I have two Arduino Unos that I can't seem to get talking to one another via serial. I uploaded the PhysicalPixel sketch to one, and another sketch which alternates the 'H' and 'L' characters (as documented in http://arduino.cc/en/Guide/ArduinoXbeeShield).
I've connected Pin 0 of one board to Pin 1 of the other, and vice-versa, and I cannot get the LED to cycle like the sketch says. One significant behavior i've noticed is that the TX LED on the sending Uno flashes every second when I have it powered via USB, but not when I have it powered externally (via a 9V battery or 12V wall wart). Additionally, if I have the Uno plugged in to external power and upload the sketch via USB, the Uno will flash the TX light every second (as the sketch says) until I plug the USB cable, then the TX light stops flashing.
What gives? Lasst week I had them talking to one another via XBee PROs, and now I can't seem to get them to talk simple serial.