B550 Tomahawk - M2_2 and PCIE_3

Tony359

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Hi all,

Just a sanity check before I return my new M2 SSD.

My B550 Tomahawk has a PCIEx4 SSD on M2_1 and a GPU installed. I also have a RAID card on PCI_E3.

When I install a new SSD on M2_2, the RAID card disappears.

The specifications say " When installing devices in M.2_2, PCI_E2 & PCI_E3 slots at the same time, PCI_E3 slot will be unavailable, and M2_2 slot only supports PCIe x2". Does that mean in "any of the M.2_2, PCI_E2 & PCI_E3" or that if something is plugged in ALL THE M.2_2, PCI_E2 & PCI_E3 simultaneously then the PCI_E3 slot is going to be unavailable?

In other words: is what's happening to me normal for this board?

Also, is PCIe x2 equivalent to about 2GB/s?

Thank you!
 

darkhawk

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That is normal. What it means is that if you populate M2_2 while having a card in PCI_E2, it will cause the M2_2 to run at PCIe x2, and PCI_E3 will be un-usable.
Take a look at page 29 of the manual.
1634392375659.png
 

Tony359

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Thanks - I didn't think of checking the manual! :D

However, I do not have a card in PCI_E2. Hence my question! The table you copied seems to suggest that I cannot have PCI_E3 anyways as long as M2_2 is used, right? Regardless of PCI_E2.

I see there are 10 PCI Express lanes available on the B550 - and only 5 max on the slots (I guess the others are used for other devices such as the SATA controller etc.)

So the first question is: my RAID controller is a PCI Express 2.0 x8. How does that work in a PCI Express 3.0 x16 slot which only has 4 lanes available?

In other words, how many PCI Express 3.0 lanes would a single PCI Express 2.0 lane use?

Interesting to see that Intel seems to have way more lanes on their chipset... :(

Thanks again!
 
Last edited:

RemusM

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The specifications say " When installing devices in M.2_2, PCI_E2 & PCI_E3 slots at the same time, PCI_E3 slot will be unavailable, and M2_2 slot only supports PCIe x2".
They didn't explain it properly.
In fact M.2_2, PCI_E2 & PCI_E3 share the same 4 PCIe lanes.
So ...
something in M2_2 ---> PCI_E3 not available
something in M2_2 + something in PCI_E2 ---> PCI_E3 not available + M2_2 downgraded from x4 to x2
nothing in M2_2 ---> PCI_E3 available at full speed
nothing in M2_2 + something in PCI_E2 ---> PCI_E3 downgraded from x4 to x2

I hope it's cristal clear now.
:biggrin:
 

darkhawk

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Thanks - I didn't think of checking the manual! :D

However, I do not have a card in PCI_E2. Hence my question! The table you copied seems to suggest that I cannot have PCI_E3 anyways as long as M2_2 is used, right? Regardless of PCI_E2.

I see there are 10 PCI Express lanes available on the B550 - and only 5 max on the slots (I guess the others are used for other devices such as the SATA controller etc.)

So the first question is: my RAID controller is a PCI Express 2.0 x8. How does that work in a PCI Express 3.0 x16 slot which only has 4 lanes available?

In other words, how many PCI Express 3.0 lanes would a single PCI Express 2.0 lane use?

Interesting to see that Intel seems to have way more lanes on their chipset... :(

Thanks again!
Obviously first see Remus's reply above but....

PCIe 2.0 and 3.0 don't really translate as you think. Even though a PCIe 3.0 x1 is the same as a PCIe 2.0 x2 in bandwidth, in the end, the CPU has limited lanes. Think of it like a highway. Even if 1 lane is much faster, you still only have '10 lanes', so in the end, that's what you get. Even if 2 lanes are twice as fast, if you use them at a slower speed, that's all you get.

Basically, as Remus explained, as soon as you populate M2_2, you will lose PCI_E3. PCI_2 only plays a part in how much bandwidth the M2_2 gets.

More or less, the B550 chipset probably doesn't have the setup you need in order to really use the hardware you want to use. An X570 chipset would have more lanes available and probably more slots available for M.2 SSD's and better suit your needs, apparently.
You'll basically have to make a decision, either use the M.2 SSD, or use the RAID card.
Or you could upgrade the motherboard and find one that would better suit the needs you have.
 

Tony359

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So ...
something in M2_2 ---> PCI_E3 not available
something in M2_2 + something in PCI_E2 ---> PCI_E3 not available + M2_2 downgraded from x4 to x2
nothing in M2_2 ---> PCI_E3 available at full speed
nothing in M2_2 + something in PCI_E2 ---> PCI_E3 downgraded from x4 to x2

I hope it's cristal clear now.
:biggrin:
Very clear - thank you a lot for taking the time to explain it to me so clearly!!

I understand that fully. I can live with that. I just never considered how important PCI Express lanes were - everything was SATA up to my previous system so I never really had an issue with lanes. Now that everything is M2 - and I have this RAID card - obviously things are different.

Thank you Darkhawk for explaining the lanes thing. Still, my RAID card says it's a x8 card. But it's on a x4 slot. Does that mean that it's "slowing" down to x4 - or that the manual is inaccurate? That's only curiosity as it's a 16 ports RAID card and I am only using 3 drives so I guess that x4 is PLENTY for that card anyways!

Thanks again!
 
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