Subject: Re: Breaker... Breaker...
To: John Refling <johnr@imageworks.com>
From: Greg A. Woods <woods@weird.com>
List: port-sparc
Date: 05/03/2002 21:11:12
[ On Friday, May 3, 2002 at 16:07:39 (-0700), John Refling wrote: ]
> Subject: Breaker... Breaker...
>
> It was my impression that there had been a sysctl
> call added which can be set from the command line (or
> in the system startup files) to disable the break
> dropping back to the ROM monitor.  There was definately
> discussion about doing this.

Any hack to disable BREAK detection is not the issue.  It is very often
the case that the special handling of an intentional BREAK is desired.

> I have sparc classics and ss5 and plug and unplug pc
> laptops into the serial ports while they are running
> with never an ill effect.

Try it on the older machines with a longer unshielded cable, and
disconnect/reconnect the laptop end....

> I can't imagine any analogue circuit which would
> reliablly "filter" out the break and pass the "desired"
> signals.

You can't filter all BREAK conditions since it is no more than simply a
normal logic-0 bit level that extends at least beyond the width of a
normal character time (eg. 10 bits wide), and most of us don't want to
filter intentional ones anyway, but with a simple pull-down resistor you
can prevent many of the conditions which are often detected as
accidental BREAK conditions.  It really does work, though it's more
reliable of course if it's installed inside the machine instead of in a
cable or dongle that can itself become accidentally disconnected.  It
also doesn't stop broken-by-design terminal devices from generating
real-looking break conditions when their power transitions -- if you
want to power-cycle your terminal device and it causes this problem then
unplug it first!

> I hope that the sysctl to disable the break action has
> been implemented... This will solve most of our issues.

It's only a bad hack and will do nothing for those who need or want the
ability to send and detect real BREAK conditions on the console port.

-- 
								Greg A. Woods

+1 416 218-0098;  <gwoods@acm.org>;  <g.a.woods@ieee.org>;  <woods@robohack.ca>
Planix, Inc. <woods@planix.com>; VE3TCP; Secrets of the Weird <woods@weird.com>