Mike Kietz wrote to my original posting.

>A 10K resistor is way too small.  Use a 1M resistor to keep the current
>flow through the internal protection diodes low.  Then you probably won't
>need the external diode.   Of course, the 12C50X pin that doubles as MCLR
>(GP4?) can't be used this way since raising it above 5V will put the chip
>in programming mode.

GP3 is the pin that doubles as MCLR.  You can run RS-232 voltages thru a 10K
resistor on any of the other five pins without problems.  I personally don't
like using very high resistances on clamped CMOS inputs because of the
danger of induced noise changing the input state (I got bitten by this once).

It doesn't seem to matter what resistance you use, I tried up to 2M, when
you input RS-232 on this pin, it is always held in reset.  The external
clamping diode seems to be the only thing that works.

As for inadvertently putting the PIC into programming mode, I haven't seen
any problems with that (I suspect the current drive capability from the
RS-232 Drivers is insufficient to put the chip into programming mode).

>Install the resistor near the PIC so the high-impedance node between the
>resistor and PIC isn't affected by stray capacitance or noise.

Always good practice.

Thanx for the comments,

myke

"I was well aware that the processes of puberty are often fatal to psychic
power."
- Sir Arthur Conan Doyle