Subject: Re: what is a serial BREAK ?
To: Manuel Bouyer <>
From: Johnny Billquist <bqt@Update.UU.SE>
List: netbsd-users
Date: 06/03/2005 12:57:35
On Fri, 3 Jun 2005, Manuel Bouyer wrote:

> 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.

Well, when you disconnect the terminal, the signal is actually free
floating. Power off (usually) leads to ground. Neither is actually a valid
voltage on the RS-232. The problem is that all RS-232 converters just
have a TTL interface on the other side, so ground have to be mapped to
'0' or '1'. If you map it to '0', you will get something that looks like
an infinite break.

> 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 :)

Fun. Shouldn't be that complicated, I guess.


Johnny Billquist                  || "I'm on a bus
                                  ||  on a psychedelic trip
email:           ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol