CMOS full duplex transmission-reception circuit

A data input/output circuit for full duplex communication incorporated in each of a plurality of digital apparatus in order for data to be transmitted among said digital apparatuses, comprising:

• a data accepting and sending circuit including an input/output terminal connected to a data processor provided within said digital apparatus, receiving transmitted data from said data processor, transmitting said data and receiving received data from the external through said input/output terminal;
• a threshold reference circuit dividing a voltage level of said data transmitted from said data accepting and sending circuit and producing a divided certain voltage level;
• a differential receiving circuit including one input terminal connected to said input/output terminal and the other input terminal connected to said threshold reference circuit, and being not operated by a zero voltage difference between said one input terminal and said other input terminal when said data is transmitted from said input/output terminal but operated by a voltage difference between said one input terminal and said other terminal when said data from the external is received through said input/output terminal, said received data being supplied to said data processor of said digital apparatus;
• an output impedance reference circuit for generating a predetermined value of the output impedance of said data accepting and sending circuit;
• and
• a voltage regulation circuit for adjusting at least a power supply voltage to be applied to said data accepting and sending circuit so that an output impedance value of said data accepting and sending circuit is changed in accordance with said predetermined value of said output impedance.