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Specification:
- Print system: in-line thermal head
- Paper width: 58 mm
- Command for automatic paper cutter
- Resolution: 4 dots / mm (192 dots / line)
- Print speed: max 55 mm / sec
- Interface: parallel (Centronics) or serial (RS232C) from PC or other device
- Power supply: 24V DC
- Print barcodes: no
- Print labels: no
- Operating temperatures: + 10ºC ... + 35ºC
For economy, this PCB was designed to function with only one communication interface: serial or parallel. In this way, when is delivered with serial interface, connector X1 is not soldered on PCB board, switch S1 is present and when is delivered with parallel interface, switch S1 is not soldered on board, connector X1 is present. Also for the economy, I used the 68 pin microcontroller version mounted on PLCC socket.
The thermal head of the printer consists of 192 heat elements and head drivers
to control the heat points. Serial printing data input from the interface is transferred to the shift register synchronously with the CLK signal, then stored in the latch register with the timing of the LATCH signal. Inputting the head activation signal (DST 1 to 3) activates heat elements in accordance with the printing data stored in the latch register. The printer mechanism has a built-in head driver IC. Applied energy is in accordance with the temperature of the thermal head and the environment the printer is used in. The thermal head has a built-in thermistor. The interface measures the temperature using thermistor resistance. Standard applied energy is based on a temperature of 25°C. The interface calculates the printing energy using equation:
E = (0.160 - TC × (TX - 25) )
TX: Detected temperature using the thermistor (°C)
TC : Temperature coefficient 0.00192
To prevent deterioration in printing quality due to backlash of the paper feed system, the motor is rotated 20 steps anticlockwise (the backlash absorption step should be input) when seen from the motor gear side, after initialization, after opening or closing the platen block, or after cutting thermal paper with the autocutter or tear bar (hand cutter). Start printing is performed after such operation. To stop the motor, the interface command for 5 ms with a phase that is the same as the final one in the printing
step. The resistance of the thermistor at the operating temperature is calculated with a liniar equation, 100 KΩ at 0°C and 2KΩ at 100°C. Abnormal thermal head temperatures are detected by both hardware and software. Paper detection is performed by a reflection type photo interrupter. The interface does not activate the thermal head and motor when there is no paper. The head activation pulse width is calculated with the formula:
t = ( ( E x R) / V² ) x C x D
t: Head pulse width (ms)
E: Standard applied energy (mJ)
V: Applied voltage (V)
R: Head resistance (Ω)
C: Head pulse term coefficient
D: Heat storage coefficient
V = Vp x (1.2 - 1.3)
Vp: Head activation voltage
R = ( ( RH +16 + ( Rc + rc ) x N )² / RH
RH: Head averange resistance, typical 176Ω
16: Wiring resistance in thermal head (Ω)
Rc: Common terminal wiring resistace in thermal head, typical 0.05Ω
rc: Wiring resistance between Vp and GND (Ω)
N: number of dots driven simultaneously, typical 192/3
C = 1 - 0.5 / ( 0.8 + W )
W: activation pulse for the previous block, 1000/motor frequency
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