Two wire interface circuit

An interface circuit having:

• a first reference terminal;
• a first input terminal for receiving a first input current signal referenced to said first reference terminal from a first input current signal source;
• at least a second input terminal for receiving a second input current signal referenced to a second reference terminal from a second input current signal source, said second input current signal having a first and second signal current level;
• said interface circuit comprising:
• first receiver means coupled between said first input terminal and a first receiver reference terminal (71), said first receiver means being responsive to said first input current signal first and second signal current levels for providing a first output signal at respective first and second levels, said first output signal being referenced to said second reference terminal (GND2) at a first signal output terminal, said first receiver means having:
• a first receiver means threshold detector having a sense resistor coupled between a terminating terminal (52) and said first receiver reference terminal (71) and a switch for coupling a threshold detector output terminal (41) to said first receiver reference terminal in response to said current through said sense resistor exceeding a predetermined limit;
• a first receiver coupler circuit having a first receiver coupler circuit sensing diode having an anode and a cathode and a current limiting resistor coupled from said first input terminal to said current sense diode anode, said sensing diode cathode being coupled to said threshold detector terminating terminal (52), said coupler circuit also having a first receiver coupler circuit output transistor having a base optically coupled for receiving light from said current sense diode in response to current passing from said current sensing diode anode to said current sense diode cathode, said first receiver coupler circuit output transistor having a conduction channel coupled between said first signal output terminal (51) and said second reference terminal (GND2), said coupler circuit being characterized to transfer said conduction channel to a conductive state in response to current passing through said sensing diode from said sense diode anode to cathode and said current exceeding a predetermined threshold level;
• a first receiver shunt switch circuit having a switching transistor having a control terminal and a conduction channel having a collector terminal and an emitter terminal, said switching transistor collector terminal (56) being coupled to said first receiver coupler circuit sensing diode anode an said output switch emitter terminal being coupled to said coupler circuit sensing diode cathode (52), said switching transistor control terminal being coupled to said interface circuit first input terminal via a resistor, said transistor switch conduction channel being characterized to be conductive in response to a first input current signal second level sourcing current from said interface circuit first input terminal to said switching transistor control terminal, said transistor switch conduction channel being characterized to be substantially non-conductive in response to said first input current signal assuming a first level;
• and
• a second receiver means coupled between said first receiver reference terminal and said first reference terminal, said second receiver means being responsive to said second input current signal having a value greater than a first predetermined level for decreasing the value of resistance between said first receiver reference terminal and said first reference terminal to a value less than a first predetermined value and for increasing the value of resistance between said first receiver reference terminal and said first reference terminal to be greater than a second predetermined value in response to said second input current signal being greater than a second predetermined level, the value of input resistance between said first receiver reference terminal and said first reference terminal being substantially independent of said first input current signal first and second signal current levels.