Arduino ATMega32u4 USB pinout

Question

Really quick question here - is it possible to use the GND/5V from the USB cable to power the Arduino through the USB connector but use the RX/TX pins to a motherboard's USB pin headers (D+/D-)?

The D+ = TX
The D- = RX

Then connect the common grounds (5V, GND and motherboard GND together).

So:

5V (psu) ---------> Arduino USB \$V_{cc}\$ connector
GND (psu) --------> Arduino USB GND connector
TX (Arduino pin) -> motherboard USB header D+
RX (Arduino pin) -> motherboard USB header D- 
GND (Arduino pin -> motherboard USB header GND

Will the GND pin on the Arduino be considered "combined" since the PSU GND is connected via the USB connector (grounding both USB GND and motherboard GND)?

The Arduino I am using is this:

Answer 1

It sounds like you want to power the Arduino with a different power supply than the motherboard. This is common for self-powered devices and the device's datasheet will have a section with the recommended schematic for that configuration. The GND will be tied together and IIRC the +5V is used to detect the connection.

You can consult the datasheet and the board's schematic to see whether you can reproduce the datasheet's schematic for self-powered devices with a hacked USB cable.

Answer 2

If you are talking about http://elecfreaks.com/store/download/Freaduino%20microV1.2.pdf board you can power from USB connector and use serial RX/TX 32u2' 5V serial pins. D+/- are high frequency USB data wires dedicated to USB only. You can make ugly hack, unsolder R3&R10 and wire D+/- to RX/TX. But it's really really awfull reusing USB connector as 5V serial.