Electrical – use a MAX232 to communicate between a PIC and a Raspberry Pi

One annoying misfeature of the UART on every PIC I've worked with is that a data overrun error will shut down the receiver until code disables and re-enables it. It is thus imperative to have code that will periodically check the overrun-error flag and, if it is set, disable and re-enable the UART receive function. Otherwise if the receive buffer overruns, you won't just lose a received byte--you'll lose all data forevermore.

I'm not really sure why Microchip designed their UARTs this way. My guess would be that in early PICs a receive overrun would cause the receive state machine to lose frame sync, and that disabling the receiver was considered preferable to receiving randomly-framed data (I might agree with them on that point, though I would consider dropping whole bytes while maintaining sync to be better still); later PICs maintained the behavior for compatibility, despite substantial redesigns of the UART subsystem.

In any case, the PIC's UART implementation is what it is. Check to ensure that the UART receiver is enabled and the receive-overflow isn't tripped. If not, disable the UART receive function and re-enable it. Note, btw, that on some PICs the master disable for the UART function will not disable the receiver; you need to clear and re-set the receiver enable.

In order to send directly data using an electrical connection you need to have a common reference voltage, that reference level is normally the ground but if the circuits are isolated from each-other you can use an optocoupler

Here is some related reading http://www.circuitstoday.com/optocoupler-devices-and-application