In message <20050603105139.GA4960@antioche.lip6.fr>, Manuel Bouyer writes:
>Hi,
>thanks to all who replied. So a serial break is just a long logical '0'.
>I wasn't sure because sun serial console gets a break when disconnected
>from the terminal or when the terminal is powered off, which lead me to
>think is was related to ground.
>

This actually goes back to where the word "break" comes from -- a line 
break on a teletype line....  As Johnny Billquist noted, it's somewhat 
arbitrary -- teletypes used a 20 milliamp "current loop", rather than 
RS-232 -- but it was certainly convenient to mimic the old behavior.

		--Steven M. Bellovin, http://www.cs.columbia.edu/~smb