Panasonic KX-TC1503B - 900 MHz Digital Cordless Phone, Service Manual, Page: 42 / 86

12.7. Auto Disconnect Circuit
Function:
This circuit is used to detect the fact that another telephone connected to the same line is OFF-
HOOK while the unit is in a receiving status or OGM transmitting status.
Circuit Operation:
The voltage DCIN of DSP is monitored. If a parallel-connected telephone is put into an OFF
HOOK status, the presence/absence of a parallel connection is determined when the voltage
changes by 0.2V or more.
When the set detects the parallel-connected telephone is OFF HOOK status, the line is
disconnected.
12.8. 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 Q7 is ON, the voltage is monitored DCIN of DSP. 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 Q7 is OFF, the voltage is monitored DCIN of DSP; the presence/absence of a
parallel connection is determined when the voltage changes by 0.2 V or more.