If the output from the other board does not go above the PIC's Vcc supply rail nor go below GND voltage, then the PIC's input will remain in high impedance.
If the limits are exceeded, the protective diodes will start to conduct and will be destroyed if the currents are excessive. Adding series resistors to limit this current would be advisable.
If the output from the other board does not go above the PIC's Vcc supply rail nor go below GND voltage, then the PIC's input will remain in high impedance.
If the limits are exceeded, the protective diodes will start to conduct and will be destroyed if the currents are excessive. Adding series resistors to limit this current would be advisable.
A +5V reg may not be designed to sink current if something else tries to raise the voltage. So if the diode to Vdd conducts, it can raise Vdd above 5v, which will mess with your ADC readings, and if it goes above 7v, total latchup that even the WDT can't fix.