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KX-TEA308NE
11.4. INTERCOM CIRCUIT
11.4.1. INTERCOM CIRCUIT
1) Composition
This is composed of the following circuits:
a) +15V power source for the extension telephones
b) Hook detect for SLT and pulse dialing detect
c) Bell ring trip section
2) Circuit operation
a) Power supply to the telephone
With the telephone off hook, a DC loop is formed, and current is supplied to the telephone. This circuit is limited to about 30mA
by Q200, Q202, R202, R204 and Q201, Q203, R203, R205.
+27V→R204→R202→Q200→RL201→telephone→RL201→Q201→R203→R205→AG
b) Hook detect for SLT and pulse dialing detect
When the telephone handset is taken off, DC loop is formed and the collector of U200 3,6 pin change to L from H.
And IC7 detects the off hook condition.
When the handset is replaced back on hook, the DC loop is interrupted and collector of U200 3,6 pin change to H from L and
EXT Pulse dialing is input either in the on hook or off hook condition, and the break number (on hook condition) is counted and
read as the dial number.
c) Bell ringing trip section
When the telephone is a signal line telephone, extension calling is executed by means of a ringing signal. When the ringing
signal is supplied, RL201 turns ON and the current flows are as follows:
Bell generation circuit → ringing signal line → RL201 → telephone → R206 → AG → Bell generation circuit
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