You didn't even need Caetla. You could do that straight-up with the original ROMs for some of the cartridge series (FCD), and X-Link from DOS (or, if you were prepared to get involved with a bit of spicy linux 2.3, bitbang the parallel port yourself from the /dev/ interface for it at the time, the protocol was really simple). You get a live memory monitor you can watch (made it really easy to make cheat codes or look inside stuff for fun), and you can write and debug anything you want - in many ways it was nicer than the official Psy-Q kit Sony adopted, I thought.
Except to be fair, I didn't have a C toolchain, but R3000 assembly language is really nice when you get used to the delay slot.
i guess you'd discover that protocol by playing with a yaroze/caetla/etc with some of of device listening in between and recording the words?
never really got into using the parallel port as a cheap dio until building science rigs years later... (and some fun experiments with using it to do cheap adc when in college)
Except to be fair, I didn't have a C toolchain, but R3000 assembly language is really nice when you get used to the delay slot.