Subject: Re: SparcStation ipx versus Sparc Classic
To: Don Yuniskis <auryn@GCI-Net.com>
From: Greg A. Woods <woods@weird.com>
List: port-sparc
Date: 05/02/2002 21:38:47
[ On Thursday, May 2, 2002 at 16:37:41 (-0700), Don Yuniskis wrote: ]
> Subject: Re: SparcStation ipx versus Sparc Classic
>
> And, if you think about it, this solution DOESN'T WORK!

Well, it does exactly what it was designed to do, nothing more, and
nothing less....

> It prevents an "open" input from glitching to SPACE and, thus, making the BREAK.
> However, the output impedance of (classic) EIA232 drivers is ~600 ohms.
> So, if the external device *drives* the line to SPACE, the resistor may as well be
> sitting in your desk drawer... it isn't going to *do* anything!

Of course!  This modification is not designed to prevent an
_intentional_ BREAK condition from being generated, and as a result it
cannot prevent an accidental BREAK condition that is electrically
identical to a real BREAK condition.

(the document I quote does claim it will prevent accidental BREAK
conditions when a poorly designed terminal device is powered off or on,
and obviously it can only do that in some cases, though from what I've
seen of such problems these happen to be quite common cases....)

> So, you have to unplug the cable before applying power to the external device
> as well as before removing power from said device -- since in either case it
> can "decide" to drive the line to SPACE while it is in transition.

And you have a problem with that?  You stated your problem was
accidentally halting your systems when you unplugged the console cable.
I.e. that you want to prevent an "open" input from generating an
accidental BREAK condition.  As you admit this fix will prevent that
problem and that's exactly what it was designed to do.

If on the other hand you have a terminal device that drives a
real-looking BREAK condition when its power source is transitioned, then
just don't turn off your terminal device when it's plugged in!

(Personally I think I've only ever encountered problems when
re-connecting a powered up terminal to the console port, and that's a
lot less troublesome since it's easy enough to immediately start the
system running again.)

If your terminal device forces a BREAK condition even despite this
counter-measure when you transition its power source then _you_ can
learn not to do that.  Meanwhile this "fix" will allow you to safely
disconnect your terminal device and move it to another system without
risking halting the system you've just been working on, and yet still
allow you to generate an intentional BREAK condition to manually halt a
system on demand.

-- 
								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>