Auto Disconnect Circuit; Parallel Connection Detect Circuit - Panasonic KX-TG2216LAS Service Manual

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)

16.7. 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.
Note:
DSP is IC201.
See CPU DATA (BASE UNIT)

16.8. Parallel Connection Detect 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.
It is like an "Auto Disconnect Circuit".
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