Telephone Line Interface - Panasonic KX-TG2650N Service Manual

12.10. Telephone Line Interface

Circuit Operation:
ANSWER / In the idle mode, Q121 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 R103 C103 Q101 IC501 pin 64. /
When the CPU (DSP) detects a ring signal, Q121 turns on, thus providing an off-hook condition
(active DC current flow through the circuit) and the following signal flow is for the voice signal. /
T D101 Q121 Q131 R138 C140 D101 L102 PO101 R
ON HOOK / Q121 is open, Q121 is connected as to cut the DC loop current and to cut the voice
signal. The unit is consequently in an on-hook condition.
SPECIFICATIONS / In the on-hook state (idle), the current flows between the telephone line and
the unit is as follows: / T L101 R103 C103 R106 C104 R104 L102
PO101 R. / The DC component is blocked by C103, C104: thereby providing an on-hook
condition. / The AC interface impedance is over 47 k ; thus, satisfying the telephone company
requirements.
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