Panasonic KX-TG6700BXB Service Manual page 21

4.5.7. Parallel Connection Detect 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 idle
state; to catch the empty line automatically.
Circuit Operation:
The circuit is same as Auto Disconnect circuit.
But this circuit is controlled by inverse RLY signal. In on-hook state, the input signal logic is "high", then the transistors (Line 1:
Q156 and Q136, Line 2: Q256 and Q236) turn on.
As a result, the circuit gain is different from off-hook state.
If the input signal to DSP is 1.65 V or higher, DSP will judge there is no use of the telephones connected in parallel.
Transformation Characteristics
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KX-TG6700BXB/KX-TGA670BXB