Thanks Mark,

Thats a good place to start. Is the AC43.13 documentation available
for download (PDF or otherwise) online?

Are the undocumented features of "cool" variety?

Not really. More like stuff you can only learn by experimenting because the
manual isn't detailed enough.

What language do you write your software in?

Asm, C, C++, Ada, VB, and others, but the language is mostly irrelevant


Whats the difference between arnic 429 and serial? Arnic digital and
serial not? Or is it speed?

Arinc 429 and RS-232 are both serial data in the sense that data bits are
transmitted and received one at a time sequentially. That's where the
similarity ends.

You'll have to go google and study on your own from here.