From Intellivision Wiki
The PlayCable is a peripheral produced by Gerrold that was designed to work with a special cable-TV station. The PlayCable allows customers to choose games to play from a rotating list maintained by the cable operator. The unit itself consists of a tuner, a specialized modem, and some RAM to which games can be downloaded.
Units were generally leased by cable operators to subscribers. Therefore, few PlayCable units exist in the wild.
The PlayCable boot-ROM was mapped at location $4800 in the Intellivision memory map. This is why the Master Component examines this location early in its boot sequence.