Panasonic KX-TC1881B, Service Manual

Page: 53 / 96

13.8. Parallel Connection Detection Circuit
Function:
In order to disable call waiting and stutter tone functions when using telephones connected in
parallel, it is necessary to have a circuit that judges whether a telephone connected in parallel is
in use or not. This circuit determines whether the telephone connected in parallel is on hook or
off hook by detecting changes in the T/R voltage.
Circuit Operation:
Parallel connection detection when on hook:
When on hook Q141, Q142 (LINE 1), Q241, Q242 (LINE 2) is ON, the voltage is monitored pin 50
(LINE 1) and 43 (LINE 2) of IC501. There is no parallel connection if the voltage is 1.65 V or
higher, while a parallel connection is deemed to exist if the voltage is lower.
Parallel connection detection when off hook:
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