Strange Cisco 6503 problem

Steven Bellovin smb at
Thu Jan 28 23:36:55 UTC 2010

On Jan 28, 2010, at 6:15 PM, Peter Hicks wrote:

> Dean Belev wrote:
>> I'm curious if some of you faced such a problem - reboot of the router caused by the console connection.
> 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 :)
> 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.  ( 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.  

		--Steve Bellovin,

More information about the NANOG mailing list