Abstract: An optical detector senses the teeth on a wheel rotating with an unwinding roll of medium in a printer and provides a signal which is used to count the sensed teeth to measure the rotation of the roll. When a predetermined proportion of a rotation is completed, the measured length of medium movement along a path through the printer during the predetermined proportion of a rotation is used to sense the condition of the medium. The conditions which may be sensed include amount of medium, medium thickness and medium malfunction. The circumference may, for example, be calculated and compared with a low medium circumference for the preselected or calculated thickness of the medium to detect a low medium condition. Periodically, the present medium position may be compared with its position when the last tooth was detected to sense a medium malfunction, either a medium out condition or a medium break.

Abstract: A toner or development unit arrangement for an electrographic printer or plotter in which each of the toner units is modular and can be readily removed and replaced by the user. In addition, the units are mounted in a rotating support, generally referred to herein as a carousel, which is compact and which rotates each of the units into the same position for printing, simplifying the movements of the medium past the development units. As a result, the moving parts within each unit are driven by the same set of drivers, to which each toner unit is coupled by coupling means when a selected toner is in the printing position. Also, disclosed is a method of quickly establishing a toner meniscus where the toner unit engages the medium surface as soon as the toner pump is activated. Further, cam operated means is provided to operate the medium cutter in conjunction with a cutter stepper motor.