Telephone Line Interface - Panasonic KX-TCD505CXV Service Manual

22.3. 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, Q4 is open to cut the DC loop current and decrease the ring load.
When ring voltage appears at the TP21 (A) and TP19 (B) leads (when the telephone rings), the
signal is transferred as follows;
- A C13 R17
- B C12 R16
ON/OFF hook circuit:
In the standby mode, Q4 is open, and connected as to cut the DC loop current and to cut the
voice signal. The unit is consequently in an on-hook condition.
When IC2 detects a ring signal or press the TALK Key onto the handset, Q5 turns on and then
Q4 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 D3 Q4
R24 IC2 Pin 119 (CID IN +)
R32 IC2 Pin 112 (CID IN -)
R34 Q8 R45 R46 D3
80
B [OFF HOOK]