Nortel Networks Circuit Card - Manual (Page 40)

Page 40 of 906 Overview
553-3001-211 Standard 3.00 August 2005
(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. Newer line cards
can sense when the message waiting lamp is not working and can report that
information back to the system controller.
Digital line interface units
The NT8D02 digital line card provides voice and data communication links
between a switch and modular digital telephones. These lines carry
multiplexed PCM voice, data and signaling information as Time
Compression Multiplexed (TCM) loops. Each TCM loop can be connected to
a Nortel “Meridian Modular Digital” telephone.
The digital line interface card contains one or more digital line interface units.
See Figure 5 on
page 41 . Each digital line interface unit contains a Digital
Line Interface Circuit (DLIC). The purpose of each DLIC is to demultiplex
data from the DS-30X Tx channel into integrated voice and data bitstreams
and transmit those bitstreams as Bi-Polar Return to Zero, Alternate Mark
Inversion (BPRZ-AMI) data to the TCM loop. It also does the opposite:
receives BPRZ-AMI bitstreams from the TCM loop and multiplexes the
integrated voice and data bitstream onto the DS-30X Rx channel.
The 4-wire to 2-wire conversion circuit converts the 2-wire tip and ring leads
into a 4-wire (Tx and ground and RX and ground) signal that is compatible
with the digital line interface circuit.
TCM loop interfaces
Each digital phone line terminates on the digital line card at a TCM loop
interface circuit. The circuit provides transformer coupling and foreign