of this chart. I want to understand what they are trying to communicate with the data in these cells. Please note, that on page 3-22 there is an explanation of: PCI Express X16_3 slot (black) bandwidth [Auto]. However, I am still confused about how to interpret the data in the configuration chart itself. I get that some things are enabled and disabled. But I am trying to understand the different modes of the Y-Axis in relation to the various columns of this chart. What does this stuff all mean?
February 12, 2012 8:46:23 PM

seems that slot (pci16_3) shares bandwith with the other devices on that chart, like the two x1 slots, usb3_34 and esata ports. setting that slot to x4 for example (for a high speed device on that slot) will disable the others on the chart because that slot at x4 dedicates all the bandwith to that single device. setting it to x1 means it only uses 1/4 of the resources and the other devices can function at x1 speed. basically if you add a device to that slot and run it at x4 you won't be able to use the others on the list. if you aren't going to use that slot just set it to auto and the other devices will work fine. if you do add a device there it will run at x1 by default unless you change it to x4 manually.
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February 19, 2012 12:13:34 AM

The reply from furry_roadkill was somewhat helpful. However, I myself have clarified what I needed to know about my original question. When I make my notes on what I discovered, I will also post a clarification here for all to see. Please standby, I will get back to this soon.
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February 19, 2012 4:05:52 AM

Here's what I found out to this point:

The PCIEX16_3 configuration chart on pages 30 and 31 of the instruction manual at section 2-13:
> The entire chart is about the use of Slot #7. See Slot Descriptions chart on page 31 of the instructions.
> The X-Axis of this chart are referencing the various cards that can go into Slot #7. That is using cards defined as: PCIEX16_3; PCIEX1_1; PCIEX1_2; USB3_34; ESATA. Note that (PCIEX1_1 and PCIEX1_2) refer to the same type of PCIe card. The underscore extentions as: ( _1 or _2 ) just refer to which slot of the seven slots are being used for those PCIe cards.
> What is in the cells of this chart are the bandwidth or speeds that the card will be running at.
> X4 means 4 times the actual card rating. So, if you’re using a PCIEX16 card in the PCIEX16_3 slot, which is Slot #7, and you’re in the X4 Mode then you’ll be running that card at 4 times the speed of the card itself. So, since this card is rated at X16 or using a 16 lane bandwidth then you’ll multiply the 16 by 4 to get 64. Each lane transfers one byte at a time per second. See, my definition of a lane.

Example of how this chart can help you when you utilize the various cards with this motherboard:
>If you put a PCIEX16 card into slot #2 then you can also put a PCIEX16 card into slot #5. Note also both these PCIEX16 cards will be running at X1. Note also that because you’re not using slot #7, you can utilize that slot for and other card referenced by this chart. But all three cards, the two PCIEX16 cards and any other single card from this chart in slot #7 running at X1, will be able to run in this configuration.
>If you put a USB3_34 card into slot #7 and you’re running the X1 Mode then you can put a PCIEX1 card into slot #1 and another PCIEX1 card into slot #3. Both of these PCIEX1 cards are version 2.0. And all three of these cards will be running at X1.
>A single PCIEX16 card in slot #2 will be a single VGA/PCIe card configuration. This card will run at the X16 rating. However you will not be using slot #5.
>I’m not sure about this, but it seems if I put a PCIEX16 card into slot #2 and I put a PCIEX1 card into slot #3 and an eSATA card in slot #7 then all three of these cards will run at X1.
>If I put a PCIEX16 card in slot #7 and I am in the X4 mode then I will not be able to use any of the other cards with this configuration.

I need somebody from this community to confirm and/or clarify my findings on this PCIEX16_3 Chart. Remember this whole chart seems to govern the way slot number 7 (see the slot chart in the manual instructions) is being used.

