Quote Originally Posted by AvionicsMaster1 View Post

You say you want to use a zener with a cap and two resistors for VD clamping (may not be right term). Most zeners require a turn on current and I'm going to use 100mA as my minimum guess of that current.
With zener diodes the target is 10% of their maximum current. So for a 5V/1W zener it's maximum current is 200mA so the minimum is 20mA. 7V drop @20mA=350ohm.

If you just want to clamp the voltage at any input the PIC have protection diodes so a series resistor is sufficient. Read the datasheet to find a reasonnable current for protection diodes. Hint: check PBP manual for the SERIN command, there is a schematic to interface RS232 without level shifter...

HTH