A mailing machine base comprising, structure for a direction of the path of travel and feeding a sheet in a path of travel, a fence defining against which the an edge of a sheet is normally registered for alignment therewith and thus in the direction of the path of travel, structure for controlling the sheet feeding structure, the controlling structure including a microprocessor connected to the sheet feeding structure, the controlling structure including structure for sensing a sheet fed into and out of blocking relationship with the sensing structure and providing a corresponding signal to the microprocessor, the signal having a first magnitude when a sheet is not disposed in blocking relationship with the sensing structure, the signal having a second magnitude when a sheet is disposed in blocking relationship with the sensing structure, the signal having a variable magnitude between the first and second magnitudes for less than a predetermined time interval when a sheet is fed into blocking relationship with the sensing structure and the sheet edge is in alignment with the registration fence, and the signal having a variable magnitude between the first and second magnitudes for at least the predetermined time interval when a sheet is fed out of blocking relationship with the sensing structure and the sheet edge is not in alignment with the registration fence; and the microprocessor programmed for successively alternately obtaining a sample of the magnitude of the signal and delaying sampling thereof for the predetermined time interval, determining whether the magnitudes of any two successive samples are both between the first and second magnitudes, and causing implementation of a shut-down routine if the magnitudes of any two successive samples are both between the first and second magnitudes.