I am also interested in how to understand and read the manual's IRQ (Interrupt Request) chart on page 31 of the manual instructions. I have no idea what the "shared" means in this chart, nor what the letters A-H means on the X-axis of this chart, nor what the items in the Y-axis of this chart have to do with the entire chart itself. So, any better understanding of what this chart is communicating to the end user means will be appreciated.
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February 19, 2012 4:10:24 AM

The definition of a Lane=

Lane = A lane is composed of a transmit and receive pair of differential lines. Each lane is composed of 4 wires or signal paths, meaning conceptually, each lane is a full-duplex byte stream, transporting data packets in 8 bit 'byte' format, between endpoints of a link, in both directions simultaneously.[5] Physical PCIe slots may contain from one to thirty-two lanes, in powers of two (1, 2, 4, 8, 16 and 32).[4] Lane counts are written with an × prefix (e.g., ×16 represents a sixteen-lane card or slot), with ×16 being the largest size in common use.[6] (Wikipedia)
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February 19, 2012 9:00:23 AM

ok let me try to explain it a little better. Some pci slots have a dedicated amount of lanes available to them. For example, on your board, slots 2 and 5 share 16 lanes. You can use one video card and allocate all 16 lanes to that one card to improve the bandwidth, or you can run two cards and allocate 8 lanes each. These 16 pci lanes are dedicated to these two slots and cannot be used any way other than the two combinations listed above.

your board is only left with 4 pci lanes to share between 5 devices, pci slots 1, 3, 7, usb3_34 and esata. usb3_34 is the internal header to connect an additional usb 3.0 bracket (see chapter 2 page 3 in your manual, it's labeled #8 on the previous page). the esata refers to the connection on the motherboards rear i/o panel. now those 4 lanes have to be distributed among those 5 devices. the first pcie slot is an x1 slot. if you use this one, it can only use up 1 lane. Same for pcie slot 3.

pci slot 7, however, is an x4 slot. meaning it can utilize all 4 lanes (remember that these lanes have to be shared between all previously listed devices. This will disable all other devices it is sharing the lanes with. That would mean you cannot use your first or third pcie slots, your internal usb3.0 header, or your esata connection. For example, you want to use this card.
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

it's a pcie ssd. That card requires alot of bandwidth and is a pcie x4 card. So you put that card in pcie slot 7. That's where your original questions comes in. What does the chart mean? It means you have 3 options when it comes to running that card in that specific slot (#7). You can run that slot in Auto, X4, or x1. If you run this card in x4 mode as it was intended, you would not be able to use the other 4 devices this slot shares lanes with (slots 1, 3, usb header, and esata). Your other option is to run the slot in x1 mode by setting it to x1 or auto in the bios. you would essentially be limiting this cards performance (as it is meant to run in x4 mode because it can utilize lots of bandwidth) but it gives you the option of using 3 more devices (out of the remaining 4).

btw running a card in x4 mode does not mean you are running it 4x as fast, simply that you are dedicating 4 lanes to that card. Whether or not it utilizes all available bandwidth is dependent on the card itself. For example, placing a dedicated sound card (usually an x1 card) in pci slot 7 and setting the slot to x4 mode does not mean the sound card will utilize 4 times the data. it will simply be using only 1 of the pci lanes, even though it has 4 dedicated to it.

i hope this clears some of the confusion up.

[edit: typo]
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February 20, 2012 11:41:16 AM

Thank you for your response. I am following some of what you say. However, I need to get my head around some other parts you speak of. Let me express my appreciation. I am new to building a computer, but I have a moderate amount of computer knowledge which helps. This process has been a learning experience for me, and it is taking me time to comprehend many new acronyms and computer engineering terminology. I've come a long way since I've started this project. And I will get to the finish line!
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February 20, 2012 1:33:30 PM

just let us know what parts are still fuzzy and myself or someone else can try to answer them. maybe with some pictures :) 
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February 20, 2012 9:37:35 PM

Oh gee, thank you so much. Please check back for a follow up post. And I will try to break my questions down for you to make your responses match the question more specifically. I will get back at you soon.
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February 21, 2012 7:53:13 AM

Let me reference the 3 charts in the manual for the Asus manual for the P8Z68-V_PRO_GEN3.
*There's the Slot Description Chart (SDC) at section 2-12, page 30.
*There's the VGA Configuration Chart (VGAC) at section 2-12, page 30 (same page).
*There's the PCIEX16_3 configuration Chart (PCIEX16) at section 2-13, page 31.
*There's the IRQ assignments for this motherboard (IRQ) at section 2-13, page 31.

