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4.0 THEORY OF OPERATION
4.1
OVERVIEW
The DP6 & DP7 Document Printers handle and print a large variety of documents. The mechanism consists
of a printhead, a printhead drive assembly and a document drive assembly. The electronics consist of an AC
PCB (printed circuit board), a power transformer, a Main PCB, two sensor assemblies and a keypad/display
assembly.
4.1.1 Printhead Drive Assembly
The printhead drive assembly positions the printhead as it prints each line of characters. The printhead and
ribbon drive mechanism are mounted on a moveable carriage which 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 carriage is lifted by a slider that rides across the document
surface and maintains a constant printhead to document gap, thereby maintaining print quality across a wide
document thickness range. A guide wheel mounted on the carriage rides across the front of the metal box and
stops the carriage from rotating around the carriage shaft.
A toothed belt clamped to the carriage is driven by a toothed pulley mounted on the carriage stepper motor
located on the right 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 maintained by the electronics and updated when a flag
on the carriage interrupts the optical slot sensor mounted to the left pivot arm.
The ribbon cartridge is driven by a shaft on which two pulleys with internal one-way clutches are mounted. A
spring tensioned steel cable mounted between the pivot arms rotates both pulleys as the carriage traverses
and the clutches are oriented so the shaft is rotated clockwise by the rear pulley when the carriage is moving
to the right and by the front pulley when the carriage is moving to the left.
4.1.2 Document Drive Assembly
The document drive assembly positions inserted documents. The document drive stepper motor mounted on
the left side of the metal box 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 spring loaded pinch roller automatically adjusting to the
document thickness. The stepper motor positions in 1/120th of an inch increments.
The presence of a document in the input tray is sensed by the first of two optical slot sensors located along
the left edge of the document path. The second sensor is used during document insertion for positioning to
the top or bottom edge of the document.
Printers with the auto alignment feature have a gate that is raised across the document path during document
insertion. At that time the printhead moves past its normal right 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. A third optical sensor, in line with the second sensor, checks
the leading document edge to insure that both sense the leading document edge within a programmable
alignment tolerance. After alignment is verified, the printhead moves to the left, lowering the gate out of the
document path and restoring normal loading force to the document drive rollers.
4.1.3 AC Printed Circuit Board and Power Transformer
The AC PCB is located under the rear of the document drive and contains a line filter, line fuse and line
switch. It provides AC to the power transformer located inside the metal box. The transformer converts 115 or
230 VAC to center tapped 48 VAC with the center tap connected to both chassis and signal ground.
4.1.4 Main Printed Circuit Board
The Main PCB is located in the rear of the printer and contains power supply, control and driver electronics.
The center-tapped 48VAC from the power transformer is converted to unreg30 to 40VDC,
