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The motherboard in question is the ASUS P5MT-M Micro ATX

The following image shows a couple sections of information taken from the official ASUS manual for this motherboard, along with an image of the entire motherboard taken from the ASUS website. (The motherboard in the image is REV:1.01, whereas mine is REV:1.03 - I don't think this matters too much for the question at hand.)

The PCIE slots are not identical - one has an x8 link and one has an x4 link. However, they are both x8 size to make this micro motherboard slightly more robust. They appear identical visually - and as you can see from the image of the motherboard, every square millimeter of this motherboard is crammed with a chip or a capacitor or something. There's absolutely no room for component labels - or anything helpful at all, really.

What I need to figure out is which one of the PCIE slots is x8 and which one is x4 - in order to maximize performance of PCIE expansion cards. I have provided a link to the most recent official manual from ASUS - but I can't find anything in there that says which slot is which. Does anyone know where this information might be found, or how I might figure it out for myself?

enter image description here

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Sure enough, I can't find ANY information on the internet stating which PCIE slot is which when I'm actually SEARCHING for it ... So now I'm planning on using JFV's tools to figure it out, and I'm searching the web for an applicable I/O shield, and whaddya think I find? A WEB PAGE SHOWING THE LAYOUT OF THE SLOTS!! The kicker is that I found this on the ASUS website: What are the chances of them making nitty-gritty specs avaiable FROM THE ACTUAL MOTHERBOARD PAGE? Haha, nothing on ASUS, of course. I just found it a little ironic. –  Giffyguy Aug 9 '09 at 5:22

4 Answers 4

up vote 8 down vote accepted

Without powering up the system, you can most likely tell from visual inspection which slot is x8 and which is x4. Each PCIe pair of traces (differential pair) has two series capacitors (AC coupling capacitors). The capacitors will be lined up on the top and/or bottom in pairs parallel to the long edge of the slot starting closest to the interior of the board. Given that the manufacturer won't populate the capacitors on the unused traces to save money, you'll see 4 pairs of capacitors for a x4 slot and 8 pairs of capacitors for a x8 slot.

Here is a picture of the an example CAD layout of one set I found through a Google image search. The rectangular pads are where the capacitors would be populated:

alt text

You can easily see the AC coupling capacitors on either side of the top slot in this picture:

alt text

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Wow! This is great information. It was much more difficult to find those capacitors on the Micro ATX, where everything is tinier than usual and crammed as tightly as humanly possible. But upon REALLY close inspection, I was able to pinpoint approximately 4 pairs of capacitors by the first slot and approximately 8 pairs of capacitors by the second slot. This fits with the documentation I found (see my comment on the question), stating that the x4 slot is closest to the CPU/RAM. –  Giffyguy Aug 9 '09 at 19:48
It is definitely something to look for when buying a new motherboard. Large PCIe slots with less than maximum lanes hooked up to them seems to be more and more common these days. –  hanleyp Sep 5 '09 at 2:12

I'm willing to bet that the topmost card is the x8 link. Motherboard manufacturers will typically place the fastest or highest performance slot at the top, closest to the board.

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That's good to know. Thanks! –  Giffyguy Aug 9 '09 at 0:52
According to the information I eventually found (see my comment on the question): This specific motherboard was actualy manufactured contrary to your prediction. The x4 slot is the one closest to the RAM/CPU. I should have taken that bet, haha. Although, it's an awesome rule of thumb. After all, this is the first time I've seen a system that DOESN'T have it's slots ordered by core speed, from fastest to slowest. Go figure... –  Giffyguy Aug 9 '09 at 5:27
Wow, that is indeed very odd. At least you found the right one after all :) –  EvilChookie Aug 9 '09 at 5:52

You can always check CPU-Z to tell you which is which. Or you can use SIW.

Both of them should give you a good look at your board. I think that SIW should be better thatn CPU-Z.


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This appears to be the best all-around solution for this problem. These tools are REALLY powerful. :) But it looks like it'll be a small hassle to make use of these tools BEFORE the core system installation. I'll have to install a simple dummy OS (Windows 2000, perhaps) on a random drive, so that I'll have somewhere for these tools to even run. I'll use the information they provide to structure my PCIE RAID setup - and then I'll have to wipe the drive and stick it back in my RAID array, and then install my server for real. –  Giffyguy Aug 9 '09 at 4:53
I think these are great tools, but in the end it appears that you CAN find this information by physically looking at the motherboard. I prefer this to a software solution, since it eliminates the need for a functional OS. After all, a technician may or may not have a functional system at the point where they need this information. –  Giffyguy Aug 9 '09 at 19:39
@Giffyguy: It would have been nice if you explained that in your original question. I assumed that there already was an OS on the box... –  JFV Aug 9 '09 at 19:46

The information is in the manual, but you really have to dig for it.

The block diagram (page A-1) shows that "x8 slot 1" has eight lanes going to it, and "x8 slot 2" has four.

The board illustration on page 2-3 shows that PCIe2 is the slot nearest the RAM DIMMs.

Therefore the other PCIe slot, the one next to the PCI slots and farther from the CPU, is the one with 8 lanes.

Not what I would have expected, but there it is.

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This does not add really any new information that @hanleyp didn't already provide years ago. –  Ramhound Sep 30 at 12:24
Not for the OP. But it does show that the info IS in the doc, and may help others find similar info. –  Jamie Hanrahan Sep 30 at 12:40
The author accepted the answer. At first i thought this question or answer appear in one of the review queues, but given everyone has more then enough reputation for that NOT to happen, how did a 5 year old thread draw your attention? –  Ramhound Sep 30 at 12:43
I don't know. It showed up when I clicked "questions". For some reason I did not see any answers let alone an accepted one. So I went and looked up the manual, expecting to find the answer in the block diagram, and I did. I hadn't noticed the date on the OP either and was surprised to see refs to Win 2K3 in the manuall... When I came back to the question I saw the answers but I wasn't going to throw the info away. Even if not needed by the OP (who may not even have that mobo any more!) I felt that the technique of interpreting the block diagram deserved a description, since no one else had. –  Jamie Hanrahan Sep 30 at 12:49
Alright. Oh well. Given the answer in that context it is decent. –  Ramhound Sep 30 at 13:39

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