The USB cable from the PCIE riser cards can plug directly into these “USB” ports on the motherboard, thus saving space and improve stability.
ASUS rolled out one of its first crypto-currency miner-friendly motherboards based on the Intel 300-series chipset platform, now that cheaper Pentium Gold and Celeron processors for this platform are available. The H370 Mining Master puts out all 20 PCI-Express gen 3.0 lanes of the H370 Express chipset out as x1 slots. It does this in a space-saving way – wiring out each PCIe “port” as USB 3.0 physically (using its 9 pins), which you wire out using USB 3.1 type-A male-to-male cables into open-ended PCIe x1 risers. You can plug in 20 graphics cards over risers, besides a 21st card on the board’s PCI-Express 3.0 x16 slot.
The rest of the board is pretty spartan, which is what miners need. The LGA1151 socket is powered by a simple 4+2 phase VRM. Power is drawn from three 24-pin ATX, and one 8-pin EPS. The CPU socket is wired to two DDR4 DIMM slots, supporting up to 32 GB of dual-channel memory, the x16 slot, and the H370 Express chipset. Storage connectivity includes just two SATA 6 Gbps ports. You get six USB 3.0 ports (four on the rear panel, two via headers). Display outputs include DVI and HDMI. Legacy connectivity includes PS/2 combo, and one serial COM header. 6-channel HD audio, and one 1 GbE interface, driven by an Intel i219-V, makes for the rest of it. ASUS threw in onboard power/reset buttons, and POST debug display to sweeten things.