While checking my new Garmin 496 power/data cable, I put a scope on the
NEMA DATAOut line. Just like the EI instruments, the Garmin puts out
TTL (0V and 4V) logic levels instead of the normal RS232 voltage levels
(-5V / -12V to +5V /+12V)

Do all aircraft systems that communicate via "RS232" use the TTL
levels? How about if I drag my Labtop out to the computer to read some
diagnostics or upload a new database?