ok I’m able to answer myself now: it’s just a matter that the Stalker board, unlike for example the Arduino Diecimila, is NOT including the serial level conversion circuitry on board!
So basically I was assuming that the Stalker was including the same conversions that I was having in the Diecimila, but this is NOT the case, the section like the following one is missing on the Stalker: freecircuitdiagram.com/2009/03/2 … ransistor/
That is also why people using the USB adapter from seeedstudio do not have issues since this section is in place in the adapter (implemented with the classic MAX232 chip plus USB conversion instead of using two transistors of course).
Ok now that this is clear I knwo that the RX and TX signals from pins 2 and 3 need to go trough a MAX232 first!
Anyway I tought it to be worth to mention here, since it may not be so obvious when accustomed to other boards you may think completely compatible…