Implemented Commands
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$PRTLN,00,Text to be printed,
Print, followed by line number, followed by text to be printed

$PRINT,00,00,Text to be printed,
Print, line number, 

$CLEAR
Clears the screen

$OFFST,00,00,
Set the horizontal and vertical offset.
Horizontal offset is between 0 and 63 (corresponding to -32 to +31
pixels in the OSD chip docs).
Vertical offset is between 0 and 31 (corresponding to -15 to +16 pixels
in the OSD chip docs).

$GETOF
Get the current horizontal and vertical offset

$VIDIN,0,
Set the video input. Hardware supports lines 0 to 3.

Possible Future Commands
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Next waypoint
$WAYPT,000.00000N,000.00000W,

Current speed
$SPEED,100.0000,

$LOCAT,000.00000N,000.00000W,