The Cable Modem To SDR Transformation

What do you do with an old cable modem in the closet? If you are [stdw] you reengineer it and turn it into a software defined radio. The modem in question was the Motorola MB7220. After looking at a similar project using a different modem, it seemed to be feasible.

The opening of the case revealed two possible UART ports, one of which was active. The result of this UART provided a lot of information. The chip was Broadcom BCM3383, which is a MIPS processor. It had eCos as the operating system. However, the bootloader eventually disabled the UART, so it was not possible to do many more investigations through the serial terminal.

The next step was to discard the flash drive. This required a small soldering operation to prevent the board from starting while the flash chip was powered. It turned out that some key credentials and configuration data are present, but they are really backups. After restoring the backup to factory settings, the correct data was visible.

After a lengthy study, the diagnostics, which builds a spectrum display, gave up its data. At first, the data was only a small sample of what was really required, but it showed a local FM station as spectrum. In the end, the rate of data loss is reduced to about 12% when streaming, which is not great, but it is good enough. You can hear audio clip on the reception. Not exactly crystal clear quality, but not bad.

Of course, no one will use this for FM radio. But this is a fascinating sight of how far you can penetrate such a device, if you have some skills and patience. There must be something in the quarantine that makes people hack old equipment, as we recently saw a similar Netkear hack. Even cheap games are not safe.