Oh, the nostalgia! I built an interface box to connect to the user port on my PET, and wrote a load of machine code software to send and receive radio teletype. Even used it on HF to do that.
I had the top-of-the-range dual floppy disk drive - £650 new! A wondrous machine. I've still got the PET, but the floppy drive I sold to the Royal Air Force some years ago - something they had still used them and they were finding it hard to locate spares.