In a previous post Serial Console on OS X I wrote about how to connect a USB-Serial converter to console into Cisco router and use the ‘screen’ command.
To perform a password recovery on a Cisco router you need to issue a “terminal break” character to interrupt the boot process. The Cisco page on terminal break shows a lot of break sequences.
The break sequence for “screen” is to use Ctrl-A and then Ctrl-B. The Ctrl-A shifts the focus back to the screen process (away from the serial console) and the Ctrl-B issues the break sequence.
You may need to break more than once. This is done by Ctrl-A, Ctrl-B, Ctrl-B, Ctrl-B.