Only one genuine PCIx16 slot is available for the AMD B350-A520.
Only one genuine PCIx16 slot is available for the AMD B350-A520.
People were designing some AM4 systems and noticed that while some boards had two or three PCIx16 slots, only the first one was fully connected, the others only had half-pin connections. I don’t recall this happening even on my old i7 3770 or i9 9900k motherboards. Why did manufacturers choose the larger PCIe slot when they weren’t using full lanes?
None of the A320/520, B350/450/550 models feature a fully x16 wired second or third PCI-e port since the lanes are managed by the chipset. The same applies to Intel; all B and H chipsets lack a dedicated x16 wired slot. Even many motherboards with Z chipsets don’t offer this capability.
However, they allocate the entire slot for the second and third positions, rather than using smaller configurations like 8x, 4x and 1x as before. It might be more economical to opt for a single x16 full slot instead. The B450 specification indicates that the remaining two slots are actually x8, but in practice, they’re filled with larger units.
In recent years, few consumer boards have completely wired the second PCIe slot due to insufficient PCIe lanes. Most consumer CPUs support up to 20 usable lanes, with four reserved for NVMe. Boards featuring more than one x16 slot typically allocate lanes from the chipset or operate in x8/x8 mode. Earlier models might have had fully wired x16 slots using PLX chips, but these are costly and limited to premium systems. This challenge is largely avoided in high-end HEDT configurations like X299 and Threadripper, where ample lanes allow full customization.
This setup supports running an x1 card in a PCIe x16 port without issues, and you can also use an x16 card with only one PCIe lane. Using a full-length slot makes it simpler to fit longer cards that don’t need maximum bandwidth. Another choice is open-backed slots, like the Z690 Unify X’s bottom PCIe slot, though this means you can’t fully clip the GPU and the second half remains exposed at the back.
The PCI-E X16 port nearest the CPU socket is linked to PCI-E tracks supplied by the CPU. The remaining PCI-E ports are provided by the chipset, which has a restricted number of these tracks. Standard Ryzen chipsets offer 24 PCI-E connections: 16 for the graphics card, 4 for the M.2 slot near the CPU, and 4 for the chipset itself. Most APU designs (those with built-in graphics) have only 8 or 4 lanes available in that X16 port since some are reserved inside the CPU. On high-end chipset boards (like x570, x470), it's feasible to divide the 16 PCI-E tracks into two sets of eight, resulting in two X16 slots with eight lanes each. The chipsets generate only 6 to 12 PCI-E connections. Certain PCI-E tracks connect to a second M.2 port, others to PCI-E X1 slots, and the X16 slot receives whatever remains, typically four lanes. This configuration enables use of larger devices such as SAS/SATA controllers or dual 10G Ethernet cards that require fewer than 16 or 8 PCI-E connections.