For the P1 connector what is the function of the black wire and what is the voltage of the black wire only?Please include sources.
The P1 connector on what? It's not clear exactly what connector you have. P1 just probably means it's connector (port) number one on some particular device. That doesn't really tell us specifically what connector you're talking about. On power connectors, the black wire is DC ground. The voltage should be zero with respect to the case/frame ground.
