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Use a coaxial cable to make this connection, ensure that there is a little slack in the cable so that there is no tension on the connectors. You can use the HDMI cable that came with the box, or any other HDMI cable. Make note of the HDMI port that you use on your TV so that you can easily switch to it later.  Many Comcast cable boxes have multiple HDMI ports. Make sure you're using the HDMI Out port on the box and not the HDMI Input ports. If you don't have an HDMI port on your TV, you can use a coax cable to connect the "To TV/RF Out" port to your TV's coax port. This will not result in high definition image. Plug it in to the Power connector on the back of the box. If you connected the box to an HDMI port on your TV, switch to the HDMI input that matches that port. If you connected the box using coax cable, switch to channel 3 or 4. If you have multiple TVs and have active coax outlets, you can use digital adapters to connect your cable to additional TVs. Each adapter will need its own coax wall outlet.  Connecting a digital adapter is pretty much the same as a regular cable box. Use a coax cable to attach it to the wall outlet, and HDMI or coax cable to connect the adapter to the TV, and then connect the power adapter. Make sure all of your digital adapters are connected and powered on before starting the activation process.

Summary:
Connect the "Cable In/RF In" connection to the cable wall outlet. Connect the "HDMI to TV" port on the cable box to the HDMI port on your TV. Connect the power adapter to the wall outlet. Switch your TV to the correct input. Connect any additional digital adapters.