13.5. Telephone Line InterfaceTelephone Line Interface Circuit:Function•••• Bell signal detection•••• ON/OFF hook and pulse dial circuit•••• Side tone circuit13.5.1. Line 1Bell signal detection and OFF HOOK circuit:In the idle mode, Q104 and Q142 are open to cut the DC loop current and the ring load. When ring voltage appears at the Tip(T) and Ring (R) leads (When the telephone rings), the AC ring voltage is transferred as follows:T → L101 → R111 → PC111 → DSP pin 3 [BELL]To provide an off-hook condition (active DC current flow through the circuit), CPU turns Q145 on.Thus Q104 and Q142 turn on. Following signal flow is the DC current flow.T → L101 → D101 → Q104 → Q161 → R164 → Q142 → D101 → L102 → P101 → RON HOOK Circuit:Q104 is open, Q104 is connected as to cut the DC loop current and to cut the voice signal. The unit is consequently in on-hookcondition.Pulse Dial Circuit:Pin 4 ofDSP turns Q145 ON/OFF to make the pulse dialing.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 Q161 iscanceled by the canceller circuit ofDSP.53KX-TG2480BXS / KX-TGA248BXS
