Arduino Nano - external 5V <-> USB communication

Hi, Im using an external 5V power supply with my arduino nano on the 5V pin. When I connect an USB cable to the Ardunio I have no communication between PC and Ardunino. If I completly disconnect the 5V power supply and connect the USB cable I can communicate with my Arduino via USB without any problems.

I have to use the 5V power supply for my other hardware and I need stable 5V on all output pins - so the external ps is essential. For further control of my hardware I need the serial connection via USB. So is there any way to use both at the same time?

Thanks for answers Jazz

What model Nano do you have? The Nano V3.0 schematic (http://arduino.cc/en/uploads/Main/ArduinoNano30Schematic.pdf) shows that everything is powered from +5V and USB power is used if other power is not available. Same on the nano V2.3 schematic. The nano description (http://arduino.cc/en/Main/ArduinoBoardNano) says :

The FTDI FT232RL chip on the Nano is only powered if the board is being powered over USB. As a result, when running on external (non-USB) power, the 3.3V output (which is supplied by the FTDI chip) is not available and the RX and TX LEDs will flicker if digital pins 0 or 1 are high.

That doesn't match the schematics but it MIGHT explain your symptoms IF your USB host isn't providing power for some reason.

exactly, that doesn't match the schematic and i can measure the 3.3V even without USB supply. By the way i have the same problem...