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The missing pulse detector approach was my first thought also. However could it be even more simple than that? For example what if the motor drive transistor where PWM controlled and you coupled the PWM signal to the base of the drive transistor through a capacitor, then as long as the PWM signal were active the motor would be driven. If the PWM signal failed, do to a microntroller failure or the microcontroller halting the PWM signal, then the motor would stop. Seems simply and easy enough to do as a safety backup so why is it not implemented by default in most drive circuits i have seen? What am I missing?