Parallel Connection Detect Circuit - Panasonic KX-TG5431BXS Service Manual

KX-TG5431BXS / KX-TGA542BXS

18.8. 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
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 Q136 is ON, the voltage is monitored at pin 35 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 Q136 is OFF, the voltage is monitored at pin 35 of IC501; the presence/absence of a parallel connection is
determined when the voltage changes by 0.2 V or more.
76