Simulated drone with pixhawk only


I have to create a simulated drone based on the ardupilot architecture using as an hardware only the pixhawk : all the devices that are supposed to be connected on the pixhawk will be simulated, and the simulated drone created with the simulated elements will work with the arducopter code within the pixhawk. So it’s a work of hardware in the loop (HITL) as you can see. So in order to simulate the usual components of the pixhawk, I’m using mission planner (APM) software to arm my pixhawk and make it able to send PWM signals (for example) on the main outputs, usually sent to ESCs to control motors. In my case, I’m reading those PWM on an arduino MEGA, in order to write a code that will simulate the motors of the drone (I’m not gonna simulate it only with the arduino, I’m gonna use a Raspberry Pi for that, but first of all, I just need to read the PWM signals from the pixhawk, htat’s why I’m using an arduino here)

Now the problem is that I can’t read anything on my arduino, the pixhawk is completely silent. It has been armed with mission planner even if there is no devices on it, so if I’m using a joystick to make the simulated drone fly and send PWM signals on the Main outputs of my pixhawk, I should be able to see something on the serial port of my arduino, but all I can get is a serie of zeros, making sure that I am not receiving what I want. I don’t know what I can do to make sure first of all that my pixhawk work correctly and give me the signals it is supposed to send. Can anyone help me please ?