Hi Jeroen,

When RI is set, which is to say when the serial port interrupt is triggered, program control is automatically vectored to, I'm guessing 23H? It's specified in the tutorial. Maybe it's 28H. You can look up the actual address. In any case that's where you need to move the character from SBUF into A (or whatever register/location you wish). Your serial port interrupt routine must be at that vector address, otherwise your code will execute whatever happens to be there (probably a bunch of NOPs). I noticed that you were careful to clear the RI flag. You also need to be sure and use RETI (RETurn from Interrupt), not RET, to return to your main program. Read the special section of the tutorial (found in the left hand column, right over there <--) on Serial Port Interrupt.