This is an FT230X-based USB-UART adapter with built-in level shifting. This allows talking to targets using different I/O voltage levels (1.8V to 5V), but it's especially useful to avoid backpowering a target through the TX line.
This board can be assembled by JLCPCB PCBA for about $5 each, see the fab/
directory for Gerber/BOM/CPL data.
The I/Os on the right-angle header (J1) are:
->(output, UART TX from the perspective of the adapter)<-(input, RX)G(GND)RTS(output)CTS(input)
The pins are arranged such that a 4-pin receptacle on the target is sufficient when designing specifically for interfacing with this adapter if flow control is not needed.
Level-shifting is configured through the 4-pin header (J2). When the I/O
reference voltage is available from the target it is recommended to connect it
to Vt and leave J2 open.
Alternatively, you can connect a jumper between 3.3 and Vt or 5 and Vt
on J2 to set the I/O voltage to 3.3V or 5V (strictly speaking USB VBUS),
respectively. However, in this case we cannot detect an unpowered target and
the usual backpowering considerations will apply. You can also just connect a
lab power supply to Vt and G.