Hi Mike,

there is already a standard serial bus with own power: DeviceNet.
Please look at:
http://www.odva.org/
for more informations.
It use 4 wire with 24V on one pair and CAN on the other.
It suggest optical insulators on every node and grounding only on a single point (typically on the 24V power supply).
All nodes must be connected in a long chain with termination resistors on both ends. The power supply should be inserted on the half of the line to get minimal voltage dropping over the line. Nevertheless the CAN drivers (82C251) are still working until a ground shift of +/-5V. They withstands also wrong connection (+/-40V on the data lines).

To get the 5V without many power loss use better step down switching regulators or DC-DC converters.

I suggest for professional usage to meet this standard.


Peter