Why? Have you never done serial? You just need 3 wires: TXD RXD and GND which corresponds to pins 2, 3 and 5 in a standard db-9 plug. You might need to swap TXD and RXD if you got it wrong. Don't ask me about usb because that's a whole new can of worms.



To clarify in case you somehow got it wrong: You can use either port, the rj45 or the 3 pin header for serial communications. "Both" should work... Using an rj45 for serial communications is old. The port knows when you plug this type of cable instead of Ethernet in devices with serial, there is nothing special about this. But in addition there happens to be a 3 pin header that appears to be the same. Just ignore the 3 pin header if you don't get it.

RJ-45 Pin	Signal	DB-9 Pin	Signal
1	RTS	8	CTS
2	DTR	6	DSR
3	TXD	2	RXD
4	GND	5	GND
6	RXD	3	TXD
7	DSR	4	DTR
8	CTS	7	RTS

https://www.juniper.net/documentation/en_US/release-independent/junos/topics/reference/specifications/port-rj45-db9-adapter-pinout.html

You could wire them all if you are bothered with hardware control, but my made by Cisco version cable didn't bother. I don't think they use it (cts/rts) anyway, or the data ready pins.