Digital Converter (over-the-air) to Digital Cable Converter

Hi,

I recently bought an "Access HD" model DTA1010D digital to analog
converter for over-the-air digital broadcasts. This works fine for over
the air digital signals, but it doesn't work on digital cable systems. The
TV cable system uses a different "format", which I have no information
about.

This "Access HD" has an RS232 serial port type of connector (DB9) which
is for "future updates or service" (a programmable system). If I can get a
service manual or schematic for this unit, and...

Digital Converter (over-the-air) to Digital Cable Converter