Intercom Mode; Telephone Line Interface - Panasonic KX-TG5571BXM Service Manual

KX-TG5571BXM / KX-TGA550BXM

12.5. Intercom Mode

1. Press the Base Unit INTERCOM button, and the handset number button ([1] to [8]), then a monitor tone is heard from the
speaker.
2. At the same time, the Handset ring. This status is called "Intercom stand-by".
3. Then press TALK button of the Handset, the status is changed to "Intercom mode".

12.6. Telephone Line Interface

Telephone Line Interface Circuit:
Function
• Bell signal detection
• ON/OFF hook and pulse dial circuit
• Side tone circuit
Bell signal detection and OFF HOOK circuit:
In the idle mode, Q141 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 → L101 → R111 → C111 → Q111 → DSP pin 3 [BELL]
When the CPU (DSP) detects a ring signal, Q141 turns on, thus providing an off-hook condition (active DC current flow through
the circuit). Following signal flow is the DC current flow.
T → L101 → D101 → Q141 → Q161 → R163 → D101 → L102 → P101 → R
ON HOOK Circuit:
Q141 is open, Q141 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:
Pin 4 of DSP turns Q141 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 Q161 is
canceled by the canceller circuit of DSP.
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