Table of Contents
Advertisement
6.2. THE LINE INTERFACE SECTION (SEE BLOCK DIAGRAM Fig. 17)
6.2.1. INTRODUCTION
This section consists of the telephone line interface, bell detector, charge-pulse detector, hook
switch, pulse dialing circuits, audio circuits, DC mask & line impedance circuits, power
supplies, and battery charger circuits.
6.2.2. TELEPHONE LINE INTERFACE (SEE Fig. 19)
The telephone line is connected to a bridge rectifier D8. Surge suppressor SA3 protects against
excessive line voltages. Test points are TP40 (A), TP26 (B). Bridge rectifier D8 provides for lines
of either polarity. The output of D8 is "Line +" (TP50) and "Line -" which is ground.
6.2.3. BELL DETECTOR (SEE Fig. 19)
The AC ringing signal is detected by phoptocoupler IC2, using its internal diode in conjunction
with D4. DC from the line is blocked by C2. The other components D2, D3, and R3 reduce
current and increase the circuit impedance in line with national requirements. When ringing is
detected IC2 will turn on, and the RING line will be dragged to a low voltage.
6.2.4. CLIP CIRCUITS (SEE Fig. 19)
The CLIP signal is detected by IC3 and sent to the BBIC through the /TXAF line as a square
waveform. The CLIP_STATE signal from the BBIC is used to provide a CLIP impedance through
22
Advertisement
Table of Contents
