Asus Z97 mainboard, important heads-up

[VJ]

Hello,

I recently got this mainboard, and noticed something. The specifications state:

2 x PCIe 3.0/2.0 x16 (x16 or dual x8)
1 x PCIe 2.0 x16 (x2 mode)
2 x PCIe 2.0 x1 *1
2 x PCI

And at the bottom:

1* The PCIe x1_1/2 slots share bandwidth with M.2 Socket 3. The M.2 Socket 3 is default disabled.

I have an M.2 disk installed, and had to enable this in BIOS. Now, I tried to plug in a Compro E900F in one of the PCIe x1 slots, and it fails to find the card. Looking around online, I found this review, where it says:

...you can use either the M.2 slot or the two PCIe X1 slots, not both at the same time (a chipset limitation) so the M.2 slot doesn’t clash with any PCI connectivity

So according to that it is either the M.2 disk, or the 2 PCIe x1 slots. Not really my definition of sharing... I expected that sharing implied that both would work at lower performance. Looking at the specs of other Asus Z97 mainboards, they have similar limitations, but are "sharing" the M.2 disk with other things, e.g. 2 SATA ports or even the SATA Express port. On other Z97 mainboards, it can be that one x16 slot is "shared" with 2 x1 slots.

This is for sure something to look out for, and I personally think it is false advertising. I checked other mainboards, but I'm not sure I would have opted for a different one had I known in advance (others have other limits), although I might have gone for one that has more PCIe and drops the PCI all together.

I will mail the issue to Asus, to ask if it is really either/or, and will post back here.

Hope this helps somebody... If it is a limitation of the Z97 chipset, then other manufacturers should have similar things. Double check!!!


Jörg

[MultimediaMan]

This is a straight limitation of how many PCI-e lanes the chipset has. They could do a better job explaining what a "shared" PCI-e bus means in terms of "or" and not "and".

[VJ]

This is a straight limitation of how many PCI-e lanes the chipset has. They could do a better job explaining what a "shared" PCI-e bus means in terms of "or" and not "and".

Well, I know that in most mainboards, using two x16 slots will make both of them running in x8 mode. This seems like sharing in the proper sense: both work, but slower...
I'm just annoyed by the fact that I went over all the specifications with a fine comb, and then just notice this when trying to plugin in a first PCIe card. The main reason for me to choose this board was to have 5 PCIe slots, but now I'm left with 3. It should still suffice, but it feels wrong.

[MultimediaMan]

Intel has been ramping up PCI-e Lanes as of late... the only thing that keeps multi-socket machines alive is Memory bus speed and PCI-e lanes. Haswell/Broadwell should fix most of the bottlenecks in time for M.2 to replace SATA/SAS.

[VJ]

Yes... Still, they worded it quite poorly... "Sharing bandwidth" is not the same as "sharing PCIe lanes"... If I had known, my choice would have been between the Z97-A and the Z97-Pro, which shares the PCIe lanes differently. It seems like on that one, you don't have all the PCIe slots at the same time (some x1 slots are "shared"), but at least it allows for some choice (e.g. in the case of double slot video cards or so). In the Z97-A, one double video card leaves me with 2 PCIx 16 (one mode x4) and one PCI. In the Z97-pro it would be 2 PCIe x16 and 2 (maybe 3) PCIe x1, because the one blocked by the double video card is shared, so can be "lost" in favour of another slot. The slots that are lost in the Z97-A are in a position where they are never blocked by anything.
Proper information would have allowed me to make a better choice. But it is too late to return this one, which is what annoys me, but I guess I'll have to make do with it.

[Umfriend]

I agree. "Sharing" implies concurrent use, not exclusive use. Sue them.

[VJ]

Interestingly, the description for the X99 mainboards look much better, and specifically mentions that enabling some devices disables others.

I contacted support, and explained my situation, and that the specifications are misleading. Let's see what they reply...

Either way, the mainboard is ok, but I would have liked more PCIe slots for future expansion.

[VJ]

Clearly they don't know the definition of sharing. I got this as a reply to my complaint:

When enabling the M.2 Socket the PCIe x1_1/2 will be disabled as they share the same bandwith which means that cannot use the same bandwith at the same time.

At least a better answer than what I got from technical support; they answered

If the other slots are in use the slots run X2.
this so it remain accessible.

Still quite annoyed by it...