I have an external cable box, (a Motorola DCT2224,) that I use with a PVR-250. Myth changes channels on the cable box and leaves my PVR-250 card on channel 4. This works for live tv and for recording programs.
The problem is that you need a way for Myth to change channels on the cable box. The method you use depends on the cable box and on your local cable company.
For my setup I have a straight-thru serial cable connected from my Myth box to a serial port on the back of my cable box (not all boxes have them.) Next, my local cable company (Comcast) has enabled the serial port on my cable box. Not all cable companies will enable the port (and not all local Comcast centers will enable them either. And the Motorola box has to have a minimum firmware version, 7.54 I believe.)
I downloaded Myth ver .13 and compiled the dct-channel program. I placed it /usr/local/bin/ (once compiled the name is just 'channel'.) In mythtv-setup’s Input Connections for my input source, I set the external channel change command to /usr/local/bin/channel and I set the preset tuner to channel value to 4 (which is the channel my cable box sends its output to.) I then restarted the Mythbackend and things just worked. (I was lucky.)
If you can't use a serial cable then you'll have to use an IR Blaster. That's a device that acts like a cable box remote control that sends the IR signals to your cable box. It also connects to your computer’s serial port.
For more information on either method you can search the Myth archives at http://www.gossamer-threads.com. Search for IR Blaster or for your cable box’s manufacturer name.
You can also search the tivo board at http://www.tivocommunity.com. They've had these discussions too. The following post discusses how to determine what firmware a Motorola cable box has: http://www.tivocommunity.com/tivo-vb/sh ... genumber=1
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