Panasonic KX-FP342BX Service Manual page 153

7.11.2. CIRCUIT OPERATION
The TAM INTERFACE circuit consists of an EXT. TAM HOOK detection circuit, CNG signal from the other party's detection circuit,
VOX detection circuit (to judge sound/no-sound) and RLY101 (to separate EXT. TAM).
1. EXT. TAM HOOK detection circuit
A bell is received at EXT. TAM and EXT. TAM is connected to the line, making a DC LOOP. Then, PC105 detects this voltage.
During detection, PC105 (4) becomes low.
(DC LOOP)
Tip → L102 → PC105 (2, 1) → L105 → Tip1 → (EXT.TAM) → Ring 1 → L104→ RLY101(5, 3) → L103 → POS101 → Ring
2. CNG/VOX signal detection circuit
The CNG signal from the other party's FAX is detected in MODEM IC505 (digital board).
(Signal path)
Refer to CHECK SHEET (P.105).
3. Remote receiving
This is the parallel-connected DTMF signal for the TEL or EXT.TEL mode between T and R. When the other party is a FAX,
the unit switches to FAX receiving.
(Signal Path)
Detects the DTMF signal in the MODEM.
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KX-FP342BX