great answer Tweakie,i guess thats it,just in that case the change of direction should always occur in same direction..instead of going -x and -y it switches to +x and +y i guess, and never it could switch from +x to -x cos for that we need 5 volts impulse...i guess. So do you know where in mach i can see for what + or - direction i need those 5 volts?...anyway..this can be answer to my old question"why my mach suddenly go crazy" cos i experienced that doing parallel finish sometimes instead of continuing going back and forth ,the tool continues to go forth and breaks...again as i remember ,it continued to +y instead of getting back to -y(which would obviously need 5 volts that he didn't get). thx