Panasonic KX-TG4771B Service Manual page 13

4.4.5. 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 → R111 → C111 → Q111 → BBIC pin 59
When the CPU (BBIC) 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 → D101 → Q902 → Q161 → R163 → R167 → D908 → D909 → Q913 → D101 → P101 → R
ON HOOK Circuit:
Q902 is open, Q902 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 6 of BBIC turns Q902 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 BBIC.
4.4.6. Parallel Connection Detect Circuit/Auto Disconnect Circuit
Function:
In order to disable call waiting and stutter tone functions when using telephones connected in parallel, it is necessary to have a
circuit that judges whether a telephone connected in parallel is in use or not. This circuit determines whether the telephone
connected in parallel is on hook or off hook by detecting changes in the T/R voltage.
Circuit Operation:
Parallel connection detection when on hook:
When on hook, the voltage is monitored at pin 17 of IC501. There is no parallel connection if the voltage is
0.54 V or higher, while a parallel connection is deemed to exist if the voltage is lower.
Parallel connection detection when off hook:
When off hook, the voltage is monitored at pin 18 of IC501; the presence/absence of a parallel connection is determined by
detecting the voltage changes.
If the Auto disconnect function is ON and statuses are Hold, receiving ICM, OGM transmitting, BBIC disconnects the line after
detecting parallel connection is off hook.
KX-TG4771B/KX-TG4772B/KX-TG4773B/KX-TGA470B
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