Build a La Fonera serial cable

These days, the Fonera 2100 router of a friend was totally bricked. There was no way to get the IP of the router to contact it through HTTP, ssh nor telnet. There was also no way to reset or hard reset it (press the reset button, wait 10 seconds and unplug the power cable, then plug the power cable while still pressing the reset button, wait 45 seconds and release the reset button, wait for the WLAN led and press again the reset button during 10 seconds). The only solution to get it working again was to build a serial cable to access the RedBoot boot loader command line.

There are many solutions to build a serial cable. Actually the serial cable for a Fonera is just a TTL to RS232 (serial) converter which works at 3.3 Volts. Most of the schematics available are using a MAX232 or a MAX3232 circuit to do the job. I didn't have such circuits and I didn't want to buy and wait to receive one. But I found an other solution which uses NPN transistors, a diode and a bunch of resistors: Lefinnois's solution (and in French with more pictures). To build it you will need these components:

• 2x NPN 2N2222 transistors
• 4x 1K Ohm resistors
• 1x 2.2K Ohm resistor
• 1x 1n4148 diode

Here is the schematics found on Lefinnois's web-page:

Hopefully these components are very common and I have them. I just had to grab a breadboard and a D-Sub DE-9 male connector and put the components on the board. Here is the result:

(two big capacitors here)
. . . . v
. t r . g
(SDRAM chip here)

• v: Vcc 3.3 Volts
• g: Ground
• r: RxD
• t: TxD

I used a simple USB-to-serial converter to plug the cable on my notebook. In the serial terminal emulator I used a speed of 9600 baud. On OpenBSD I just had to type the following command:

# cu -s 9600 -l /dev/ttyU0

...and it just works!