# DID line interface signaling

There are no dependencies between DID transmit signaling and DID receive signaling.

## DID transmit signaling

The following illustration shows transmit signaling for DID line interfaces:

This table summarizes the transmit signaling for DID line interfaces:

Bit | Hex bitmask | Description
--- | --- | ---
A bit | 0x08 | Reverse battery polarity. 0 = Normal polarity, tip positive. 0x08 = Reverse polarity, tip negative.
B bit | 0x04 | Must be 0.
C bit | N/A | Reserved.
D bit | N/A | Reserved.

## DID receive signaling

The following illustration shows receive signaling for DID line interfaces:

This table summarizes the receive signaling for DID line interfaces:

Bit | Hex bitmask | Description
--- | --- | ---
A bit | 0x08 | Loop current detector. 0 = No current detected. 0x08 = Current detected.
B bit | 0x04 | Same as A bit.
C bit | N/A | Reserved.
D bit | N/A | Reserved.