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becomes Low according to selected L1 (or L2), and Q130, Q131 (Q230, Q231) turned on and the
Line Switch for that line (Line 1, Line 2) closes.
Line current flows through Q130. As a result, a loop is formed through D101 (D201)
Emitter of
Q130 (Q230)
Collector of Q130 (Q230)
D131 (D231)
L101
R154
Collector of Q155
Emitter of Q155
R163
D152
Collector of Q131 (Q231)
Emitter of Q131 (Q231)
D130 (D230)
D101 (D201).
During a conference, the line current is the sum of the currents in the two lines.
If the line current exceeds 60 mA, the voltage across R154 increases, turning Q153 ON. As a
result, current is supplied from Q153 Collector
R157
R158
base of Q154, turning Q154
ON. Consequently, current flows through R156, preventing the DC resistance of the unit from
rising.
12.2. Reset Circuit
Function:
The reset circuit is a detection circuit which is used to detect the power supply voltage and apply
a reset to the microprocessor when the circuit changes from an ON-HOOK status to an OFF-
HOOK status.
Circuit Operation:
The reset conditions are as follows:
1. When AC Adaptor into outlet.
2. When the power given out;
a) ON-HOOK
OFF-HOOK
In the case of one of above conditions, a reset signal will be sent to the microprocessor IC801.
12.3. Tone Dial Circuit
44
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