Panasonic KX-TG2559LBS, Service Manual

Page: 62 / 90

12.7. 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
→
R139/R140
→
C139
→
RX
•
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
Ω
; thus, satisfying the telephone company requirements.
62
KX-TG2559LBS