Nortel NT1R20 Description page 55

Description Page 31 of 290
The line interface unit (Figure 10) has a relay that applies the ringing voltage
onto the phone line. The RSYNC signal from the 20 Hz (nominal) ringing
voltage power supply is used to prevent switching of the relay during the
current peak. This eliminates switching glitches and extends the life of the
switching relay.
The off-hook detection circuit monitors the current draw on the phone line.
When the current draw exceeds a preset value, the circuit generates an
off-hook signal that is transmitted back to the system controller.
The message waiting circuit on message waiting line cards monitors the status
of the message waiting signal and applies –150 V dc power to the tip lead
when activated. This voltage is used to light the message waiting lamps on
phones that are equipped with that feature. The high voltage supply is
automatically disconnected when the phone goes off-hook. Later line cards
are able to sense when the message waiting lamp is not working and can then
report that information back to the system controller.
Line cards Description