Adding commands to Ardupilot firmware

Hello everyone,

I am in the process of modifying an S1000 with a Pixhawk 1. There will be another system onboard the vehicle which the Pixhawk needs to communicate with over CAN bus. This other system should be able to send statuses to the Pixhawk and the Pixhawk should react to these.

For example, the other system detects a fault and tells the Pixhawk to reduce the control rates, return home to land, sound the buzzer, etc.

Can anyone suggest where to start with this endeavor?

Thank you!