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6.2.5.
HOOKSWITCH (SEE Fig. 20)
T8 is the hookswitch, driven by T9. When the phone is “off-hook”, the HOOK control signal from the BBIC will be a high logic level
(+2.8V), and both transistors will be on, thus T8 will “loop” the line. The zenner diode D10 protects transistors T11 to T13 against
transient line voltages.
(+2.8V), and both transistors will be on, thus T8 will “loop” the line. The zenner diode D10 protects transistors T11 to T13 against
transient line voltages.
6.2.6.
PULSE DIALING (SEE Fig. 20)
During pulse dialing the hookswitch (T8, T9) is used to generate the pulses using the HOOK control signal, which is set high
during pulses. To force the line impedance low during the “pause” intervals between dial pulses, the PULSE-DIAL signal turns
on T11.
during pulses. To force the line impedance low during the “pause” intervals between dial pulses, the PULSE-DIAL signal turns
on T11.
6.2.7.
AUDIO CIRCUITS (SEE Fig. 20)
The loop current (typically 40mA) passes through T13, R57 and R56. R54 and R55 provide dc biasing for the base of T13.
The line output signal from the BBIC RXAF is DC decoupled by C47 and amplified by T13. The emitter load (R57, R58 and C46)
is complex in order to achieve the right frequency response with the complex line impedance.
is complex in order to achieve the right frequency response with the complex line impedance.
The line input signal TXAF is taken from the junction of R41 and R49. Phase cancellation is provided at this point by R70 so
that only the incoming line audio should be passed to the BBIC on TXAF.
that only the incoming line audio should be passed to the BBIC on TXAF.
6.2.8.
DISCONNECT (SEE Fig. 20)
The disconnect (DSC) circuit detects whether a second telephone receiver has been picked up while the TAM is operating on
the same loop.
the same loop.
The second telephone will cause the current through the unit to drop. This is manifests as a reduced voltage across R56 which
is detected at the AD3 input port (pin 140) ofthe BBIC.
is detected at the AD3 input port (pin 140) ofthe BBIC.
Circuit Diagram
18
KX-TCD725RUM
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