Please note: I will use my abbreviations in () above to try to make things easier for us.

The following will be closed ended questions to make you job of answering my questions easier. Please just answer what I am asking directly. If I need to be corrected then please just explain the subject of my question better without going beyond (cause that's what can add to my confusion). Thanks.

With this in mind:

1. The PCIEX16 is a chart that focuses on just the Slot #7 in the SDC. I understand that Slot's #2 and #5 can also accept the PCIX16 card too, but this chart seems to be focused only upon Slot #7. This is why Slot #2 and Slot #5 have the "_1 and _2" added as a suffix to the PCIEX16. And why Slot #7 has the "_3" suffix. Am I correct here? Yes or No, please explain.

Okay, so let's say my first question is accurate, then:

2. The X-Axis of the PCIEX16 shows the five options of PCI cards that can go into the PCIEX16_3 or Slot #7 location. Is this correct? Yes or No, please explain.

So, let's say my second question is accurate, then:

3. The Y-Axis of the PCIEX16 shows the different configurations for Slot #7?
Is this correct? Yes or No, please explain.

4. The different configurations of the Y-Axis are chosen by the BIOS? (Note: I haven't got up to reading about this in the manual yet. But I'll get there! Free time is limited to learn about all this stuff.)
Yes or No, please explain.

5. Now, if I'm right about my reading of this PCIEX16 chart and if I am using the X4 mode, then I will be required to use Slot #7 or PCIEX16_3 for my PCIEX16 card. And, I will not be able to utilize any other PCI card (see the X-Axis of the PCIEX16 chart) at the same time, even in any other Slot 1-6. Is this correct? Yes or No, please explain.

6. If I am using the Auto Mode of the PCIEX16 chart then a PCIEX16 card in Slot #7 will only run at 1 lane? Yes or No, please explain.

7. In addition, if I am using the Auto Mode of the PCIEX16 chart and Slot #7 contains a PCIEX16 card then Slot #'s 1, 2, 4,5 and 6 are available for the other types of PCI cards? Slot #7 will use only 1 lane (question #6). The other cards can be: a PCIEX1 card, a USB3_34 card or a ESATA card.
And, any other PCI card option used will run at 1 lane each. It seems you can only use a maximum of four PCI cards on any of the three different modes. Is this correct? Yes or No, please explain.

8. If there are a total of 16 lanes which can be used then is each X1 = to 4 lanes? This means that using X1 with four different PCI cards that equals 4+4+4+4 = 16 lanes? Is this correct? Yes or No, please explain.

9. The "X#" is not a multiple of how many bytes are sent down a particular lane. It is just an indication of how many of the 16 lanes are being used?
Is this correct? Yes or No, please explain.

10. Using the X1 Mode, only Slot #'s 7, 1, and 3 can be used, even for a USB3_34 PCIE card or PCIE ESATA card. Is this correct? Yes or No, please explain. (Note: By this question I think you're getting an idea what is causing me confusion in looking upon this PCIEX16 chart.)

11. The VGAC shows that there are only two ways you can configure a PCIEX16 card(s). If you're using a Single then you must put that PCIEX16 card into Slot #2, which will use all 16 lanes. The other PCIE cards must use the other available slots. Is this correct? Yes or No, please explain.

12. The VGAC allows for a Dual configuration, which requires you to use Slot #'s 2 and 5, which will only use 8 lanes each. The other PCIE cards must use the other available slots. Is this correct? Yes or No, please explain.

I think you can understand by now how confusing this stuff can be to somebody new to building their first computer. Gosh, I sure wish the manual and tech writer would realize that not everybody is a computer engineer. Yet, basic consumers go into a computer store and are faced with computer parts that they have no idea about. And getting educated on this stuff is quite a task.

I'm giving myself an award for getting this far! If others can do this then so can I.

