Re: what is a serial BREAK ?

On Thu, 2 Jun 2005, Manuel Bouyer wrote:

> Hi,
> not strictly NetBSD-related (although my primary concern is to be able
> to enter NetBSD's ddb from the system I'm designing :) but I know there are
> some knowlegeable peoples here.
>
> What is, from an electrical POW, a BREAK on a serial line ?
> A long period at high voltage level, a long period at the low voltage
> level, or a long period at ground ?
>
> I plan to use a MAX232 to convert the TTL signal to RS232 levels,
> and I don't know if I can generate a BREAK by driving the
> MAX232 input, or if I need some extra circuitery.

I assume you know how normal asynch characters look on the line.
You don't need any extra circutry. It's a long logical zero. It's like if
you were to send a nul character, but instead of getting the stop bit at
the end (which always is a 1) you continue to transmit 0.

So electrically, nothing different from normal levels. Protocol wise, it's
a framing error. A break can be as long as you care to. Once you get a
framing error, and the data is all zeroes, you have detected a break.

Strictly electrical, it's a positive signal. The exact level is rather
free in RS-232 (I think the spec says something like 5-15V. Positive for a
zero, and negative for a one)

	Johnny

Johnny Billquist                  || "I'm on a bus
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