catatung:
As I understand it, when you power the Arduino from an external supply, the serial functions do not work. The Serial pins only work when the Arduino is powered by USB.That is not correct. The serial functions still work either by wiring external TTL voltage level comm wires to pins 0 and 1, or by USB if plugged into a PC, even if using external power. You must have some misunderstanding.
I came up with a possible solution, was wondering if anyone has already tried it though:
I'm thinking of cutting a USB cord and abandoning the power supplied by the computer. This is on 2 of the 4 pins. Leave them disconnected.
Next, provide the +5v wires on the arduino side of the cut plug with an external supply of +5v. Tie GND from the PC side of the cut USB cable and the external 5v supply.This should, in theory, provide the arduino with 5V on the USB cord and enable serial routines, while having the data lines of the USB still connected to the PC. This way, (I hope) when the computer is off the arduino can run from the external power supply. When the computer is on, that will not change but serial routines should be available on the arduino.
Has anyone tried it this way before? In theory it seems it would work correctly, but was wondering if anyone has attempted this yet.
Again I think you are over thinking the issue and seeing a problem that doesn't exist. If an arduino has both USB power and external power avalible at the same time the on-board auto-voltage selector circuit uses the external power via the on-board +5vdc voltage regulator and turns off the USB +5vdc via a mosfet switch. The USB chip still gets power in either case and communication can still happen either by using the USB link to a PC or using pins 0 and 1 to an external TTL serial device. The arduino comm capablities and it's powering options are two totally seperate things that are in no way interdependent on each other.
Lefty