Nortel NT1R20 Description page 57

Description Page 33 of 290
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
voltage protection between the TCM loop and the digital line interface circuit.
It also provides power for the digital telephone set.
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 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
the system controller to operate the digital line interface circuits during calls.
The circuits receive outgoing call signaling messages from the controller and
return incoming call status information to the controller over the DS-30X
network loop.
Line cards Description