Subject: Re: what is a serial BREAK ?
To: Manuel Bouyer <email@example.com>
From: Johnny Billquist <bqt@Update.UU.SE>
Date: 06/02/2005 12:54:38
On Thu, 2 Jun 2005, Manuel Bouyer wrote:
> 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 Billquist || "I'm on a bus
|| on a psychedelic trip
email: firstname.lastname@example.org || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol