Thanks to everone who answered my question. Here is more information about it:

The 16F84 is usually disconnected from the "external circuit". It is like an electronic lock door, or something similar.

First, when you connect both circuits, the external ciruit (let´s call it READER from now on) starts injecting a clock signal to the 16F84, and it replys to the reader, giving it the 255 bits of information to a line in the reader wich is for input and output, too. There is an aditional line wich indicates if you are reading or writing in the 16F84.

For this thing, I use RB0/int for detecting the clock signal from the reader, RA0 for giving the 255bits to the reader, and RB1 for seeing if it is a read or a write operation.

When this is done, the reader waits for X seconds and then, starts to write in the 16F84 using the input/output line. WAIT!!! While I´m writing these few lines, I can see what I have to do!!!!!!! This is it!!!!!!!!!!!!

I will write soon. Hope to solve it.

Thanks again, Bye.-