The bridge has a UART serial port interface that can be used to access the console. This serial port is the key to accessing the bridge and making our own changes.
The serial interface is a simple, three-pin port on the bridge. To access it, you have to remove the bridge's case. Looking at the top surface of the bridge circuit board (the side with all of the chips on it), you'll see the word Serial and a box stenciled around three holes to the right and above the Ethernet port.
The pinout for the serial port is, from left to right: RX, TX, Ground.
To use the serial port, I recommend you solder on some header pins so you can more easily attach jumper wire to the serial interface. I chose to put three male header pins on, and use M-F jumpers to connect to my USB-serial interface.
The serial port communicates at 115200, 8N1 (115,200bps, with 8 data bits, no parity bits, and 1 stop bit). This is a pretty common config.
The serial port on the bridge is a standard UART interface, which means you can hook it up to almost anything. But I wanted to hack the board, I so decided to connect it to my computer. There are a few things that I needed to do this: a serial interface, and a terminal emulator.
For the serial interface, I chose the common FTDI USB-serial interface, in particular a Sparkfun 3.3V FTDI Basic Breakout. You could select a different serial interface, but make sure it's 3.3V compliant, as that's what the bridge needs.
For the termial emulator, I'm using picocom. It's small, light and stable. If you're on a Mac and use homebrew (and if you don't, why not?!), you can install it with
$ brew install picocom
On Linux, you package manager will most likely have picocom. On Debian-style distros, you can do
$ sudo apt-get install picocom
On Windows, I would use PuTTY.
If you're using an FTDI breakout like the one I selected, you need to connect the pins on the bridge to the corresponding pins on the breakout. This is simple:
Bridge FTDI
------------------------
RX <-> TX
TX <-> RX
ground <-> ground
In case you've get Serial port dyslexia: don't forget that you need cross the streams! It's RX to TX and TX to RX.
Ok, once you've got that set up, plug the FTDI into a serial port on you computer. Fire up your terminal emulator and connect to the port. With picocom on my Mac, this looks like
$ pico --b 115200 /dev/cu.usbserial-AAAAAAA
You'll connect to the port, but not see much. At this point, you should connect the Ethernet and power to the bridge board. Once you do, check back with your serial emulator. You should see the board begin it's boot sequence. You're on your way to hacking the bridge!