Panasonic KX-TC1743B, Service Manual

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4. The transmission signal flows:
MIC C235, C236 R213, R214 pins 46 and 47 of IC201 RF
13.6. Telephone Line Interface
Telephone Line Interface Circuit:
Function
- Bell signal detection
- ON/OFF hook and pulse dial circuit
- Side tone circuit
- Auto-disconnect circuit/Parallel connection detection circuit
Bell signal detection and OFF HOOK circuit:
In the idle mode, Q4 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 L1 R30 C16 Q6 DSP pin 27. [BELL]
When the CPU (DSP) detects a ring signal, Q4 turns on, thus providing an off-hook condition
(active DC current flow through the circuit) and the following signal flow is for the voice signal.
T D4 Q4 R1/R2 C1 R3 R5 D1 D4 L2 POS1 R
ON HOOK Circuit:
Q4 is open, Q4 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:
DSP (2) turns Q4 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 Q1 is canceled by the chancellor circuit of DSP.
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