> Using a 4 mhz crystal, my instruction time is 1us right ??

Yes.

> Wanting to transmit data at 9600 baud I will have a period of
> 104us, right?

Each bit time is 104 microseconds.

> That means that to transmit a "1" I would have to drive a pin
> high for 52us and low for 52us right ???

No.  That would be NRZI or phase encoding or something similar.

RS-232 is a simple voltage interface.  The data line should be
held high or low for the _entire_ bit time.  Transitions occur
between bits.

> To transmit a "0" I would just drive the pin low for 104us?
> I know that regular RS232 is low marking

Right, but the output stage may invert the signal.  In other
words, depending on the driver IC you are using, you may have
to "lower" a PIC pin to generate an RS-232 high (+3 to +25V).
And conversely, you may have to "raise" the PIC output pin to
generate an RS-232 low (-3 to -25V).
                                                Lee Jones

-------------------------------------------------------------------
Jones Computer Communications             lee@frumble.claremont.edu
509 Black Hills Dr, Claremont, CA 91711         voice: 909-621-9008
-------------------------------------------------------------------