Northern Telecom NT5K17, Description, Page: 20 / 38

2-6 NT5K18 Exchange Line Trunk Card
553-3001-173 UK Trunk cards description
Call placed by Meridian 1
The Meridian 1 places a call by applying ground to the B wire and the
Public Exchange acknowledges by applying ground to the A wire. The
Meridian 1 then places the trunk unit into the on-line state (low impedance
loop), and removes the ground from the B wire. When the Public
Exchange is ready to receive digits, it returns dial tone. The Meridian 1
sends digits in the form of Dual Tone Multifrequency (DTMF) tones or
dial pulses.
Public Exchange disconnects first
The Public Exchange disconnects a call by removing the ground on the A
wire of the trunk. The Meridian 1 detects the drop in current and responds
by returning to the idle state.
Meridian 1 disconnects first
The Meridian 1 disconnects a call by removing its on-line state and
drawing only a very low current from the A and B wires.
Loop start operation
Loop start operation is configured in software and is implemented in the
card through software download messages.
Idle state
When the Exchange Line trunk is idle, it provides a high impedance toward
the public exchange for isolation and dc detection. The public exchange
applies -50V dc on the B wire of the trunk and ground on the A wire.
Call placed by Public Exchange
The public exchange initiates a call by applying ringing to the B wire and -
50 V dc to the A wire. The Meridian 1 ringing detector is tripped and the
Exchange Line trunk card place a low resistance dc loop between the A and
B wires. This low resistance loop serves as a called subscriber answer
signal (CSA) to the public exchange. The public exchange responds by
removing the ringing signal and returning the A and B wires to their
normal conditions.
Call placed by Meridian 1
To place a call from a telephone set or attendant console, the Meridian 1
places a low resistance loop across the A and B leads. When the public
exchange is ready to receive digits, it returns dial tone. The Meridian 1
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