I have an Arduino Mega 1280 which I was using to debug some sensors. Now I can't program the mega to run any other code. I'm guessing the all the traffic is causing the malfunction. I have an Arduino Duemilanove and I was trying to use it to reprogram the Mega using ArduinoISP but to no avail.
When trying to program the Arduino Mega with Serial USB cable, Arduino IDE hangs on uploading and the RX and TX lights blink a few times (3 I think) and then the TX light is constantly on.
When I use ArduinoISP, the IDE gives me either:
avrdude: stk500_getsync(): not in sync: resp=0x15 or
avrdude: stk500_getsync(): not in sync: resp=0x00
Try holding the Reset button, unplugging and re-plugginf the USB cable, clicking the Upload button and releasing Reset when you see the RX light blink.
The same problem happened to me.
When I tried to load the program the IDE gives 'Uploading...' forever and the TX Light was constantly on.
What happend was that my APC220 was connected to the Mega and I needed disconect the cables of '5V' , 'RX' and 'TX' pins.
This resolved my problem, I could uploading the code to de Mega normally after that.