I am unable to get a Nano 3.0 to be recognized by Windows USB port when I apply 5v to the regulated input or 12v to the VCC input. All my Nanos behave this way. I've seen this problem described in other posts. Apparently, this issue may be specific to the Nano. I disabled the 5v power line on a USB cable, but the Nano received no USB power when itis powered by my circuit board. So it seems like Nano can get power from its USB, but USB can't get power from Nano. I haven't reviewed the schematic, but I'm guessing there's a diabolical diode in there somewhere.
I have two Nanos on this board.
Even more curiously, I can't get the USB working when the Arduinos are seated in the board and there is no power applied.
I do have the serial ports connected together so the Arduinos can talk, but I don't think this is the problem, because it happens when they are not mounted, or when only one is mounted.
My board is too complicated to breadboard this test, but what do you gurus think about putting a jumper switch that will disable the 5v input trace to the Nanos when connecting to USB and uploading sketches?
Has anyone dealt with this?