KX-NCP1174XJ (serv.man2) - Panasonic PBX Service Manual (repair manual). Page 12

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The off-hook status is detected when the telephone is off-hook. When a telephone is off-hook, change in voltage is detected by the
RING terminal in HIC, and this information is transmitted to the HOOK terminal (Example: 9-pin IC200A) as OFF-HOOK=L/ON
HOOK=H signal. This digital signal is received by CODEC IC (Example: 35-pin IC201) and recognized as off-hook signal. The off-
hook status is analyzed by software. 

When the BELL signal is sent out, the off-hook signal from a SLT is detected, and then it breaks the BELL relay.
When the handset of a ringing telephone is picked up, a DC loop is formed in the sequence of "Original bell signal", "Internal electri-
cal resistance of a telephone", "Intercom bell load resistance" (R203A-P:1K

Ω), and then "Ground" (Since DC30V superimposed in

the BELL signal). As a result of this, a voltage rise occurs in Intercom bell load resistance" (R203A-P:1K

Ω) in the intercom circuit,

and a voltage is placed on the 9-pin RING terminal. This change in voltage turns off the driver transistor of the BELL relay in HIC,
and it breaks the BELL relay.