US Patent 7952359 Test apparatus having bidirectional differential interface

First and second resistors are provided between a first input/output terminal and a power supply terminal, and between a second input/output terminal and the power supply terminal, respectively. Third and fourth resistors are connected to the second and first input/output terminals, respectively. First and second current-switching switches couple either the first input/output terminal side or the second input/output terminal side with a first current source and a second current source, respectively, according to the value of pattern data. A level shift circuit shifts the electric potentials at the second terminals of the third and forth resistors by a predetermined level. A comparator circuit compares the electric potentials at the second terminals of the third and fourth resistors level-shifted by the level shift circuit with those at the second terminals of the fourth and third resistors, respectively, and generates first and second comparison signals according to the comparison results.