Avionic buses

RS232 is a very well adopted standard globally. Parts are inexpensive and you find loads of engineers with experience.
Arinc 429 is a special purpose bus where parts are much more expensive and the engineers that are really experienced can be carried by one bus (globally).

Therefore the question should rather be: Which applications are so critical that they really require Airinc and can not be done by RS232.

I’ve asked myself the same question. E.g. the Garmin G5 has CAN-bus and RS232 interfaces. To interface a G5 configured as HSI to Garmin’s own GPS navigators, you also need a separate CAN-bus/ARINC gateway (the Garmin GAD29B) as a HSI G5 needs both RS232 and ARINC connections to the navigator!?

The answer to this depends on your background and what exactly you need to learn.

To start with the pompous version, there is the ISO 7 layer networking model

At the bottom of that is the physical layer. Or if you like, the “interface”.

Things like RS232 RS422 RS485 are interfaces; just the physical layer. The data flowing through these can mean anything but in most cases is fairly easy to deal with. These are cheap to implement; under say €0.50. These go back to the 1960s/1970s.

CAN is a bit of several things; it specs the physical layer (which is a bit like RS485) and some of the stuff above that, but most implementations are proprietary so almost nothing “just works together” unless specifically intended to. It costs a bit more than the above to implement, but much more in software terms. CAN was originally going to take over the universe but ended up used just in cars. I guess anyone who tried to read the spec grew a beard while doing that… a bit like USB but nobody actually implements USB; you always buy a chip and use free drivers for it and if that doesn’t work you try another chip and another set of free drivers.

ARINC429 is a very old avionics bus and is a physical layer (and is unlike anything else; reception can be done with a software UART but transmission is messy and is usually done with dedicated chips which are expensive) but also extends into packet definition. Compatibility is ok in some application areas, poor in others where you would expect it to work, probably due to crap software. Cost is of the order of €10-€50 per channel; can be done for less. I have just done a design which has some ARINC429 interfaces and the chip cost $100.

A lot of the G1000 stuff AFAIK uses ethernet, and the protocol is totally Garmin-proprietary. Normal ethernet is very popular for networking and works well, with excellent compatibility. A lot of software though to actually get things to talk to each other if not using a common operating system to do all the dirty work.

Of all these interfaces, only ethernet is isolated and thus really robust. The others are usually 2 wires plus a ground.

When developing a product, you choose the interfaces mostly according to what you want it to talk to / the intended market.

@wigglyamp, if he’s still around these days, can post a history of this stuff in aviation, and a lot more detail.