Hi @yohan, One of the fun parts of Serial Comms to Ard is trying to see or imagine what Ard is doing with the string you’ve given it. Various techniques:
Make your Processing sketch print the string. Now save it as text and stop Processing. In Ard IDE, Serial Monitor, paste in the string and enter. Now you can use print statements in the Ard sketch and see what’s happening.
Which Ard are you using? Some have extra serial ports where you could print progress messages and view them via TTL to USB adaptor. You could use OLED or LCD screens.
You could post your Ard sketch. (And the Processing string you’re sending.)