Panasonic KX-TG6440PK, Service Manual

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KX-TG6440PK/KX-TGA641S/KX-TGA641T/KX-TGA740B
4.4.7. Parallel Connection Detect 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 con-
nected 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 86 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:
When off hook, the voltage is monitored at pin 90 of IC501; the presence/absence of a parallel connection is determined when
the voltage changes by 0.2 V or more.