Over on his github blog, user stdw has uploaded a comprehensive post explaining how he investigated and turned an old Motorola MB7220 cable modem that was sitting in his closet into a fully functional software defined radio.
To begin the investigation stdw first opened the case and looked for a serial UART port. After finding one he connected the UART up to a Raspberry Pi and was almost immediately able to connect to the device's terminal. From the information displayed during the boot process, stdw was able to determine that the modem was running the eCos operating system on a Broadcom BCM3383 SoC. Unfortunately after receiving that information the UART connection is dropped, preventing any further terminal investigation.
To get around this issue, stdw decided to dump the flash memory via an SPI memory chip he saw on the board. Again using the Raspberry Pi he was able to connect via SPI and use the flashrom tool to read the memory. Next using a tool called bcm2-utils, stdw was able to parse and actually modify the configuration information stored in the flash memory. With this he was able to modify the configuration so that the serial connection did not drop after boot.
With terminal access gained, stdw was now able to reverse engineer the firmware, and after a lot of searching eventually find a console command which would perform a bandpower measurement for a given frequency range. He found that IQ data for this scan was stored in a buffer which he could then stream out via a TCP connection. With the IQ data finally available on another PC he was then able to use Python libraries to compute an FFT and actually visualize the scanned spectrum. Some further investigation yielded actually demodulated FM audio, and the realization that the usable bandwidth is 7.5 MHz.
Unfortunately there were some limitations. There is only enough RAM to store less than a second of data at a time at max bandwidth and precision, which meant that a lot of data needed to be dropped in between captures. Further investigation yielded methods to reduce the sample rate down to 464 kHz which meant that only 12% of data was ever dropped - enough to stream a wideband FM radio signal.
If you wanted to try investigating the modem yourself, the Motorola MB7220 is available second hand on eBay for prices ranging between US$15 - US$40, and new on Amazon for $46.99. Although the usability of the modem for any real SDR applications may not be great, further investigation may yield better results. And if not, following along with the process stdw took looks to be a great reverse engineering learning experience. Other modems that use similar Broadcom chips may also be worth investigating.