>> Is there anything I can add to protect the chip from unwanted negative v=
oltage?

Well, to start with, there should be a substantial resistor; about 10k.
This will rely on the internal protection diodes to clamp the voltages to l=
evels that won't cause problems.  There is plenty of reason to assume that =
this is insufficient, but microchip does publish app notes where this is do=
ne (apparently successfully.)

Um, you know, the RX input of the PC ought to be a high impedance input, wi=
thout providing any dangerous currents to the PIC.  Are you sure you have t=
he correct pin connected?  You shouldn't see -9.6V on the receive pin; it s=
hould be floating...
It ought to be sending data in the other direction that causes problems.

The other thing I've seen is getting the connector wiring wrong, and using =
DCD as GND and DTR as RxD (to mention the most likely error on a DB9 connec=
tor.)  Make sure that the signals you are using are the ones you think you'=
re using.   A "breakout board" is valuable:  http://www.flickr.com/photos/5=
8843278@N00/6645963575/  An LED/resistor connected between GND and the PC's=
 RX signal should NOT light up in either direction!

BillW


--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.