Nortel NT1R20 Description page 65

Loop Start Mode
In Loop Start mode , the A and B bits have the following meaning:
Transmit from LTI:A bit = 0 (tip ground on)
B bit = Ringing (0=on, 1=off)
Receive to LTI: A bit = Loop (0=open, 1=closed)
B bit = 1 (no ring ground)
When a T1 channel is idle, the line-side T1 card simulates a ground on the tip
lead and -48Vdc on the ring lead to the terminal equipment by setting its
transmit A bit to 0 and transmit B bit to 1. Accordingly, an on-hook channel
on the terminal equipment simulates an open loop toward the line-side T1
card, causing the line-side T1 card's receive bits to be set to A = 0 and receive
B = 1.
Incoming calls to terminal equipment attached to the
Incoming calls
line-side T1 card can originate either from stations that are local (served by
the Meridian 1 PBX), or remote (served through the public switched
telephone network). To provide the ringing signal to a telephone the line-side
T1 card simulates an additional 90V on the ring lead to the terminal
equipment by alternating the transmit B bit between 0 and 1 (0 during ring on,
1 during ring off). When an incoming call is answered by the terminal
equipment going off-hook, the terminal equipment simulates tripping the
ringing and shutting off ringing, causing the line-side T1 card's receive A bit
to be changed from 0 to 1.
During outgoing calls from the terminal equipment, a
Outgoing calls
channel is seized when the station goes off-hook. This simulates a
low-resistance loop across the tip and ring leads toward the line-side T1 card,
causing the line-side T1's receive A bit to be changed from 0 to 1. This bit
change prepares the line-side T1 to receive digits. Outward address signaling
is then applied from the terminal equipment in the form of DTMF tones or
loop (interrupting) dial pulses that are signaled by the receive A bit pulsing
between 1 and 0.
Description Page 41 of 290
Line cards Description