Patent ID: 8704527

Claim:
A comparison judgment circuit configured to judge the level of a signal received from a communication partner device via a transmission line, comprising: an input/output terminal connected to the transmission line; an attenuator circuit configured to attenuate the voltage at the input/output terminal so as to generate an attenuated voltage; and a level comparator configured to compare the attenuated voltage with a predetermined threshold voltage, and to generate a level judgment signal that corresponds to the comparison result; wherein the attenuator circuit comprises: a first resistor having a first terminal and a second terminal, arranged such that the first terminal thereof is connected to the input/output terminal; and a second resistor having a first terminal and a second terminal, arranged such that the first terminal of the second resistor is connected to the second terminal of the first resistor, and such that a first voltage is applied to the second terminal of the second resistor, wherein the voltage that occurs at a node that connects the first and second resistors is output as the attenuated voltage; and wherein the comparison judgment circuit further comprises a protection circuit configured to monitor the attenuated voltage at the node that connects the first and second resistors, and to forcibly cut off or change the attenuated voltage input to the level comparator when the voltage to be monitored deviates from a predetermined voltage range.