I suggest one gets their SD card reader and LCD display working independently prior to hooking up to Arduino up to the Atari. The software works fine without an Atari present - of course it will dump error messages to the serial monitor. All the connections are described on the original SIO2Arduino webpage. I posted my activities on the Antic Podcast Facebook Page and Michael Glaser of the Atari XEGS Cart-by-Cart Podcast joined in with his experience. It was nice to have a partner in crime. He made this nice pin-to-pin connection cheat sheet for hooking up the Mega 2560.
The idea of attaching the Arduino to my Atari made me pretty nervous. The last thing I wanted to do was blow the I/O pins on the POKEY. The creator of SIO2Arduino points out the standard colors of the Atari Data In (orange) and Data Out (green) wires in the SIO cable in his hookup guide. Fortunately, one of the SIO cables I have has machine screws instead of rivets attaching the connector back shells. Removing half of the back shell let me double check my connections. I found that the pins on the jumper wires from the Sunfounder Arduino clone kits are a nice fit for the sprung sockets in an SIO plug. Here's a photo of these connections.
One of the peculiar behaviors I found was directories need to have less than 256 files. SDrive can scroll thru them, but SIO2Arduino will mod-256 the file number and you won't load the file you intended.
I am an avid fan of the BASIC 10-Liner Game Competition. As Kevin Savetz says, "It's the most wonderful time of the year." At last, I can test my own programs on real hardware - after developing them with modern tools, of course. The first thing I tried was mounting and booting TurboBASIC XL. That was successful. I could even write a short program ("Hello World" anyone?) and save it to the disk image. Sweet! Then I played a couple rounds of my ersatz Interceptor - pretty nice on real hardware.
If you have a few minutes more to waste, check out my video of it in action.
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