Telephone Line Interface; Parallel Connection Detect Circuit/Auto Disconnect Circuit - Panasonic KX-TGC210LAB Service Manual

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 5 [BELL]
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  Q141  Q161  R163  D101  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 6 of BBIC 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 BBIC.
+3.0V
C173
R172
680k
0.1u
C175
560p
Q171
Pin 28 of IC501
R121
R122
C120
1000p
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
KX-TGC210LAB/KX-TGC212LAB/KX-TGC213LAB/KX-TGC220LAB/KX-TGC222LAB/KX-TGC223LAB/KX-TGCA20LAB
R160
820
Q161
C
B
E
C163 NC
R177 R164 R161
1k 2.7k 100k
R151
100k
C184
1u
390k C121 680p
390k C122 680p
VDDC
*Q141
R141
100k
Q142
C
B
E
Hook Hook
Pin 6 of IC501
+3.0V
BELL BELL
Pin 5 of IC501
Q111
15
C101 680p
D101
1 2
4 3
C102 680p
NC
*C115
1
*R115
NC
2
L1T
*P101
L1R