AboutThe serial connection is probably the simplest way of flashing a fonera, and it is surely the only way if it is "bricked". This article explains how to build your own serial adapter for the fonera model 2100 and 2200.
Some more infoAll the fonera models have a serial connector, that can be used for accessing redboot directly, and to see the kernel logs when booting. It can also be used to have access to a Linux shell, just like with telnet/ssh. The fonera model 2100 has a 10 pin connector, but only 4 pins are actually used. The 2200 model has a 4 pin connector. Regardless of the model, one pin is ground, one is connected to the 3.3V voltage regulator of the fonera, and can be used to power some small circuit (probably no more than 15 or 20mA) and the last two pins are TXD and RXD, serial TX and RX lines. TXD and RXD lines are 0/3.3V logic levels, while a PC serial port is at +/-12V logic levels, so it is not possible to directly connect the fonera to a computer. The MAX3232 chip does level conversion, and can be powered with the 3.3V line that comes from the connector, so no external power is required for the level converter. The following image shows the circuit schematic, and connector pinouts for both fonera models.
Here is an image of the serial level converter
An image showing the connector and a fonera 2100
An image showing the connector and a fonera 2200
USBModern computer no longer have a serial port, so what to do? Luckily there are USB to serial converters that work well in this case. I made all the tests with the USB converter, and everything worked well.
Software partThis short guide shows how to access the redboot prompt from Ubuntu linux.
1) Connect the 10 pin cable to the fonera, and the other end of the cable to the computer:
With USB converter
Fonera<----(10 pin cable)---->Serial level converter<---->USB converter<----(USB cable)---->PC
Without USB converter
Fonera<----(10 pin cable)---->Serial level converte<----(Serial cable)---->PC
Do not yet connect the fonera power cable
2) Open a terminal and type
screen /dev/ttyUSB0 9600if you used the USB converter, otherwise type
screen /dev/ttyS0 9600screen is the program that creates a virtual terminal over serial port, /dev/tty* is the port name and 9600 is the port speed. The port speed must match the speed used by the fonera, otherwise you won't be able to connect. Redboot (the bootloader that starts every time you turn on the fonera) always uses 9600bit/sec, while DD-WRT uses 115200
3) Connect fonera power cable.
4) After some seconds you will see some hardware info, and then a message asking to type CTRL-C to activate redboot shell, so type CTRL-C.
5) RedBoot> Here is the prompt :-)