Nortel Circuit Card, Installation Manual, Page: 41 / 908

Overview Page 41 of 906
Circuit Card Description and Installation
voltage protection between the TCM loop and the digital line interface circuit.
It also provides power for the digital telephone.
To prevent undesirable side effects from occurring when the TCM loop
interface cannot provide the proper signals on the digital phone line, the
system controller can remove the ±15 V dc power supply from the TCM loop
interface. This happens when either the card gets a command from the
NT8D01 Controller card to shut down the channel, or when the digital line
card detects a loss of the 1 KHz frame synchronization signal.
Each TCM loop interface circuit can service loops up to 3500 ft. in length
when using 24 gauge wire. The circuit allows for a maximum ac signal loss
of 15.5 dB at 256 KHz and a maximum DC loop resistance of 210 ohms.
Signaling
The digital line interface units also contain signaling and control circuits that
establish, monitor, and take down call connections. These circuits work with
Figure 5
Digital line interface unit block diagram
4-wire
to
2-wire
conversion
Digital
line
interface
circuit
TCM loop
interface
and
protection
Tip
Ring
DS-30X loop
Tx PCM
1 kHz frame
sync
Rx PCM
553-6154
±15 V dc power supply