The Cable Modem To SDR Transformation

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

Cracking open the case revealed two likely UART ports, one of which was active. The output from that UART provided a lot of info. The chip was a Broadcom BCM3383 which is a MIPS processor. It had eCos as an operating system. However, the bootloader eventually disables the UART, so there wasn’t much more investigation possible via the serial terminal.

The next step was to dump the flash memory. That required a little solder surgery to prevent the board from starting while the flash chip had power. It appeared that some key credentials and configuration data were present, but they were really backups. After doing a factory reset to remove the backups, the right data was apparent.

After some lengthy exploration, the diagnostic that builds a spectrum display gave up its data. At first, the data was just a small sample of what was really required, but it did show a local FM station as a spectrum. Eventually, the data loss rate was down to about 12% when streaming which is not great, but good enough. You can hear an audio clip of the reception. Not exactly crystal-clear quality, but not bad.

Of course, no one will use this for an FM radio. But it is a fascinating view into how far you can hack into a device like this if you have some skills and patience. There must be something about quarantine that is making people hack old gear, as we just recently saw a similar Netgear hack. Even cheap games aren’t safe.

5 thoughts on “The Cable Modem To SDR Transformation

  1. Wasn’t the device an SDR to begin with? If the hardware is there, then what’s really happening here is that someone’s trying to change the software to something they want, or one might imagine something more universal.

    An SDR is defined by a certain collection of hardware, and the software defines what can be done with it. So sometimes it means some specific use device, that can’t be modified much. It may mean those now cheap shortwave portables that offer no way to customize, but become cheap because it moves away from analogy processing. Or it could mean very general hardware that allows general software to tell it what to do.

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