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7

KX-DT321 uses a 16 characters by 1 line LCD.

Power is supplied to LCD from +5V rail, formed as follows:
Pin 24 of IC6 is made to switch, forming a square waveform which is input to IC8 pin 2. The output of IC8 pin 4 switches between
+3.3V and 0V. When the output is low, C603 charges via D61. When output is high, anode of D62 rises to 6V and C601 is charged
via D62. D63 limits the voltage across C601 to +5V. D5V powers the LCD and also IC10.

IC10 is a converter circuit to allow the 3V output of IC6 to drive the LCD, which uses 5V inputs.

LCD contrast is adjusted by means of voltage level supplied at pin 3 of LCD module.
It is adjusted to one of 3 settings by means of a bias circuit (R600, R622) and IC6 pins 19 and 59, according to the following table:

Data is sent to the LCD module via IC10 from the processor using a 3-wire interface (IC6 pins 13, 14, 15).

Contrast

IC6 pin 59

IC6 pin 19

High

L

L

Middle Z

L

Low

Z

L

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The lighting of the LEDS is controlled by pin 14 to 18 (Row) and pin 8 to 13 (Colum) of IC2.
The LED lights up in a dynamic lighting system.
The duty ratio is 1/8 (ON time 1.5ms).

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Reset Signal is outputted from IC7 and inputted into IC2, IC3 and IC6 when the TEL cord is plugged in.

10

Power to the digital telephone set is provided from the PBX.
D.C. voltage is fed via a split transformer (T1) to diode bridge D1.

The PSU circuit includes a switching regulator (IC1) to produce the +3.3V output rail.
Q4 control the power supply circuit to prevent IC1 from starting up until the PBX feeds a 40V supply to the set (some other sets
work at 15V). They reduce the current consumption whilst the PBX is applying 15V, allowing the PBX to progress to 40V supply.

• Input voltage 21V or less 

→ Q4 Off →  IC1 pin 2 High → IC1 Off.

• Input voltage 21V or more 

→ Q4 On →  IC1 pin 2 Low → IC1 On.