The experience gained by working on this project led to my cassette port - using project called CBM Tape Pi, please check it out!
Raspberry Pi and Commodore PET / CBM communication via GPIO and user port.
Current transfer speed (using ASM receiver on PET): Almost 60 bytes per second (limited by Linux!).
Working features (>v1.1.3):
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Transfer files (e.g. ASM applications) from Pi to PET's non-BASIC memory (e.g. to tape buffers) by using BASIC receiver application (see main.bas file). 
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Transfer BASIC PRG files from Pi to PET's BASIC memory by using C or Python sender and ASM receiver application (see main.asm file). 
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Autostart retrieved PRG files on PET. 
How to:
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Use (e.g.) CBM prg Studio to create PRG files from BASIC (main.bas) and ASM (main.asm) receiver applications. 
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Use C sender application on Pi (see main.c file, etc. - use Makefile to build). 
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As an alternative to C use Python sender application on Pi (see main.py file). 
Features to be implemented (will probably never happen, because there is CBM Tape Pi, now):
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Sending data (PRGs) from PET to Pi. 
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(Automatically) relocate ASM receiver application in PET memory "out of the way" to be able to store more than BASIC applications in memory during retrieval from Pi. 
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...