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KX-TG4753B/KX-TG470B/KX-TGA405B
4.4.5.
Telephone Line Interface
Telephone Line Interface Circuit:
Function
• Bell signal detection
• ON/OFF hook and pulse dial circuit
• Side tone circuit
Bell signal detection and OFF HOOK circuit:
In the idle mode, Q141 is open to cut the DC loop current and decrease the ring load. When ring voltage appears at the Tip (T)
and Ring (R) leads (When the telephone rings), the AC ring voltage is transferred as follows:
T → L101 → R111 → C111 → Q111 → BBIC pin 59
When the CPU (BBIC) detects a ring signal, Q141 turns on, thus providing an off-hook condition (active DC current flow through
the circuit). Following signal flow is the DC current flow.
T → L101 → D101 → Q141 → Q161 → R163 → R167 → D101 → L102 → P101 → R
ON HOOK Circuit:
Q141 is open, Q141 is connected as to cut the DC loop current and to cut the voice signal. The unit is consequently in an on-
hook condition.
Pulse Dial Circuit:
Pin 6 of BBIC turns Q141 ON/OFF to make the pulse dialing.
Side Tone Circuit:
Basically this circuit prevents the TX signal from feeding back to RX signal. As for this unit, TX signal feed back from Q161 is
canceled by the canceller circuit of BBIC.
3.0V
R175
R172
C178
R178
C175
RX
Q171
Pin 16 of IC501
Hook
R181
Pin 6 of IC501
TX
Pin 28 of IC501
BELL
Pin 59 of IC501
Q161
C165
(
)
C163
(
)
C173
R164
R161
R177
C184
Q141
C101
D101
Q142
3
4
C102
3.0V
Q111
14
L101
2
1
L1T
L102
P101
L1R
TEL JACK
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