Modern Homebrew "Cassette" Data Communication

[No True Scotsman]

That Bit Boffer circuit looks familiar. I think I saw XOR gates used in a similar way in a DIY audio meter. As I recall, the meter was used to determine if anything would go missing if a stereo recording were played back on a mono system.

Incidentally, the TRS-80 Model 100 portable bit-banged cassette and telephone modem from two serial pins on the 80C85 itself. If memory serves, the "transmitter" was simply a low-pass filter.

Here's a KCS implementation from Steve Ciarcia's book, BuildYour Own Z80 Computer.

Kansas City Standard (KCS) receiver

A KCS transmitter

The author warns that it takes a lot of tweaking to get a higher rate than 300bps from this circuit.

If it were me, I'd build the interface from a Raspberry Pi Pico or similar.