# Subscriber loop line interfaces

There are three states to which the subscriber loop module may be set:

State | Description
--- | ---
Off | The A and B bits of the transmitted stream are set to 0 (off). No battery is applied to the line and no ringing is applied. Audio can not be passed through the system.
Idle or Active | The A bit is set to 1. This is required for the audio path to function. The B bit is set to 0. The board is active and applying battery to the line. The loop current detector is active and audio passes through the system. This is the normal operation state for the subscriber loop module.
Ringing | The A bit and the B bit are set to 1. This state is used to close the ringing relay when it is desired to ring the attached line/phone. The B bit must set the ring signal cadence. Ring-trip is done automatically by the hardware.

## Subscriber loop transmit signaling

The following illustration shows transmit signaling for subscriber loop line interfaces:

This table summarizes the transmit signaling for subscriber loop line interfaces:

Bit | Hex bitmask | Description
--- | --- | ---
A bit | 0x08 | Puts battery voltage on tip/ring pair. 0x08 = Apply battery. 0 = No battery.
B bit | 0x04 0x00 | Applies ringing voltage/signal to tip/ring. No ring.
C bit | N/A | Reserved (should be 0).
D bit | N/A | Reserved (should be 0).

## Subscriber loop receive signaling

On the receive signaling path, the A bit indicates whether or not the loop current is flowing. When the current flows on the line and the port is in the active state or the idle state, the A bit will be high. When the port is in the ringing state and ring-trip occurs, the A bit will go high.

The following illustration shows receive signaling for subscriber loop line interfaces:

This table summarizes the receive signaling for subscriber loop line interfaces:

Bit | Hex bitmask | Description
--- | --- | ---
A bit | 0x08 | Denotes loop current is flowing. This bit is used to determine when the far-end is off-hook. 0x08 = Current is flowing (off-hook). 0 = No current is flowing (on-hook).
B bit | 0x04 | Same as A bit.
C bit | N/A | Reserved (should be ignored).
D bit | N/A | Reserved (should be ignored).

Off-hook conditions during ringing is detected in 50 to 150 ms.