Subject: Re: what is a serial BREAK ?
To: Johnny Billquist <bqt@Update.UU.SE>
From: Dan LaBell <>
List: netbsd-users
Date: 06/02/2005 18:52:28
On Jun 2, 2005, at 2:53 PM, 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
> break.
There was a post from '97 in alt.folklore.computers that described 
historical use
of ASCII codes for teletypes,  says 'NULL 00 ^@ is always ignored...' ( 
Subject was: OLD ASCII - FS GS RC etc? , is googlable ) so if a break 
is passed up to software as NULL it should be dropped.

HMM, then again, isn't the signal inverted on Rs232, +V is 0, -V is 1 ? 
It supposed to look like a cable break isn't? (historical telegraph 
antecedent )

Also from usenet:
>  "8250 trick", from
>  all the PCs that couldn't send break. Set baud rate to 34 baud, length
>  to 10 bits, send a 0xff (or was it a null), set baudrate back