I wrote code to connect arduino with pic , I don't want to explain things don't matter, but the main problem is that arduino always receives data although I don't connect any wire to pins(TX3,RX3).
this simple example , I uploaded it to arduino, and the result is that on terminal monitor
always print "the fire in the room : " .
and sometimes print stranger characters or nothing as shown image in down .
Serial.println("the fire in the room : ");