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Dot Matrix Impact Printer
A dot matrix printer consists of following subassemblies.
1. Power supply
2. Logic Card
3. Print mechanism
The power supply is responsible for generating required DC voltages for the logic card and the motors.
Most of the older models of printers use Linear Power Supply. These power supplies have an input step down transformer, a bridge rectifier and the filter circuit. The linear power supplies are bulky and less efficient. Most of the DMPs require 24 volts to run the motors and +5V to run the logic board circuitry. But the voltage requirement depends on the model and the make of the printer.
SMPS is used widely today. The SMPS consists of a input EMI/RFI filter, a switching circuit, a switching circuit (switching transistors and a high frequency transformer) and a feed back circuit. Also these power supplies have high voltage and high current protection circuitry. EPSON LX - 800's SMPS supplies various voltages to different sections of the printer as listed hereunder.
This card has a CPU, which does the entire job by fetching the instructions from the ROM available on this board. The CPU normally used is 7810H. This card is primarily responsible for accepting the data from the PC and putting the data into its internal buffer (RAM), understanding whether the data sent is in text or graphics, converting the data to the required form to be sent to the print mechanism. Also the IC has to generate the required signals for running the stepper motors in required steps, accept the signals from the sensors for status information and act on it, generate required signals to the printer head which finally prints the data onto the ribbon and which in turn hits the paper to form an impression. Following are the logics on the logic card:
This generates the reset signal, which is used by the CPU and other logic to ensure proper signals during power on transients.
The CPU or the micro controller executes the program stored in the EPROM. It has a powerful instruction set and control logic. The address bus of the CPU is decoded to select various devices like memory, Interface and other chips, CPU commonly used are 7810 TMP 90C041F.
PROM and RAM
The PROM contains the actual firmware of the printer and the character generator information. The RAM serves as input buffer for the data as well as working area for the CPU.
CR motor and Control logic
The printing is done on the paper with horizontal motion of the carriage. The CR control logic generates the sequence required by the CR motor for its operation. It also controls the motor, by switching the motor in various modes like hold, run etc. as required by the micro-controller.
PF Motor control logic
After printing each line the paper is fed forward by the PF motor. The amount of paper fed and the speed of paper movement are controlled by this logic.
Print head control logic
The print head has pins (9 or 24) which hits the ribbon (in turn the paper) to form an impression on the paper. The pins get fired due to the current passing through the print head coil. The print head control logic controls the firing of head pins.
This is responsible for communicating with the PC. The standard interface in all printers is centronics interface. Optional serial or RS 232C interface are provided in most of the printers.
Control panel / Front Panel
This panel has some switches and are used for following
o Choosing various fonts and print styles
o Loading/Ejecting paper
o Entering self test mode, setting the configuration
o Pause the printing
The carriage mechanism consists of print head, the carriage, the timing belt, the carriage motor, and the platen.
The characters or graphics image is formed of dots in Dot Matrix Printers. The print head has 9 pins or wires in a single column on it.
The print head consists of a 9 head driving coil, 9 dot wires and the actuating plate. When the head-driving coil for a dot wire is energized, the actuating plate engaged with the dot wire at one end is attracted to the iron core and drives the dot wire towards the platen. The dot wire thus driven finally strikes the ribbon and the paper against the platen to print a dot in the paper. When the print head coil is de-energized the actuating plate returns to the initial position under the action of actuating plate spring.
Friction feed mechanism
The paper is held by two papers feed rollers and by print cover. The paper feed motor is driven to rotate the platen gear, via the paper feed reduction gear which feeds the paper due to friction from the paper feed rollers and the platen.
Because the paper is held against the platen by the spring force of the paper feed rollers, the paper can be released by manipulating the paper release lever to its forward settings.
Tractor feed mechanism
When using the sprocket (tractor mechanism), holes of the paper are set over the sprocket pins along the sprocket wheel. The paper feed motor is driven to rotate the gears, via the pinion on the shaft of the motor. With the sprocket feed method, the paper feed rollers against the platen is released by manipulating the release lever to its forward settings.
Ribbon feed mechanism
The ribbon feed mechanism consists of the ribbon cartridge and the ribbon feed section. A ribbon driving gear turns counter-wise, via the following gear trains, irrespective of the direction of the timing belt.
Home position sensor
The home position sensor functions to determine the home position of the carriage and sends a reference signal for print start position. The home position sensor is a mechanical switch, which is operated by lever at the bottom of the frame. When the detection plate pushes the lever of the sensor, the mechanical switch closes
Paper end sensor
The paper end sensor detects the presence/absence of paper. The paper end sensor mechanism consists of a mechanical switch, which is operated by paper end sensor lever.
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