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February 21, 2012 7:55:00 AM

Oh, I almost forgot:

How do I read the IRQ chart?
This is another mystery.
Thank you.
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Best solution

February 22, 2012 4:24:29 AM

#1. correct.

#2. no.

The 5 devices on that chart aren't options that you put in slot 7. those are physical slots or headers on your motherboard. like this:



#3. correct.

#4. correct. (you can set slot 7 to run in auto, x1, or x4 in the bios.

#5. no. it means if you place a card in slot #7 (green arrow in pic above) and you set it to x4 mode, then you cannot use any of the other devices marked with an arrow in the picture above.

#6. correct. setting slot 7 to auto in the bios will automatically run in x1 mode, regardless of the card you place there.

#7. this part " In addition, if I am using the Auto Mode of the PCIEX16 chart and Slot #7 contains a PCIEX16 card then Slot #'s 1, 2, 4,5 and 6 are available for the other types of PCI cards?" is correct. This part is where you are confused is "The other cards can be: a PCIEX1 card, a USB3_34 card or a ESATA card. ". you are confusing usb3_34 and esata for cards. They are the header and port colored purple and yellow in the picture above. if #7 has a card and it's set to AUTO, you will still be able to use the black, purple, and yellow ports yes.

#8. no. an pciex1 slot can only have one lane. a pciex4 slot can only utilize a maximum of 4 lanes. a pciex8 slot can utilize a maximum of 8 lanes. a pcix16 slot can utilize a maximum of 16 lanes. Note however, i said MAXIMUM.

#9. the first number after the 'pciex' part of the slot means the physical size of the card that can be used. the second set of numbers is the amount of lanes that can be used by that slot. For example, a pciex16 x8 slot means it can accomodate a pcie16 card (the large ones) but it will only have 8 lanes available to it. a pciex16 x4 slot means it can accomodate a pci16 card, but it will only have 4 lanes available to it.

#10. no. see answer #7.

#11 errr...sort of. vgac chart refers to ONLY slots 2 and 5. These two slots share 16 lanes. These 16 lanes are not included in the 4 that are shared by the devices in the picture above. They are separate. What this chart means is if you are only using one pcie16 card, you have to place it in slot 2 so that one card can utilize all 16 lanes. This is because all 16 lanes are physically connected to slot #2. 8 of those 16 lanes then continue to slot 5. Slot 5 only has 8 lanes physically connected to it. Therefore if slot 2 is left empty, but u put an x16 card in slot 5, you are only taking advantage of 8 lanes. The other 8 that are wired to slot 2 are left unused. see crude drawing below.



each one of those red arrows represents 8 lanes. Notice how both arrows physically pass through slot 2? That means slot 2 can utilize all 16 lanes. only 1 arrow (again, think 8 lanes) reaches slot 5. So how much sense would it make to leave slot 2 empty, but put an x16 card in slot #5 that only uses 8 lanes. it makes more sense to put a single card in slot 2 and utilize all 16 lanes. by the same token you should see that if you are going to use both slots with x16 cards (for sli or crossfire) then each would use 8 lanes. So if you read the vgac chart again you can see that if you are only going to use one video card, it recommends you place it in slot 2 (your motherboard lists slot 2 as pci 2.0 16_1 because it's the first pci16 slot. slot #5 is the second pci16 slot, hence pci 2.0 16_2.

by the way, that chart is called 'vga configuration' because most (if not all) x16 cards are video cards. slots 2 and 5 have the most lanes physically wired to them so it makes sense to make those available to video cards (video cards require lots of bandwidth)

as far as irq's go, you really don't need to worry much about these. They basically go hand in hand with the 'shared lanes' concept from above. each device is assigned an irq (think of an irq as a number between 1 and 20) so that the system knows which device needs to use a resource (like raising your hand to take your turn to speak). Since many of the devices share lanes, they must also share irqs, since their requests travel along the same wires (lanes). This is an extremely simplified explanation. Way back in the day, irq's were a pain in the butt. many times requiring the user to manually set them to resolve conflicts. nowadays, alot of people get by building and messing with computers without really knowing anything about them, so you shouldn't 'worry' too much about this chart. certainly knowing more about irq's can help, it's not the most important thing you need to learn right away.

btw i really hope im helping and not confusing the hell out of you. :( 


[edit: forgot to answer irq question]
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February 22, 2012 7:05:01 AM

i'm just browsing he posts and found this one. i guess that i just don't care about this stuff before but this is quite important. i really learned something today

thanks Lotus100 for bringing this up and thanks furry_roadkill for explaining it in details (thats quite a long Q&A, but i like it).

