The DCT-2000 doesn't use "serial" it uses "infrared" (at least mine does) to change channels. The setup is clearly outlined in the user guide. What you need to do is connect the IR blaster cable to your Tivo and position it over/in front of the IR receiver on the cable box. You may need to use a flashlight to locate the IR receiver on the cable box as it's usually behind the semi-transparent faceplate on the front of the box. Once you have the cable connected and positioned you need to go into setup and tell your Tivo that you have a Motorola cable box connected to your Tivo. This is all covered step-by-step in the user guide.
Hope this helps,
Ken
Hope this helps,
Ken