---Russell McMahon <apptech@CLEAR.NET.NZ> wrote:

>I am unaware of the particular circuit you are using(what is the app
note number?) but many people on this list saythey have used RS232
straight into a PIC pin via a series resistor and  have said that this
worked for them and that the PIC protection diodes "catch" any voltage
overswing. However, this method of input protection is not guaranteed
in the datasheet. Generating substrate current via the protection
diodes can conceivably cause unexpected results and in my experience,
it did.

>I used a series input resistor for RS232 receive with a 16F84 and had
endless problems due, as it turned out, to the current into the
protection diodes. This can be overcome by either placing reverse
schottky diodes (eg BAT85) to supply and ground from the input pin or
by splitting the input resistor into 2 and using 2 silicon diodes from
the midpoint to ground and supply. Both schemes stop the protection
diodes conducting during normal use.

This was one of my problems.  I added a NPN transistor to convert and
invert the RS232 levels from +10/-10 to standard TTL levels.  My other
problem was the data origin had to be changed from 08h to 0Ch to
support the 16C84 instead of the 16C54 PIC.  Everything seems to be
working now.  By the way this Parallax PIC application note #2 from
there book.  The source code can be found in the pic_apps.zip
file(rcv232.src) on there ftp site.

Thanks for the help.
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