Subject: Re: what is a serial BREAK ?
To: Johnny Billquist <bqt@Update.UU.SE>
From: Robert Elz <kre@munnari.OZ.AU>
List: netbsd-users
Date: 06/02/2005 20:28:32
    Date:        Thu, 2 Jun 2005 12:54:38 +0200 (CEST)
    From:        Johnny Billquist <bqt@Update.UU.SE>
    Message-ID:  <Pine.LNX.4.62.0506021249200.4692@Psilocybe.Update.UU.SE>

  | Once you get a
  | framing error, and the data is all zeroes, you have detected a break.

That's a risky assumption - some detection hardware/software distinguishes
between a framing error that just happens to occur on a nul character,
and a true break, which is generally required to be a lot longer (perhaps
100ms or so).

As others said though, for Manuel, any RS232/RS422 line driver chip
can generate breaks, it is just a long continuous sequence of 0 "bits"
(with no 1's at all).

Most uarts have a "send break" or "send long space" command that generates
a long sequence of 0's, so if you have a uart connected to the line driver,
it should just be programming.   Even if the uart is lacking this trivial
feature, the normal technique to simulate it is to switch to an absurdly
slow bit rate (50bps, maybe 110bps) and send a 0 character.   At the receiver,
still running at a rational rate, that usually looks just like a break
condition (even though 50bps is still only 20ms).