A simple serial modem with an RS323 interface built from an ESP01 module.
Here you will find a schematic that uses an ESP01 module and a a Sipex/Exar SP3232EEP-L -- which is
available from Mouser pretty inexpensively and only requires a few external capacitors to run the
charge pumps and so on -- to build an wireless modem for an old computer system with an RS323 port.
Any other level converter (especially the MAX232, which may in some cases be a drop-in substitute
should also be ok. The ESP01 can (and should) probably should be programmed with Bo Zimmerman's
excellent C64Net firmware, which allows the ESP to support some lower baud rates and handles actual
telnet sessions and maybe a few other nice features. You can find his work here:
https://github.com/bozimmerman/Zimodem
So far I have built exactly one of these. Photographs in the repository are of that board, and the schematic should represent its layout reasonably well. It does work reasonably well. Because it's based on the ESP01, it doesn't really handle hardware flow control.