Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!uw-beaver!rice!sun-spots-request
From: auspex!guy@uunet.uu.net (Guy Harris)
Newsgroups: comp.sys.sun
Subject: Re: Sun 3 console question
Keywords: Hardware
Message-ID: <1142@auspex.UUCP>
Date: 24 Mar 89 00:11:11 GMT
References: <8901202003.AA04397@frith.egr.msu.edu> <20329@lll-winken.LLNL.GOV> <10452@bcsaic.UUCP>
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Organization: Auspex Systems, Santa Clara
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Original-Date: 10 Mar 89 19:43:18 GMT
X-Sun-Spots-Digest: Volume 7, Issue 210, message 1 of 14

>I believe the circuit that Carl referred to is a kluge to prevent DTR from
>dropping when you unplug (or power off) the console terminal.

No, it's a kludge to prevent RxD from dropping and coming back up when you
unplug the console terminal and plug it back in again.  If RxD drops, it
looks as if the machine on the other end of the wire is sending a 0 bit.
If a zero bit is sent for more than some amount of time (200 ms is a
figure I've heard), that's a "break condition" - that's what a "Break" key
on a terminal generally produces.

>Without such a circuit, unplugging the console results in a watchdog
>reset.

I've rarely, if ever, seen it result in that.  It results in the CPU
serial port chip detecting what appears to be a "break condition" on the
console port, and the kernel recognizing that this was, in fact, what
happened on the console port and dutifully calling the PROM monitor.
Typing the "c" (for "c"ontinue) command to the PROM monitor causes it to
return back to the kernel.

Watchdog resets are a different kettle of worms altogether; you can't just
"c" and pick up from where you left off.