On Fri, Jun 03, 2005 at 08:30:20AM -0400, Steven M. Bellovin wrote:
> In message <20050603105139.GA4960@antioche.lip6.fr>, Manuel Bouyer writes:
> >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.

Many RS232 receivers have a threshold level of +0.3 V or so and so
0 V is interpreted the same as -12 V, which the UART sees as a logical
one - so power off / broken cable is NOT seen as a break condition.

But it depends on the receiver chip, so you can't rely on this, unless
you are making the hardware.

  -jm