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From: jeff@abnji.UUCP (jeff)
Newsgroups: net.micro.cpm
Subject: re: need a break key
Message-ID: <182@abnji.UUCP>
Date: Wed, 16-Jan-85 12:47:12 EST
Article-I.D.: abnji.182
Posted: Wed Jan 16 12:47:12 1985
Date-Received: Fri, 18-Jan-85 02:17:30 EST
Organization: ATTIS, NJ
Lines: 46

[I'm new to USENET,so please be kind]

	Causing a BREAK without a break key is tricky since BREAK is not
a character but a line condition.  An active RS232 line (transmit or
receive data) is normally in the MARKING state (quiescent state)
(binary 1, voltage between -3 and -25 volts).  When a character is sent,
the line sends a start bit (goes SPACE, binary 0, voltage between +3 and
+25 volts), then the data bits (and parity if enabled), then a stop
bit (back to MARKING).

	A framing error is caused when the end bit isn't received
when it is expected, usually suggesting mismatched speeds.

	A break condition is when the line is put in the SPACE state
for the time it takes to transmit a character (including start and
stop bits) and thus cannot be confused with a character transmission.
Some serial communication chips will detect the start and end of
a break (since it may last a looooong time) and interrupt on both.
Others just give a framing error with 00h data.

	The UNIX Administrator's Manual under TERMIO(7) under IGNBRK
states that a break condition is a framing error with data 00h.  Non-null
data causing a framing error is considered a framing error.

	A break key causes the line to go spacing for about 1.1 to 2x
a character time length (usually a long time - around 200 milliseconds).

	Your trick of going to a slower speed may work if the software
treats framing error as a break (regardless if the data is 00h or not).
If they are treated differently, then you must transmit a NULL
character, usually control @.  This will be undistinguishable from a
break to all but the fussiest of receivers.  This also points out
that a framing error on a null character will be misinterpreted
as a break.

	I trust this definitively answers your question.  Ask anything
more technical and I will refer to the EIA Standard for RS-232-C
"Interface Between Data Terminal Equipment and Data Communication
Equipment Employing Serial Binary Data Interchange". So there!

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			|  Jeff 'oh my gawd - it's one of THOSE!' Skot	|
			|     at beautiful downtown Somerset NJ		|
			| 	      AT&T Info Systems			|
			|  		..!abnji!jeff			|
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