KX-TS2356CAB, KX-TS2356CAW - Panasonic Telephone Service Manual (repair manual). Page 8

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Circuit Operation
Bell Detector Circuit
When the bell signal is input from LINE to T/R (when the telephone rings), the signal is output at the buzzer/speaker via the fol-
lowing path :T / R 
→ R1 / C1 → D1 / LED1(2AP) → IC1 (pin1) → IC1 (pin8) → PC1 → C5 → T1 → SW2 → Buzzer
Dummy Ringer Circuit
When [AOH] button is pressed during incoming call (Caller ID AOH mode), dummy ringer circuit will be activated.
The bell signal from LINE stops and Caller ID is received.  
The ringer will keep on ringing by the dummy ringer circuit until the handset is picked up.
The dummy ring signal is generated from IC801 via the following path:
IC801 (pin 57) 
→ Q5 → Q3 → SW2 → Buzzer
Line Interface
In talk status, SW101 become ON and Q102 base changes to high level, causing Q102, Q103 and Q101 to turn on and resulting
in a line loop.The loop current flows from D101(+) 
→ Q101 → Q107 → R119 → D103 in that order,  A pulse signal that repeated
switches between high and low logic is output from pin 73 of the CPU.This switches the line loop on and off, generating the dial
pulse signal.
Telephone Line Interface
Circuit operation
• On hook
Q101 is OFF, on-hook condition where DC loop is disconnected and voice signal flow is cut.
• Off hook
Q101 is ON*, providing an off-hook condition (active DC current flow through the circuit) and the following is the signal flow:
→ POS101 → L101 → D101(+) → Q101 → Q107 → R119 → D103 → D101(-) → L102 → R
*  Handset mode : SW101 ON after handset off-hook
• The receiving signal flows:
→ Q101 → C109 → C401 → R401 → Q401 → Q402 → Q403 → SP (handset)
• The transmission signal flows
MIC (handset) 
→ Q422 → R433 → C432 → Q107 → Q101 → LINE
• Pulse Dial
A pulse signal switches repeatedly between high and low logic is output from IC801 (pin 73), on and off the line loop, generating
the pulse dial singal.
Tone Detect
This circuit is used to sense the status of the line (busy tone or dial tone) during dummy ring generation.
Circuit operation
→ Q101 → C201 → R201 → R205 → Pin 5 of IC201 → Pin 1 of IC201 → D202 → R208 → Q201→ Pin 87 of IC801
When the subscriber hangs-up, check the intermittent tone.  If cycle tone is detected, the collector of Q201 goes to a low logic.
Initializing Circuit
This circuit is used for to initialize the microcomputer when it incorporates batteries.
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