Accessing the serial console on a Cisco 7911 IP phone

Published on 8th Jun, 2018 by David Young

When configuring Asterisk and tftpd for use with physical phones it can be useful to have access to the serial console on the hardware to aid in debugging - in fact, the serial output can provide valuable information on the state of the hardware and how it is trying to talk to the Asterisk and tftp servers.

In my case, I had a few Cisco 7911 IP phones (which will be the subject of a later article if I ever get them working) and wanted to get them talking SIP rather than SCCP. Handily, whilst the phone does not have an external serial port, the PCB does have it broken out to some pads. I discovered this by looking first for header pins, and then for unpopulated pads that looked like they should have header pins. After probing the pads with a multimeter and FTDI breakout board, all was revealed.

Unfortunately, the pads on the board are at a smaller pitch than the standard 0.1" header pins I have spare, so some creativity was required to get proper access to the interface. By bending the bottom sections of the pins and mounting the connector diagonally it was possible to mount the 0.1" headers.

Not the prettiest, but it works

Header pins mounted diagonally to the board with some patch wires plugged in

The pins are labelled on the board as 1-4, identified to be Gnd, Rx, Tx, 3v3.

FTDI board plugged in and ready to give us some sweet debugging information

Then just hit the interface with screen and away you go.