Back in 2009, a cisco 876W console port had failed.
The symptom
The router when connected to a serial terminal (PC with a terminal emulator) reported boot-up messages but had no response to any key-stroke.
The problem
No ROM monitor mode and no possibility to reset a forgotten password.
Possible cause of failure
… seemed to be an accidentally connection of console port to the ISDN line.
Fault finding and Repairing
The router would had one or more ICs as RS-232 line drivers/receivers and some of them would failed.
- The router opened.
- Top cover removed.
- Metal screen removed also.
- At the back right corner is located a Maxim MAX3243E RS-232 Transceiver
- After tracing the PCB connections to the console port founded the SMD resistor R133 of 220 Ohms to be “open-circuit”
- Part of the schematic as traced manually
- Replaced with an identical resistor and console port is functioning again.
