Panasonic KX-TC1503B - 900 MHz Digital Cordless Phone, Service Manual, Page: 41 / 86

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, and press the TALK key onto the handset. 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.
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.