From LedHed's Wiki
NOTE: Both of these scripts need to be compiled see the corresponding README located in:
/usr/share/doc/mythtv-backend/contrib/channel_changers
SA3250HD
External Channel Change command:
sa3250ch -n 0 -p 1 -s <Channel Number>
-n = node -p = port -s = Send as single digit
SA4250HDC
/usr/local/bin/sa4250hd <Channel Number>
NOTE: This script works with the sa4250HDC but I had to add the right vendor ID's to the code prior to compiling.