David Olson wrote:

> I've got some switches externally located off my PIC board (about 2
> feet) and I'm purposely driving my port pins high when the switches
> are closed (due to the fact that I don't want a short to give me an
> unwanted trigger) and I'm not sure how to size the pulldown resistors.

  This is rather confusing.  What I *think* you're saying is that you
are using low-value pull-up resistors in case of cable/ switch leakage
which you pull up with another port pin *only* when you wish to poll the
inputs.

  This is a current saving method often proposed (by Mark) here.  Fine.
PIC can drive 20mA, so you can use 270 ohm multiplied by the number of
resistors paralleled to the various sense lines.

  What it will obviously *not* do, is distinguish between a contact
closure of the remote switch and a cable short.  Obviously it cannot.

  There are ways of distinguishing a valid signal from a short, but if
we're talking microcontrollers, the obvious one is to put a 12C508 at
the end of the cable to monitor the switch closure, and have it send a
unique serial code on signal closure (or various codes to signal various
actuations).  If all switches are together, but remote from the main
board, this makes eminent sense.

  There are many delightful variations of this; power for the remote
PIC can be derived from a diode and capacitor like the DOW (Dallas One
Wire) protocols.

  In fact, you can use a DOW encoder and not bother developing your own.
(That said, a long cable is going to have far too much capacitance for a
real DOW device - you'll probably want a much lower baudrate.)
--
  Cheers,
        Paul B.