Subject: Re: what is a serial BREAK ?
To: Johnny Billquist <bqt@Update.UU.SE>
From: John Clark <>
List: netbsd-users
Date: 06/02/2005 12:31:46
Johnny Billquist wrote:

>On Thu, 2 Jun 2005, Robert Elz wrote:
>>    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).
>I don't think there is a specified time for a break character. And older
>hardware have no way to differ between a NUL with framing error, and a

I was not able to find a spec on how long break signal was on a Teletype 
33 or similar. But one
number I saw for a different system, gave 500ms as the time for a 'break 
signal'. The origin of
the break dates to the use of the telegraph, and detecting a 'broken 
wire' condition, or a 'break
in messages'.

A quick check on the ubiquitous 16450 serial chip used everywhere seems 
to indicate that
one of the control bits generates a 'break signal', and as long as that 
bit is asserted, a break
will be asserted on the serial line. A complementary status/interrupt 
bit is assocated with
detecting a break signal, which should be longer than a 'NULL' character 
at 110 Baud.

The more 'recent' use of 'break' has been used to synchronize modem 
speeds on ancient
modems, where the protocol was, 'hit the break key, until a reasonable 
message appears'...