18
TK-785
CIRCUIT DESCRIPTION
■
Key Matrix Circuit
The TK-785 front panel has ten keys. Each of them is
connected to a cross point of a matrix of the KEY1 to KEY7
ports of the microprocessor. The KEY5 to KEY7 ports are
always high, while the KEY1 to KEY4 ports are always low.
The microprocessor monitors the status of the KEY1 to
KEY7 ports. If the state of one of the ports changes, the
microprocessor assumes that the key at the matrix point
corresponding to that port has been pressed. Unused
points (KEY1 to KEY7) are also used for foot switch (FSW)
input.
■
Encode
The QT, DQT signals are output from LSDO of the CPU
(IC511) and go to the D/A converter (IC5) of the TX-RX unit.
The DTMF and single tone signals are output from HSDO of
the CPU and goes to the audio processor (IC504). An MSK
signal is output from the audio processor according to the
data (AFDAT) from the CPU. The signal is summed with a
MIC/MSK signal by the audio processor (IC504), and the re-
sulting signal passes through an analog switch (IC506) and
goes to the TX-RX unit (MO).
MO is summed with the external pin DI line by the sum-
ming amplifier (IC3) and the resulting signal goes to the D/A
converter (IC5). The D/A converter (IC5) adjusts the MO
level and the balance between the MO and TO levels. Part
of a TO signal is summed with an output signal from pin 3
(MO) of IC5 and the resulting signal goes to the MD pin of
the VCO. This signal is applied to a varicap diode in the VCO
for direct FM modulation.
X1
VCXO
IC5
D/A
A1
VCO
IC3
SUM
AMP
IC506
Analog
SW
IC504
Audio
processor
IC300
PLL
MB
MD
MO
TO
HT
DI
MIC
TX-RX UNIT
AFDAT
LSDO
HSDO
IC511
CPU
Fig. 12
Encode
Q512
SW
Q508
SW
Q509
SW
Q513
SW
KBLC
LG
LR
IC508
Shift
register
D511
GRN
RED
D512~517
LEDA
Fig. 10
Display circuit
IC511
CPU
KEY7
KEY6
KEY5
KEY4
KEY3
KEY2
KEY1
(CLEAR)
A
D
CALL
SYS
DOWN
SYS
UP
B
C
VOL
UP
VOL
DOWN
FSW
Q511
Fig. 11
Key matrix circuit
■
Display Circuit
The CPU (IC511) controls the shift register (IC508) and
display LEDs. When the LG line goes high when the trans-
ceiver is busy, Q508 turns on and the green LED on D511
lights. In transmit mode, the LR line goes high, Q509 turns
on and the red light lights. Backlighting LEDs for the key
operation unit (D512~D517) and LCD are provided.
When the KBLC line goes high, Q512 turns on, then
Q513 turns on, and the key illumination LED lights. A volt-
age is applied to the LEDA line to turn on the LCD backlight.
■
Decode
The signal (DEO) detected by the TX-RX unit passes
through two low-pass filters of IC501, goes to LSDI of the
CPU (IC511) to decode QT, DQT.
The MSK signal passes through IC504. IC504, includes
band-pass filter and MSK decoder, converts the input signal
into a square waveform (0 and 5V). This square waveform is
then fed to AFRDT line of CPU (IC511).
IC511
CPU
IC501(2/2)
LPF
IC501(1/2)
LPF
LSDI
IC507
DTMF
DECO.
STD
AFRDT
DEO
IC504
AUDIO
PROC.
Fig. 13
Decode
