Jim Ruxton wrote:

>Using 2 I/Os (RB0 and RB1 on a 16C505) I am trying
>to drive two back to back LEDs and read a switch at the end of  2 long
>wires . Each pin has a 470 ohm resistor in series with it and a .01 uF
>cap shunted to ground after the resistor. At the end of the two long
>wires there is a switch in series with a parallel combo. of  a  red and
>a green led (pointed in opposite directions) and a 10K resistor. There
>is also a 100k pullup at RB1. When I want to drive the back to back LEDs
>( if the switch is closed) RB0 and RB1  are outputs . When I want to
>read the switch RB0 outputs 0 and RB1 tries to read whether the switch
>is closed ie. RB1 sees 1 if it is open , 0 if closed.
>The wire I am driving(reading) is quite long approx. 300 ft. I am having
>
>troubles reading the switch.I can read the switch ok if  I don't change
>RB1  to an output . But thats no good because I have to be able to
>switch the LEDs.I tried putting nops between my TRIS statements etc. A


Jim:
        The first impression that I get is that you're not leaving enough
time for the cable to charge up (if it's going to). If your cable is
50pF/ft and you have two 0.01uF effectively in series across this, you have
~0.02uF and 100K. The time constant is thus ~2msec. Combined with the fact
that the cable may have to discharge completely, then recharge to the other
polarity you should be allowing possibly 10 *milli* seconds before checking
the input.
        The first thing I'd try is adding straight delay to your program.
Good luck,


        Jim Nagy
        Elm Electronics
  ICs for Experimenters
http://www.elmelectronics.com/