Line interface unit retrofit circuit

A retrofit circuit for communicating one of three conditions from a line interface unit to a receiver unit, with an external connection between said line interface unit and said receiver unit, said line interface unit having a detection circuit responsive to a received signal on said line interface unit using extended superframe format and with no yellow alarm for outputting a first input signal at a first terminal, responsive to the received signal on said line interface unit using extended superframe format and with yellow alarm for outputting a second input signal at the first terminal, responsive to the received signal on said line interface unit using superframe format and with no yellow alarm for outputting a third input signal at a second terminal, and responsive to the received signal on said line interface unit using superframe format and with yellow alarm for outputting a fourth input signal at the second terminal, comprising:

• status means responsive to the first input signal at the first terminal and the third input signal at the second terminal for outputting a first status signal at said external connection, responsive to the second input signal at the first terminal and the third input signal at the second terminal for outputting a second status signal at said external connection, and responsive to the second input signal at the first terminal and the fourth input signal at the second terminal for outputting a third status signal at said external connection;
• and
• comparator means having a first threshold and a second threshold, responsive to the first status signal at said external connection exceeding the first threshold for outputting a first output signal at a first output terminal and a third output signal at a second output terminal, responsive to the second status signal at said external connection between the first threshold and the second threshold for outputting a second output signal at the first output terminal and a third output signal at the second output terminal, and responsive to the third status signal at said external connection below the second threshold for outputting a second output signal at the first output terminal and a fourth output signal at the second output terminal.

A retrofit circuit comprising:

• status means responsive to a first input signal at a first terminal and a third input signal at a second terminal for outputting a first status signal at an external connection, responsive to a second input signal at the first terminal and the third input signal at the second terminal for outputting a second status signal at the external connection, responsive to the first input signal at the first terminal and a fourth input signal at the second terminal for outputting the second status signal, and responsive to the second input signal at the first terminal and the fourth input signal at the second terminal for outputting a third status signal at the external connection;
• and
• comparator means having a first threshold and a second threshold, responsive to the first status signal at the external connection exceeding the first threshold for outputting a first output signal at a first output terminal and a third output signal at a second output terminal, responsive to the second status signal at the external connection between the first threshold and the second threshold for outputting a second output signal at the first output terminal and a third output signal at the second output terminal, and responsive to the third status signal at the external connection below the second threshold for outputting a second output signal at the first output terminal and a fourth output signal at the second output terminal.

A method using a retrofit circuit for communicating one of three conditions from a line interface unit to a receiver unit, using an external connection between said line interface unit and said receiver unit, said line interface unit having a detection circuit responsive to a communication signal using extended superframe format and with no yellow alarm for outputting a first input signal at a first terminal, responsive to the communication signal using extended superframe format and with yellow alarm for outputting a second input signal at the first terminal, responsive to the communication signal using superframe format and with no yellow alarm for outputting a third input signal at a second terminal, and responsive to the communication signal using superframe format and with yellow alarm for outputting a fourth input signal at the second terminal, comprising the steps, using said retrofit circuit, of:

• outputting to the external connection a first status signal in response to the first input signal at the first terminal and the third input signal at the second terminal;
• outputting to the external connection a second status signal in response to the second input signal at the first terminal and the third input signal at the second terminal;
• outputting to the external connection a third status signal in response to the second input signal at the first terminal and the fourth input signal at the second terminal;
• outputting a first output signal at a first output terminal and a third output signal at a second output terminal connection;
• outputting a second output signal at the first output terminal and a third output signal at the second output terminal in response to the second status signal at the external connection;
• and
• outputting a second output signal at the first output terminal and a fourth output signal at the second output terminal in response to the third status signal at the external connection.