vt220 question

From: Jay West <jwest_at_classiccmp.org>
Date: Tue May 4 10:26:41 2004

John wrote...
> Well, two things. First, a 'real electrical "BREAK" signal' is not a
> signal per se, but a momentary interruption in the actual signal current
> flowing in the terminal circuit. This makes sense in a 'current loop'
> environment, such as DC-loop teletype circuits.

> Since I have it hooked up using the RS232
> port, and since there is no such thing as a real interrupting 'break' in
> RS232
yes there is.

> But it still doesn't tell me what the hardware does when pressing
> 'BREAK'. IF no one knows, I'll have to get out the break-out box and
> see if I can trap it.

A "BREAK" in RS232 is a "signal" at least in the sense that most receiving
hardware can detect it and do "special things" upon receipt. I am going from
foggy memory, but as I recall, a break is a long space or mark, longer than
any valid word format timing would normally indicate with no logic state
changes inbetween. So there is no "BREAK" "character" as it's not a
character with start & stop bits. I seem to remember that most UARTS (8250
is my experience) will detect a break condition and set a flag in the status
register to indicate receipt of a break. A bit of squirrelyness though, on
many terminals you can set the length (in ms) of the break signal to various
values as some hosts require the "long character" to be at least X ms.


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Received on Tue May 04 2004 - 10:26:41 BST

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