KX-TDA100DUP - Panasonic PBX Service Manual (repair manual). Page 127

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KX-TDA100DUP
16.2.3.5. End-Call Signal Detection
This is detected by the photo coupler (pin 4 of PC4) at part of making DC loop. PC4 4-pin is High level during a call. However, at
end-call, PC4 4-pin converts from High to Low.
It detects an end-call with measuring of the signal width of Low level by CODEC (e.g. pin 35 of IC6, CPCDET).
The available setting values for detection width are none, 0.5msec~8msec(at 2msec intervals) and 16~896msec(at 8msec inter-
vals).
16.2.3.6. Polarity Reversal Detection
Configured by the photo couplers (PC1, PC2) that enable to detect each current in Tip
→Ring, Ring→Tip direction. (PC2 is used
in common as the bell detection photo coupler). Using Software, it stores the current direction (the detected photo coupler) at
acquiring line and recognizes the action by changing of the current direction (the detected photo coupler changes).
When the current flows in the direction of Ring to Tip, 4-pin of PC1 is Low and 4-pin of PC2 is High.
• Ring
→L1→R2→PC1(1-2)→D3→RL3→Q4(C-E)→D4→D3→L1→TH1→Tip
When the current direction (polarity) is reversed from Tip to Ring, 4-pin of PC1 turns to High and 4-pin of PC2 turns to Low. 
It recognizes the action by changing of the current direction (the detected photo coupler changes).
• Tip
→TH1→L1→D3→D4→Q4 (C-E)→RL3→D3→PC1 (1-2)→R2→L1→Ring
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