Hi @MikeFair -
If your intention is only to spin the motors for testing purposes, an approach using the motor test mavlink messages for control makes sense…This post has relevant information.
If however you intend to drive the vehicle via this direct motor control, that is definitely not a good approach! The ArduRover stack already handles running the motors in control loops that are located “closer” to the hardware. Thus, if you want your own autonomy running the show it could simply be setting the desired heading, velocity, or even waypoint rather than the low level control you’re pursuing…
And of course, this code could run in BlueOS as an extension - no need for the second Pi and power consumption increase!