13.5. Telephone Line InterfaceTelephone Line Interface Circuit:Function•••• Bell signal detection•••• ON/OFF hook and pulse dial circuit•••• Side tone circuitBell signal detection and OFF HOOK circuit:In the idle mode, Q141 is open to cut the DC loop current and decrease 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 → C111 → Q111 → DSP pin 3 [BELL]When the CPU (DSP) detects a ring signal, Q141 turns on, thus providing an off-hook condition (active DC current flow throughthe circuit). Following signal flow is the DC current flow.T → L101 → D101 → Q141 → Q161 → R163 → D101 → L102 → P101 → RON HOOK Circuit:Q141 is open, Q141 is connected as to cut the DC loop current and to cut the voice signal. The unit is consequently in an on-hook condition.Pulse Dial Circuit:Pin 4 of DSP turns Q141 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 of DSP.54KX-TG2448BXF / KX-TG2448BXS / KX-TGA254BXF / KX-TGA254BXS
