Let me jot this down before I forget it again, I’ve looked this up so often 😉
First we hook up the CON port of the borked device with the AUX port of a working router (the normal Cisco console cable with RJ45 connectors on both ends). Then we go to the router and configure it’s AUX port with these settings:

# conf t

# line aux 0

(config-line)# modem InOut

(config-line)# transport preferred all

(config-line)# transport input all

(config-line)# transport output all

(config-line)# stopbits 1

(config-line)# end

The router has to have a loopback address and it must be up (duh). Now we have to find out which port we are going to have to telnet to. Do a show line from the enable mode, the output should look something like this:

Tty Typ Tx/Rx A Modem

0 CTY – –

* 97 AUX 9600/9600 – inout

* 98 VTY – –

Our line is number 97, telnet to the loopback address and port 2000+our line. If our loopback had the IP 192.168.100.200 the command would be telnet 192.168.100.200 2097