ASUS Prime z390-P, PCIe lanes/speed question.

[Gunslinger]

So i've been asked to help a friend identify a second M.2 drive to use on this motherboard (ASUS Prime z390-P) and whilst looking into this, it seems that his second m.2 socket for storage drives is restricted to x2 speeds.

The system has one GPU in the standard x16 slot (unknown type) and one pcie wif card in one of the lower x1 slots (don't know why they didn't use the dedicated E Key'd m.2 slot when the system was built)

So I've posted a question in the storage forum regarding the consequences of connecting a x4 m.2 ssd to a x2 slocket, and will see what the reply from there is.


But it made me wonder, if i did want full speed from the planned additional m.2 drive, could i use an adapter card in the second x16 slot?

so i went and looked in the specs, and found something that I thought i'd ask here for clarity on.......

1 x PCIe 3.0/2.0 x16 (x16 or x8+x4+x4)
1 x PCIe 3.0/2.0 x16 (max at x4 mode)
4 x PCIe 3.0/2.0 x1
1 x M.2 Socket (Key E), supports type 2230 Wi-Fi/BT and Intel® CNVi (Integrated Wi-Fi/BT) module*
*For 3 SSD on CPU support, install a Hyper M.2 X16 card (sold separately) into the PCIeX16_1 slot, enable this card under BIOS settings.
* The Wi-Fi/BT module is purchased separately

That is a excerpt from the specifications for the board (linked above)

The Hyper card, i'm not familar with at all, but would guess it's some sort of ssd raid card or similar enterprise derived ssd storage card, which is not going to be relevant, especially as it has to go into the 1st x16 slot, which is occupied by his GPU.

The bit that caught my eye where the 2 lines referring to the x16 slots, and obviously where my interest lies, so the second slot appears to be x4 speed only, not a problem that's exactly what i'd need. BUT, am I reading the 1st slots details correctly?

(x16 or x8+x4+x4) meaning that the 1st slot will run at x16 speed, but will shift down if you put any card in the 2nd x16 slot ?

also that configuration of three values after the initial 'x16 or' speed reference genuinely made me go and look if the board had a 3rd x16 slot!

could someone please explain that (x16 or x8+x4+x4) to me in layman's terms, because right now i "think" it means x16 if slot 2 isn't used, if it is, then it'll be x8 for GPU plus x4 for other card, or if used for crossfire then it'll be x4.

how far off the mark am I? TIA

 

[Radox-0]

it is an odd board for z390 in how its wired looking at the page but may be due to being more budget friendly. Normally what happens is the top x16 slot is wired to x16 bandwidth then the second x16 slot down is wired to x8 on most z390 midrange+ board. With a 16 lane Intel CPU would split lanes x8 and x8 to each x16 slot and remaining x1 PCIe slots on board would be wired via chipset.

This is not the case with this board however, the top x16 slot is wired to x16 as expected. The second PCIe slot down is only wired to x4 PCIe which suggests it goes via the chipset. This also explains why the board only supports crossfire which is not as restrictive and can work via chipset whereas as require at least x8 from the CPU.

The manual does make things a bit more clear https://dlcdnets.asus.com/pub/ASUS/mb/LGA1151/PRIME_Z390-P/E15011_PRIME_Z390-P_UM_v3_web.pdf Chapter 1-10

What it seems to suggest is with a x16 GPU in top slot, it will get the full x16 lanes from the CPU, the second x16 slot would have 4 PCIe lane via the chipset.

When you use a Hyper M.2 x 16 card which has space for 4 M.2 drives, that's where the weird (x16 or x8+x4+x4) comes in. You would plug said card into the top slot inplace of a M.2 drive as with such a card and its where the weird PCIe breakdown comes from as that card can slot up to 4 M.2 drives.

In case of your friend however, looking at page 1-20 in the manual, it does seem both M.2 slots on the board are capable of providing 32 GB/s. Only caveat is using the top M.2 slot by the CPU, Sata port 2 gets disabled.

 

[Gunslinger]

ahhh, i never went looking into the manual, didn't think it would explain any better (wrong there!) I did think it might be related to how the slots may be wired in relation to chipset vs CPU lanes direct.

Forgot to say, I think his CPU is an i7 9700.

Your explanation of the x16 for GPU direct from CPU and the 2nd slot being x4 from the chipset would make sense, and also would allow for a regular x4 PCIe card to be used for the second m.2 drive should he wish to ensure full x4 speed compatibility... wonder if there would be any latency differences given it seems to be running through the z390 chipset.

Thanks for the help, and explaining stuff, i'll have a look of that manual, as i'll be interested to know how you get the full 32gb/s bandwith if the socket is limited to x2 speed.

oh, and yes, I think you may have been right ref the budget friendly comment, I think that was sort of where the prime range was designed to sit in the market place if you didn't need all the bells and whistles, but can leave you interesting little conundrums like this down the line.