Robin2:
I don't understand that, or more accurately, I don't know what was in your mind that gave rise to that comment....R
I'm sorry about my English....
Somebody said "You can't have the Serial Monitor and your Processing code running at the same time. The COM can only work for one at a time."
Instead of the port monitor, I use a led for showing the status of x and y, and there is no problem with it.