[121858] in North American Network Operators' Group
Re: Strange Cisco 6503 problem
daemon@ATHENA.MIT.EDU (Steven Bellovin)
Thu Jan 28 18:37:41 2010
From: Steven Bellovin <smb@cs.columbia.edu>
In-Reply-To: <4B621AA2.1060002@poggs.co.uk>
Date: Thu, 28 Jan 2010 18:36:55 -0500
To: Peter Hicks <peter.hicks@poggs.co.uk>
Cc: nanog@nanog.org
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
On Jan 28, 2010, at 6:15 PM, Peter Hicks wrote:
> Dean Belev wrote:
>=20
>> I'm curious if some of you faced such a problem - reboot of the =
router caused by the console connection.
>=20
> I once managed to send a BREAK signal to a 3640 by plugging in a =
console cable. At the time, it was a pretty key router in the network =
and sat at the rommon> prompt :)
>=20
> I had that down to static somewhere, as it's the only explanation I =
could find.
Actually, it's not at all surprising, but it depends on the UART or =
equivalent.
A serial line has two states, "mark" -- a 1-bit -- and "space" (guess). =
Normally, the line is at "mark", which corresponds to a voltage of =
-3V:-25V at the receiver. Space is +3V:+25V; -3V:+3V is undefined. =
(http://www.lammertbies.nl/comm/info/RS-232_specs.html is pretty good, =
and as far as I remember quite accurate, though it's ~20 years since I =
used a breakout box.)
Now -- a break signal is normally a "long space", a 0 signal that lasts =
too long, often about .25 seconds. It originally got the name because =
it looked like a break in the teletype line; teletypes used a current =
loop standard (don't ask). More precisely -- an asynchronous byte is =
followed by a "stop bit", which isn't so much a bit as a time interval =
-- one bit-time -- during which the signal must be in the mark state. =
If you're sending at the wrong speed or send break -- something that's =
holding the line at space for long enough that it will run into the stop =
bits at any speed -- the UART will detect the problem; this is sometimes =
known as a "framing error".
So -- when you disconnect the cable, the voltage at the pin goes to 0. =
How should that be interpreted? If the board has a pull-up resistor to =
a +5V line, it will appear as a space signal; if it doesn't, it's up to =
the UART or equivalent, since it's undefined by the spec. =20
--Steve Bellovin, http://www.cs.columbia.edu/~smb
