Based on recent experience, the only way I appear to be able to hook up my set top box using HDMI or component is by paying $10 a month extra for the HD Technology package. I don't want HD, I just want to use a technology the box already supports to display a clearer picture than I get with the A/V or RF out. It even output video through HDMI when I initially connected it, but it went away after I activated it, so I know it's not the box. Am I stuck paying the $10 extortion, or is there a way to make this work? I have a Samsung PN60F5500 TV with the Motorola RNG200N box.
You could try component video (the red, green, blue video connectors) on the Xfinity box, which will give you 480p or maybe even 720i analog resolution, better than AV or RF for sure.
SInce you have an older plasma Samsung, you should be able to use those as well. And since component video is an outdated technology, you can probably buy the cables real cheap at your local dollar store.
I tried component cables and the Xfinity tech who came out did too with a different cable, and we couldn't get it to work. He left with a recommendation to pay for the HD service.
The only other thing I can think of is that there might be a setting on the Comcast box to enable component video output for SD - barring that, I'm not sure what else to tell you.