I said:

>> RS232 input to a pic can be through a single resistor [...]

Lee said in reply

Snip Snip..

>
>If the PIC is receiving data via RS232 from another device, then this is
>true.  The sender will generate both positive and negative voltages.  You
>should have a resistor to limit pin current and a diode to ground to keep
>the PIC's input pin from being driven to far below 0V.  Be aware that you
>are giving up some noise margin, but it should work fine.
>

Me again...

Microchip describe how to interface a PIC to AC power lines with just a
single resistor, or two in series for added protection.  According to there
information no clipping diodes are required except for one to Vdd in the
case of RA4.  I described this in my first reply.

I think if it works for 110VAC then for +/- 12VDC we could probably get away
without  the external clipping diode. It's a question of limiting the
current to a safe level with a suficiently high enough resistor.

Regards

Jim