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February 22, 2012 12:01:36 PM

OM Goodness! I just did a quick read of your answer to me furry_roadkill. You must be one of those electonic comets in the sky! I have to spend more time digesting what you said, which at my first reading, has made a hit in correcting my earthly understanding of things. Geepers, this stuff is so complicated, but ya gotta love this stuff. Anyways, I need more time to review this in more detail. Don't worry, I all ready have a few more follow up questions, so keep ontop of this thread.

h2_studio: I am most glad to learn that I've help elevate your awareness to this level of engineering knowledge. In my opinion and from my own computer experience as a RP&S training specialist AKA: instructor......, I'll have you know that it is quite to your advantage to understand the motherboard and other components which you choose to use and purchase. It will give you a better command of the device in question, and it will give you much greater benefits as an end user. Knowing how to set things up and what to do with "stuff" you buy before you make a major purchase, also gives you the full warranty advantage of the component in question. It also insures your understanding against making sloppy errors which can be costly mistakes. Never stop learning! Got it? Good.

Sometimes I wonder what these computer engineers eat for breakfast? Don't you? There's got to be something special in their diets! Cause I just don't know where they come up with all this stuff. But, as I learn more, I am amazed at what these things can do. For example, if you read your router manual in its entirety than you can't help at marvel at what that little itty bitty box can do.

Gotta go now, but stay tuned.......For now: "That's all folks!"

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February 25, 2012 2:12:50 PM

furry_roadkill:

A quick follow up question: Given the fact that there are 7 PCIe slots on this MB and that select configurations of a graphics card or cards will utilize one or two slots and that there are headers (which I think are better named female connectors, but then I didn't name them) for the USB3_34 and ESATA, then what are the remaining five PCI slots for? Why do I need them?

For example, if I am using a PCIEX16 card in slot #7, using either the Auto or X1 mode then why would I want to put another PCIEX1 card in Slot #1? Furthermore, why would I want to use USB3_34 and ESATA cards (they do exist according to my local computer retail representatives), using the Auto mode? Or two additional PCIE1 cards in slots #1 and #3 and an ESATA card in some other available open slot?

All these questions come up because ASUS tech people put forth this chart in the manual.

Needless to say, furry_roadkill, you have been helpful. Thank you.
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March 1, 2012 12:19:00 PM

furry_roadkill:

A quick follow up question: Given the fact that there are 7 PCIe slots on this MB and that select configurations of a graphics card or cards will utilize one or two slots and that there are headers (which I think are better named female connectors, but then I didn't name them) for the USB3_34 and ESATA, then what are the remaining five PCI slots for? Why do I need them?

For example, if I am using a PCIEX16 card in slot #7, using either the Auto or X1 mode then why would I want to put another PCIEX1 card in Slot #1? Furthermore, why would I want to use USB3_34 and ESATA cards (they do exist according to my local computer retail representatives), using the Auto mode? Or two additional PCIE1 cards in slots #1 and #3 and an ESATA card in some other available open slot?

All these questions come up because ASUS tech people put forth this chart in the manual.

Needless to say, furry_roadkill, you have been helpful. Thank you.

Please note: I am repeating this question again here, but this time I learned that I can reply directly to your post. I started second post recently because I thought you may have forgotten about this original question. For this reason, I found it necessary to start that second post. Hope you understand, Thank you.
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March 2, 2012 11:23:41 AM

Best answer selected by Lotus100.
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a b Ĉ ASUS
a c 328 V Motherboard
March 3, 2012 1:16:48 AM

This topic has been closed by Nikorr
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