KX-TDA1176X (serv.man2) - Panasonic PBX Service Manual (repair manual). Page 13

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2 out of 16 lines have switching relays (RL200A-B) installed in order to enable the outside call during a power failure. The line con-
nected to the outside call card will be directly connected to the CN201 located in forward part of the board via a 4-conductor TEL
cord. In the normal operation mode, it breaks the power failure switching relay, and during a power failure, it makes the power fail-
ure switching relay to initiate Power failure direct connection mode.

In ringing mode, the ringing DC offset and the ringing signal are summed digitally within CODEC in accordance with the pro-
grammed values. 
This signal is then converted to an analog signal and applies to the SLIC.
The SLIC amplifies the signal and supplies the line with 65 Vrms.
The bell signal is generated with +80V(VHR) and -30V (VBATL) supplied to SLIC.

In Message Waiting (MW) he MW signal is summed digitaly within CODEC in accordance with the programmed values.
The SLIC amplifies the signal and supplies the line with 90V.
The MW signla is generated with +80V (VHR) and -30V (VBATL) supplied SLIC.