Telephone Line Interface - Panasonic KX-TCD412NZM Service Manual

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 R2
- B C3 R1
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 D2
R29 IC2 (DLP) [BELL & CLIP]
R30 IC2 (DLP) [BELL & CLIP]
Q2 R8 Q3 D2 B [OFF HOOK]
54