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.

I havn't problems applying a long '0', as the RS232 line driver will be
connected to a microcontroller. The microcontroller's pin can be
configured either as UART TX or general I/O from software.

For the curious: I'm building a (small and low-cost) management board similar
to what can be found in some servers (e.g. Sun's ALOM) for ATX power supply,
which will allow, from the serial console, to control power and drive
a logical reset switch. I'll make the schematic available once tested :)

-- 
Manuel Bouyer <bouyer@antioche.eu.org>
     NetBSD: 26 ans d'experience feront toujours la difference
--