Rockwell famously used 6502-based cores in modems for many years, but that doesn’t limit the use of other 6502s. John Iannetta’s “35-cent modem” from 1998 used the SID chip to modulate data through a C64’s sound output to a receiving modem. Recently, the author revisited this project using a Commodore SX-64 and a WorkSlate. Despite little personal information known about John, his programming contributions to the Commodore community were vast. The “35-cent modem” project is a simple build involving replacing telephone tip and ring connections with an RCA phono jack to modulate audio frequencies for data transmission. Technical details of the program involving FSK modulation and baud rates are discussed in depth, showcasing a unique and clever DIY modem project.
http://oldvcr.blogspot.com/2025/01/the-35-cent-commodore-64-softmodem.html