Integrating troubleshooting results from quite a few people describing power problems with Pixhawk or Pixhawk getting out of control (linked to ESCs' signal wires connected without a ground reference) I thought the current wiki information should be completed/updated with a synthesized diagram that says it all clearly and explicitly.
This diagram represents optional components that you may have or not but the wiring logic remain the same.
The basic fact to know to start with is that the power port ground is NOT (connected to) the ground of the main output ports on the Pixhawk board, contrarily to what we were used to on the APM board (where the ground was the same on all ports, inputs or outputs).
Acronyms on the diagram: PDB on the diagram means Power Distribution Board. PM block means the power port of Pixhawk. PM/Atto means an optional power module from 3DR or the Attopilot alternative for higher than 4S battery voltages.
Any comments to add eventual useful tips that would be missing are welcome. I'm not the best at drawing stuff, so if Jethro (known for his excellent illustrations) or some other guy here on the forum has graphic design talents, this diagram could be made more sexy.
Hope this will help some folks to do a safe wiring.