Telephone Line Interface - Panasonic KX-TCD410AXM Service Manual

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20.3. Telephone Line Interface

<Function>
- Bell signal detection
- Clip signal detection
- ON/OFF hook circuit
- Audio circuits
Bell & Clip (: Calling Line Identification Presentation: Caller ID) signal detection:
In the standby mode, Q2 is open to cut the DC loop current and decrease the ring load.
When ring voltage appears at the TP3 (A) and TP40 (B) leads (when the telephone rings), the signal is
transferred as follows;
- A C4
- B C3
ON/OFF hook circuit:
In the standby mode, Q2 is open, and connected as to cut the DC loop current and to cut the voice
signal. The unit is consequently in an off-hook condition.
When IC2 detects a ring signal or press the TALK Key onto the handset, Q3 turns on and then Q2
turns on, thus providing an off-hook condition (active DC current flow through the circuit) and the
following signal flow is for the loop current.
- A R77
R2 R29 IC2 (DLP) [BELL & CLIP]
R1 R30 IC2 (DLP) [BELL & CLIP]
D2 Q2 R8
Digitized in Heiloo, Holland
Q3 D2 B [OFF HOOK]
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11/8/2014