Importance

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Abstract

A retrofit circuit for communication one of three conditions from a line interface unit (LIU) to a receiver unit (RU), using only one external connection between the LIU and the RU. The LIU has a detection circuit and two resistors. The RU has a voltage comparator. The detection circuit outputs a first input signal in response to a signal having ESF and no yellow alarm, a second input signal in response to a signal having ESF and yellow alarm, a third input signal in response to a signal having SF with no yellow alarm, and a fourth input signal in response to a signal having SF with yellow alarm. The resistors output to the external connection a first voltage level in response to the first and third input signals, a second voltage level in response to the second and third input signals, and a third voltage level in response to the second and fourth input signals. The voltage comparator outputs a first output signal and a third output signal in response to detecting the first voltage level at the external connection, a second output signal and a third output signal in response to detecting the second voltage level at the external connection, and a second output signal and a fourth output signal in response to detecting the third voltage level at the external connection.

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