Current mode I/O for digital circuits

A digital input/output interface for use with two digital circuits connected by a transmission line having a characteristic impedance Z.sub.o, including:

• a current driver in one of the digital circuits comprising a high impedance field-effect transistor connected to the transmission line in an open-drain configuration and configured to generate a relatively small current mode signal in the transmission line when a digital signal is applied to the current driver;
• and
• an impedance transforming receiver in the other digital circuit including:
• a transimpedance amplifier having a low input impedance Z.sub.in connected to the transmission line at an input node and configured to receive the relatively small current mode signal,
• a high-impedance load element connected to the field-effect transistor and configured to convert the relatively small current mode signal into an output binary voltage having a noise margin large enough for digital communication, and
• a feedback amplifier configured to adjust the input impedance Z.sub.in to match the characteristic impedance Z.sub.o of the transmission line based on changes in voltage at the input node.

A method of communicating between a first digital circuit and a second digital circuit over a transmission line having a characteristic impedance Z.sub.o, the second digital circuit having an input impedance Z.sub.in that actively terminates the transmission line, the method including:

• generating a current in the transmission line in response to a digital signal in the first digital circuit;
• receiving the current in the second digital circuit;
• converting the current into an output voltage representing the digital signal;
• and
• adjusting the input impedance Z.sub.in of the second digital circuit to match the characteristic impedance Z.sub.o of the transmission line.

A method of communicating between a first digital circuit and a second digital circuit through a transmission line having a low characteristic impedance, including:

• generating a relatively small current in the transmission line in response to a digital signal in the first digital circuit;
• receiving the relatively small current in the second digital circuit through a low input impedance that matches the low characteristic impedance of the transmission line;
• and
• passing the relatively small current through a high impedance element to generate an output voltage having a noise margin large enough for digital communication.