> PIC out pin ----------|----------------->RX 3.5V device
>                       |
>                      ___/
>                     / Z
>                       E
>                       N 3.5V
>                       E
>                       R
>                       |
>                      GND

I don't like this at all.  It will draw significant current from the PIC
when it tries to send a high level.  If your receiving device's thresholds
are above ground (many are, although that is not required by RS-232 and will
cut down on noise immunity), then just run the PIC output straight to the
receiving device.

Again, I don't like this scheme because it can fail with a perfectly
compliant receiver.  I have built devices that had such receviers.

> PIC RB input <--------------+
> with internal               |
> pullup acivated             |
>                             C
>                     BC547    B<---+------TX 3.5V device
>                             E     |
>                             |     _
>                             |     ^
>                             |     D1 1n4148
>                             |     |
>                             +------
>                             |
>                            GND

There should be a resistor in series with the TX line.  1Kohm would be fine
as long as you can still get the speed you need.  You could go much lower,
to 150ohm or so if needed.  Also, I'm not sure the internal port B pullup is
activated for the RX line when the UART module is enabled.  Check the docs
or do an experiment before assuming this.

> but I'm not convinced about the D1 1n4148 because its presence do not
permit
> at the signal to reach zero level. What happen if I remove it and stay
only
> with the BC547? (I suppose that the junction BE also do not permit to rech
> zero level; it is true?)

Huh?

The diode (with the missing resistor) prevents excessive negative base
voltages.  If you transistor is guaranteed to -20V B-E and 25V C-B then you
can lose the diode.


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