Panasonic KX-TG4011AGT, Service Manual

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KX-TG4011AGT/KX-TG4012AGT/KX-TG4013AGT/KX-TGA403AGT
4.4.5. Parallel Connection Detect Circuit/Auto Disconnect 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, the voltage is monitored at pin 17 of IC501. There is no parallel connection if the voltage is
0.54 V or higher, while a parallel connection is deemed to exist if the voltage is lower.
Parallel connection detection when off hook:
When off hook, the voltage is monitored at pin 18 of IC501; the presence/absence of a parallel connection is determined by
detecting the voltage changes.
If the Auto disconnect function is ON and statuses are Hold, BBIC disconnects the line after detecting parallel connection is off
hook.