AMD B350 -A520 only one true PCIx16????

AlfredManner · May 24, 2022

Hi guys was building a few am4 systems and I realised that although some of these motherboards have two or three full PCIx16 slots, only the first one was fully connected, the other ones only had half pin, don't remember this even on my old i7 3770 or my i9 9900k motherboards. Why did these companies use the full size pici slot if they weren't going to use full lane?

--SID-- · May 24, 2022

None of the A320/520, B350/450/550 have a fully x16 wired 2nd or 3rd PCI-e slot because it get lanes from the chipset. The same with Intel. All B and H chipsets don't support a x16 wired 2nd and 3rd PCI-e slot. Even a lot of motherboards with Z chipset lacks that support.

AlfredManner · May 24, 2022

33 minutes ago, AlfredManner said:

Hi guys was building a few am4 systems and I realised that although some of these motherboards have two or three full PCIx16 slots, only the first one was fully connected, the other ones only had half pin, don't remember this even on my old i7 3770 or my i9 9900k motherboards. Why did these companies use the full size pici slot if they weren't going to use full lane?

But then why do they use the full slot for the second and third, why not just use 8x, 4x and 1x slots like the old days??

Maybe cheaper to just use x16 full slots? Like this b450 the specification says the other 2 slots are x8 but the used full slots.

RONOTHAN## · May 24, 2022

No consumer board in the past ~4 years has fully wired 2nd slots because there just isn't enough PCIe lanes to allow that to work. The most PCIe lanes a consumer CPU has is 20 lanes usable by the consumer, with 4 of those dedicated to the NVMe slot. For the boards that do have more than 1 PCIe x16 slot, the second slot either gets lanes from the chipset or it has PCIe and runs the first two x16 slots in x8/x8 mode.

Older boards might have had fully wired x16 slots if they were using a PLX chip, though PLX chips are very expensive so it was limited to just the top end of motherboards.

this isn't really an issue on HEDT setups like X299 and Threadripper because there's so many PCIe lanes that you can wire up everything no problem.

RONOTHAN## · May 24, 2022

7 minutes ago, AlfredManner said:

But then why do they use the full slot for the second and third, why not just use 8x, 4x and 1x slots like the old days??

Maybe cheaper to just use x16 full slots? Like this b450 the specification says the other 2 slots are x8 but the used full slots.

That's done for compatibility sake. You can run an x1 card in a PCIe x16 slot no problem, and you can run an x16 card on just 1 PCIe lane. Having the full length slot means it's easier to slot in a full length card that doesn't particularly need all its bandwidth. Another option for that is to have open backed slots (I know the Z690 Unify X has a open backed x4 slot for its bottom PCIe slot), but then you have the issue of not being able to clip in the card and you just have the second half of the GPU hanging out the back.

mariushm · May 24, 2022

1 hour ago, AlfredManner said:

Hi guys was building a few am4 systems and I realised that although some of these motherboards have two or three full PCIx16 slots, only the first one was fully connected, the other ones only had half pin, don't remember this even on my old i7 3770 or my i9 9900k motherboards. Why did these companies use the full size pici slot if they weren't going to use full lane?

The pci-e x16 slot closest to the CPU socket is connected to pci-e lanes provided by the CPU.

The other pci-e slots receive pci-e lanes from the chipset, which has a limited number of pci-e lanes.

The regular Ryzen processors have 24 pci-e lanes : 16 go to video card, 4 go to the M.2 connector closest to CPU and 4 lanes connect to the chipset.

Most APU processors (processors with integrated graphics) only have 8 (or 4) lanes available for that pci-e x16 slot, because a few pci-e lanes are used inside the CPU

On the high end chipset motherboards (x570, x470 etc) it is possible to split the 16 pci-e lanes from cpu into 2 groups of 8 pci-e lanes and that's how those motherboards have two pci-e x16 slots with 8 pci-e lanes.

The chipsets create only 6-12 pci-e lanes. Some of these pci-e lanes go to a second M.2 connector, other to pci-e x1 slots, and the pci-e x16 slot gets what is left, usually 4 lanes.

They use a pci-e x16 slot because it allows you to install bigger cards, like for example a SAS/SATA controller or a 2 x 10g ethernet card - those will work with less than 16 pci-e lanes or less than 8 pci-e lanes.