SIGNAL TRACING DECISION CIRCUIT

PURPOSE:To prevent the unstable operation due to noise at the time of no signal is inputted by switching a reference voltage to a decision circuit depending on the signal input and no signal input. CONSTITUTION:When no signal IN is inputted to a reception module 1, a base- emitter voltage -VBE of a transistor (TR) 10 is inputted to a terminal B of a line receiver 13. On the other hand, since the input to a carrier sense 16 is at a high level, a TR 12 is turned on and a reference voltage VTH inputted to a terminal C of the line receiver 13 is decided by resistors 7, 9, the output Vout of the line receiver 13 is stopped by selecting the voltage VTH larger than an output voltage VS of the TR 10. In increasing the difference between the voltages VTH and VS larger than the noise component superimposed on the voltage VS, the unstable operation due to the noise component is prevented.