4.0THEORY OF OPERATION
4.1OVERVIEW
The DP8 and DP9 Printers handle a large variety of documents. The two printers are identical except that the DP9 is 3.2” wider. The mechanism consists of a printhead, a carriage drive assembly and a document drive assembly. The electronics consist of a switching power supply, a Main PCB, a sensor PCB and a keypad/display assembly.
4.1.1 Carriage Drive Assembly
The carriage drive assembly positions the printhead as it prints each line of characters. The printhead is mounted on a moveable carriage that is supported by a carriage shaft. The carriage shaft is mounted to two pivot arms that allow the carriage to lift away from the platen and accommodate different thickness documents. The printhead is held against the document by the carriage shaft weight and springs. The carriage is lifted by a roller that rides across the document surface and maintains a constant printhead to document gap, thereby maintaining print quality across a wide document thickness range. An upper shaft stops the carriage from rotating around the carriage shaft.
A toothed belt clamped to the carriage is driven by a pulley mounted on the carriage stepper motor located on the left side of the metal box. The stepper motor moves the printhead in 1/120th of an inch increments along the print line. Position information is updated when the carriage is detected by an optical sensor on the Sensor PCB.
The ribbon cartridge is driven by a shaft which is in turn driven from the carriage motor shaft through a gear train which pivots so that the ribbon drive shaft rotates clockwise regardless of the direction of carriage motor rotation. The entire carriage drive assembly lifts away from the document drive and the ribbon shield and ribbon guides mounted to the carriage lower away from the printhead for easy access to the ribbon path when replacing cartridges.
4.1.2 Document Drive Assembly
The document drive assembly positions inserted documents. The document drive stepper motor mounted on the right side of the document drive is connected to the two drive rollers through toothed belts. A pinch roller is spring loaded against each drive roller providing friction feed for documents of varying thickness. Documents are fed between the drive and pinch rollers, with the
The presence of a document in the input tray is sensed by the frontmost of four optical sensors located above the document path. When auto alignment is active a gate is raised across the document path during document insertion. At that time the printhead moves past its normal left home position, activating an arm that raises the gate. The rising gate reduces the spring loading force on the front document drive rollers, allowing documents to rotate and align against the gate. Three optical sensors in line behind the front rollers check the leading document edge to insure that it is sensed within a programmable alignment tolerance. These sensors are also used to position the top or bottom edge of the document relative to the printhead. After alignment is verified, the printhead moves to the right, lowering the gate out of the document path and restoring normal loading force to the document drive rollers.
4.1.3 Switching Power Supply
The switching power supply is mounted to the inner front surface of the metal box. AC line voltage is wired through a power switch to the power supply. The power supply provides regulated +38VDC to the Main Printed Circuit Board.
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