Hi Adolfo (Adolfo Cobo (Ingenieria Fotonica)), in <3.0.3.32.19970909112408.00789824@193.144.187.40> on Sep 9 you wrote:

>   And now the problem: the current from the pin of parallel port at logic
> level high seems to feed the entire PIC circuit, because it runs even
> without power.

The current you apply to the port pin flows through a protection diode to
the VDD rail and powers your PIC.  This is not a good thing!  CMOS devices
are very sensitive to that condition.  If there were no protection diode in
the chip, you might possibly have destroyed it already.

Look at your DC spec datasheet to find that voltage on port pins may never
exceed VDD+0.6 volts (for the 16C84 part, I assume the figure is similar
for the 16F84 but you better check yourself) and never fall below GND-0.6V.

Your posting does not point out if the PC needs the pullup in case that the
PIC is not connected, or vice versa.  If the latter applies (PIC needs
pullup), you can use a PORTB pin instead.  Those have optional pullup
resistors built-in (activatable with an OPTION_REG bit).  If you use them,
you can toss the external pullup and forget about the problem.

Otherwise you should isolate the port pin by transistor, or change polarity
of your signal so you can use a pulldown instead of a pullup resistor.