Telephone Line Interface; Auto Disconnect Circuit - Panasonic KX-TG2226B Service Manual

17.6. Telephone Line Interface

Telephone Line Interface Circuit:
Function
- Bell signal detection
- ON/OFF hook and pulse dial circuit
- Side tone circuit
- Auto-disconnect circuit/Parallel connection detection circuit
Bell signal detection and OFF HOOK circuit:
In the idle mode, Q4 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, R L1, L2 R30, R31 C16, C17 Q6 DSP (BELL) [BELL]
When the CPU (DSP) detects a ring signal, Q4 turns on, thus providing an off-hook condition
(active DC current flow through the circuit) and the following signal flow is for the DC line current.
T D4 Q4 Q1 R5 D1 D4 L2 POS1 R [OFF HOOK]
ON HOOK Circuit:
Q4 is open, Q4 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:
DSP (RYL) turns Q4 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 Q1 is canceled by the echo canceller of DSP.
Note: DSP is IC201.
See CPU DATA (BASE UNIT) ()

17.7. Auto Disconnect Circuit

Function:
This circuit is used to detect the fact that another telephone connected to the same line is OFF-
HOOK while the unit is in a receiving status or OGM transmitting status.
Circuit Operation:
The voltage DSP (DCIN1) is monitored. If a parallel-connected telephone is put into an OFF HOOK
status, the presence/absence of a parallel connection is determined when the voltage changes
by 0.2V or more.
When the set detects the parallel-connected telephone is OFF HOOK status, the line is
disconnected.
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