After 3 months of testing and about 17 years after my initial release for x86 PCs I'm glad to announce that I just released the first version of AtariSIO that officially supports the Raspberry Pi:
atariserver, an Atari 8-bit peripheral emulator, now works both with the RPi UART (connecting the SIO port of Atari 8-bit computers to RPi GPIO via 3.3V<->5V level shifters) and SIO2PC USB adapters.
atarixfer, a tool to read/write disk images from/to Atari 8-bit disk drives, also works both with GPIO UART connections (again, level shifter is needed) and 1050-2-PC USB interfaces.
Device tree overlays to enable the RTS and/or CTS lines of the RPi on-board UART on GPIO are included in the source distribution, details about the required GPIO connections can be found in README.RPi.
The source code is also available from GitHub https://github.com/HiassofT/AtariSIO but you'll need ATasm installed to build the 6502 assembler code. The tarballs on my website contain pre-assembled 6502 object files, so in general it's easier to use just these as you just need a few standard Debian/Raspbian packages to compile the source.
so long & have fun enjoying some nice Atari game served from your Raspberry Pi,