Interface circuit having receiving side circuit for controlling logical threshold values

An interface circuit having a transmitting side circuit, a receiving side circuit and a transmission path connecting the transmitting side and receiving side circuits in a transmission system for transmitting a predetermined signal from the transmitting side to the receiving side, said receiving side circuit receiving a transmitting signal, a supply voltage signal and a ground potential signal, said receiving side circuit comprising:

• a receiver circuit having a first circuit which has different logical threshold values for directions of level changes of the object signal and controls said different logical threshold values and a second circuit which is set with an operating characteristic in which a threshold value corresponding to a high level is lower than a predetermined reference value on the high-level side and a threshold value corresponding to a low level is higher than a predetermined reference value on the low-level side and has a characteristic inverse to that of said first circuit;
• reference voltage generating means for generating high level and low level control signals for controlling logical threshold values from the the supply voltage signal and the ground potential signal;
• control signal deciding means for deciding whether the high level and low level control signals are equal to predetermined voltage levels;
• high-level deciding means responsive to the high-level control signal and the supply voltage signal to decide whether the supply voltage signal is equal to a predetermined high level;
• low-level deciding means responsive to the low-level control signal and the the ground potential signal to decide whether the ground potential signal is equal to a predetermined low level;
• and
• supply level deciding means for determining whether an output signal of said control signal deciding means indicates that the high level and low level control signals are equal to the predetermined voltage levels, an output signal of the high-level deciding means indicates that the supply voltage signal is equal to the predetermined high level, and an output signal of the low level deciding means indicates that the ground potential signal is equal to the predetermined low level, and delivering a decision result.