Panasonic KX-TDA0174, Service Manual, Page: 24 / 76

8.9. RING TRIP FEATURE
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 electrical resistance of a telephone", "Intercom bell
load resistance" (R203A-P:1K?), 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?)
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.
8.10. SURGE PROTECTION
ZNRs installed between T and R, and between ToR and FG protect from extraneous surge.
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