Peter wrote:
TTL (0V and 4V) logic levels instead of the normal RS232 voltage levels
(-5V / -12V to +5V /+12V)

No idea. There are some suspect practices in avionics though, like
wiring multiple receivers to a single RS232 transmitter. They should
be using RS422 for that.

RS-232 is being BADLY abused these days. The spec only requires more
than 3 Vdc or less than -3 Vdc, and a lot of modern laptops settle on
3.3 as "okay." It's rare to find one that actually puts out +/- 12 any
more.

I too have "cheated" for quick and dirty interfaces. TTL out will
usually drive an RS-232 input (although not always). Inputs? Put a
couple of protection diodes and a resistor in front of a CMOS gate and
you can receive RS-232 without any special stuff - but you give up half
your noise immunity.

I agree - for aircraft differential RS-422 is much better (or ARINC
429).

jmk