Citizen CL-E720DT, Technical Manual

Page: 39 / 189

Chapter 2 Operating Principles
2-2.
Operation of Control Parts
2-25
CL-E720DT
Thermal resistance check:
When the printer is turned ON, the thermal resistance check is conducted for the “SA Head”. If
any fault is found, the LED on the operation panel blinks in red and the LCD displays “Alarm
Head Check”.
During the thermal resistance check, +24 DC (VHD) to the “SA Head” stops, but +5V DC is
supplied instead as pin E5 (VHCON) of the FPGA goes to “High” level and Q2 and Q1 turn
ON.
The following is a simplified circuitry under checking, where Q1 turns ON and the first thermal
element “R1” is selected. At the point “A”, the voltage divided by R52 and the thermal element
resistance R1 is developed. The CPU (U1A) monitors this voltage at pin 94 (HDRES), and
checks if the voltage is within the allowable range. (For example, if R1 is broken and
disconnected, the voltage at point “A” (pin 94, HDRES) will be about +3.3V as the line is
clamped by the diode D1.)
Each thermal element is successively checked in this way.
(3)
Buzzer driver
This circuit drives the buzzer.
Buzzer will sound at the time of key operation or error occurrence. To sound the buzzer, the
CPU (U1A) outputs a pulse from pin 137 (BUZZER). The transistor Q3 turns ON and OFF, and
the buzzer sounds.
+5.0V
R52
R1
A
R1, R2, ... ,Rn: Resistance of thermal elements
Q1
Thermal Head
HDRES
AN0
94
+3.3V
R2
OFF
CPU
U1A
D1
D2
VHD
[SA Head]
ON
CPU
BUZZER
+24V
[SA Main PCB]
P9_5
137
R56
Q3
DTC114EM
BZ1
U1A