Dsc Circuits; Logic Circuits; Power Supply Circuits - Icom IC-M304 Service Manual

4-4 DSC CIRCUITS

4-4-1 DSC ENCODE CIRCUIT
(LOGIC UNIT AND MAIN UNIT)
The DSC signal is created at the CPU (IC1, pin 9). The
signal passes through the buffer amplifier (IC6A, pins 1, 3),
and is then applied to the D/A convertor (R176–R179) to
convert into the analog signal. The analog signal passes
through the low-pass filter (MAIN unit; IC10B, pins 5–7),
and is then applied to the VCO circuit to modulate the DSC
signal.
4-4-2 DSC DECODE CIRCUIT
(MAIN UNIT AND LOGIC UNIT)
The AF signal from the FM IF IC (IC1, pin 9) are filtered at
the low-pass filter (Q5, R52–R58, C83–C86) to suppress
unwanted higher noise signals. The filtered signals are
converted analog signals into digital signals at the DSC
decoder IC (IC3), and are then applied to the CPU (LOGIC
UNIT; IC1, pin 22) via the "DSCDEC" signal.
4-5 LOGIC CIRCUITS (LOGIC UNIT)
• CPU
IC1 is an 8 bit single chip micro-computer and contains LCD
driver, serial I/O, timer, A/D converter, programmable I/O,
ROM and RAM. The CPU controls to display characters on
the LCD too.
• SYSTEM CLOCK CIRCUIT
X1 is a ceramic oscillator and oscillate 7.9872 MHz system
clock for the CPU (IC1).
• RESET CIRCUIT
IC5 which is a reset IC outputs a reset signal ("HIGH"
pulse signal) to the CPU (IC1, pin 33) from pin 1 when the
transceiver turns power ON.
• LOW BATTERY DETECTOR
"HVS" voltage is divided by R14 and R15, and is applied to
the low battery detector section in the CPU (IC1, pin 1).
• DIMMER CIRCUIT
CPU (IC1), Q2, Q3 and Q4 compose dimmer circuit. The
circuit controls 4 steps LCD backlight (DS2–DS7).
• UART PORT
IC-M304 has the UART port which is used for cloning data
and NMEA interface. The signals pass through the photo
coupler circuit (IC2), and are then applied to the buffer
amplifier (IC3, pin 2). The amplified signals are applied to
the CPU (IC1, pin 20).

4-6 POWER SUPPLY CIRCUITS

Line
HV/
The voltage from the connected DC power supply.
HVS
Common 5V converted from the HV line at the L5V regulator
(LOGIC UNIT; IC9).
5V
The converted voltage is applied to the CPU (LOGIC
UNIT; IC1)
Transmit 5V controlled by the T5 control circuit (Q31, Q32)
T5
using the "SEND" signal from CPU (LOGIC UNIT; IC1, pin 52).
Receive 8 V converted from the VCC line at the R8 regulator
R8 (Q35, Q36).
The converted voltage is applied to the receiver circuits.
Receive 5V controlled by the R5 control circuit (Q33, Q34)
using the "RCV" signal from CPU (LOGIC UNIT; IC1,
R5
pin 53).
The controlled voltage is applied to the receiver circuits.
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Description