Panasonic KX-TG6700B Service Manual page 17

4.5.5. Telephone Line Interface
Telephone Line Interface Circuit:
Function
• Bell signal detection
• ON/OFF hook and pulse dial circuit
• Side tone circuit
4.5.5.1. Line 1
Bell signal detection and OFF HOOK circuit:
In the idle mode, Q141 and Q142 are open to cut the DC loop current and 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 → PC111 → DSP pin 72 [BELL]
To provide an off-hook condition (active DC current flow through the circuit), CPU turns Q145 on.
Thus Q141 and Q142 turn on. Following signal flow is the DC current flow.
T → L101 → D101 → Q141 → Q161 → R163 → Q142 → 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 on-hook
condition.
Pulse Dial Circuit:
Pin 73 of DSP turns Q145 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 DSP.
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KX-TG6700B/KX-TG6702B/KX-TGA670B