Lane Switching Considerations
The X370 chipset offers the ability to run the CPU's x16 PCI-E lanes as x8/x8 for multi-GPU setups. The B350 chipset only allows one x16 slot from the CPU. SLI support will differ depending on vendor and product, but you should see it in the manual or the marketing as it's not free to certify a motherboard for SLI.
Now that we understand that those two first x16 slots will turn into x8/x8 with two cards installed, it's important to realize where the last x16 slot and the x1 slots get their bandwidth. A quick way to see if PCI-E lanes are coming from the chipset or the CPU is to see if they are PCI-E 3.0 or PCI-E 2.0. The CPU only provides PCI-E 3.0 while the chipset only provides PCI-E 2.0.
The X370 offers eight lanes while B350 offers six, and in the case of the pictured motherboard, the last slot is x4 PCI-E 2.0 wired to the X370 chipset. The x1 slots each share their bandwidth with the x4 slot. That last x4 from the chipset might also be shared between a second M.2 slot and an x4 PCI-E slot in the form of an x16 slot. The only way to know is to read the footnotes in the specifications or manual.
Here you can see a breakout tree diagram on the left and another on the right. We can see on both motherboards the x4 slot shares all of its bandwidth with the x1 slots. We can also see what devices come from what buses or complexes. Diagrams like these can be found in reviews or manuals of motherboards, but not many motherboard vendors off the diagrams. You can read the fine print on the specification page of each product for where the lanes are shared, and you should read the fine print if you are going to use most of the slots and ports at the same time.
Fan Control Considerations
Fan headers on most motherboards are now 4-pin all around. However, not all 4-pin headers are made equal. We have both 3-pin (called voltage or DC mode) or 4-pin (PWM) headers. PWM fans are fed a constant 12v and are regulated by the 4th pin that sends a signal to the regulator on the fan. Voltage/DC mode fans rely on the motherboard to regulate their voltage. Both report RPMs back to the motherboard through their third pin.
While the header on the right and left look identical on a motherboard, you need to dig deeper into the manual to see if they support only PWM, only DC, or a hybrid. We can see on the left that the power pin is just +12v, meaning if you plug a voltage/DC mode fan into the header it will run 100%. On the picture from another motherboard's manual on the right, we can see that it's a hybrid PWM/DC header and can support either type of fan with the proper control for either. The manual is the best place to look for fan information.
The manual is the best place to look if you have any questions about an AM4 motherboard. As this guide was meant to clarify different platform features and provide an insight into the ICs used and their performance impact, I hope you learned something. If you have any questions or comments, please email me or leave a comment.
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