[MarkusWandel]

No need to junk a perfectly good Arduino for that. Just load a trivial sketch that disables the Arduino's TX pin and just access the serial link through the D0 (RX0) and D1 (TX0) pins. Reversed of course; what's RX to the Arduino is TX to the serial chip.

[fstokesman]

This is the most use that board has and would have gotten in the last 10+ years. I like to think it laying down its FTDI chip for a greater cause was an honour for it.

[stereo]

My go-to serial adapter is an esp8266 dev board with EN connected to GND to stop the chip itself from booting. The TX and RX pins are then conveniently printed on the board - I just have to remember that, in this case, TX goes to TX and RX to RX, because it shows what's usually the chip's side of the dialog.

[argulane]

You don't even have to load a sketch to the Arduino. Just connect the RESET pin to GND. That will keep the Atmega IC on stop state and the TX/RX pins are free for what ever you need them to use for.