I am trying to send and receive data from a PIC16F88 to an Arduino Uno. My problem is that the serial communication stops when the Arduino is not connected to the USB cable. For my application I need the Arduino to still perform its operations with a battery. I have tried both software and hardware serial. Is it possible for the Arduino to send/recieve data when not plugged into the USB cable?
You need a common ground between the Arduino and the PIC. Connect both grounds one to each other.
Is the Arduino running at all when on battery? You could be trying to pull too much current from the battery and killing the circuit. Another cause is that some programmers pull the reset pin to low when unplugged; took me a while to figure out that one.