Hi! Currently working on a project where I need to have communication between a PX4 and Nvidia Jetson NX using a USB cable. This is easy enough to achieve, but my question is whether or not it is safe to have the PX4 powered through its Power Distribution Board, and the Jetson powered similarly, off of the same battery, and then connecting the two using the usb cable. Will there be any unintended current flow?
It is may not like what you thought.
An USB equipment is either a power source or a sink, in either way there will be current flow in between.
A properly designed hardware should have USB Power net and system power net separated, So the question you are concern should not be a real issue.
However if the hardware does not have a decent design, that will cause problem.
between a PX4
this flight controller?
https://docs.px4.io/master/en/flight_controller/pixhawk4.html
PX4 is the firmware/software
I need to have communication
using the following?
https://docs.px4.io/master/en/peripherals/companion_computer_peripherals.html#ftdi-devices
I’ve done things like this before with a Raspberry Pi. It should be possible to connect them without unintended consequences by following the companion computer setup guide.
One thing to note is that a Jetson Xavier NX can draw up to 4A from a 5V supply. The PM07 power module that comes with the Pixhawk 4 is rated to source 3A. Drving both from the same power module, without a seperate voltage regulator for the Jetson, you may experience voltage drops and “brown-outs” during flight that could cause crashes if the flight controller loses power.
I’d recommend using a dedicated BEC connected directly to the main battery.
This is exactly what I was looking for! Thanks robwaat + everyone else 
I’ll be using different BECs to power the Jetson and Pixhawk 4, was simply worrying about connecting the two using a USB cable that could transmit power as well as data.