21.3. Telephone Line Interface
<Function>
- Bell signal detection
- ON/OFF hook circuit
- Audio circuits
Bell 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 R29 IC2 (DLP) [BELL]
- B C3 R1 R30 IC2 (DLP) [BELL]
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 Q2 R8 Q3 D2 B [OFF HOOK]
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