Date   : Wed, 16 Jan 1985 17:47:12 GMT
From   : jeff <jeff%abnji.uucp@BRL-TGR.ARPA>
Subject: re: need a break key

[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!

                       +-----------------------------------------------+
                       |  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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