In differential signal circuits (specifically, RS-485), there is usually an isolation between the signal processing / generation stage (eg, microprocessor) and the line driver chip (something like a MAX485 or SN75176).
This is usually accomplished with an isolated DC-DC power supply, or a galvanically isolated AC-DC transformer / power supply.
What I don't understand, is why isolating the power between the microprocessor and driver chip is neccesary, if instead the entire circuit is powered by a galvanically isolated power supply.
Does this not achieve the same level of isolation, if there are no other external connections sharing this power supply?
(My understanding is that the isolation is to prevent ground-loops / large voltage potential between the transmitter / receiver - and the isolated power allows the receiver to be grounded (here I'm assuming the trans/rec are connected with d+, d- and ground) at the same level as the transmitter. In my case, the galvanically isolated power supply would also be connected to the transmitter's ground along with the data wires)