KX-NSX1000, KX-NSX2000 (serv.man2) - Panasonic PBX Service Manual (repair manual). Page 14

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Part 4 contains a description of the circuit that transmits AC and DC signal.
(1) Alarm Circuit Power Source

Consisting primarily of Q105 and D114, the alarm circuit power source stabilizes 32V output voltage described in Part 2 to
approximately 27V.

(2) AC Alarm Signal Transmission Section

If AC input is detected by the AC voltage detection circuit described in Part 1 (5), Q111 is turned on by secondary side transistor
of PC3 being turned on, and a low signal is transmitted to AC alarm output. 
On the contrary, if AC input is not detected, Q111 is turned off by secondary side transistor of PC3 being turned on, and "open"
(open collector output) is transmitted to AC alarm output. 

(3) DC Alarm Signal Transmission Section

If 15V is outputted properly, IC102 is turned on, relaying to Q108 then to Q106 transmitting low signal to DC alarm output 
If there is a problem on 15V output, IC102 will turn off. Q108 and Q106, which in turn transmit open to DC alarm output.