Connected processing systems including mutual power off and signal path disconnect detection

In a first control unit comprising a first logic circuit and a first interface circuit for the first logic circuit and including a first power terminal, for connecting to a similar second control unit comprising a second similar logic circuit and a second similar interface circuit for the second logic circuit and including a second power terminal, said interface circuit for allowing communication between the two logic circuits, each of said logic circuits having a logic output terminal and providing a logic level output signal thereat, and a logic input terminal, and wherein each logic output terminal voltage floats when its associated logic circuit is not receiving power, wherein each of said interface circuits includes:

• a neutral terminal connected to the neutral terminal of the similar interface circuit;
• a data transmitter circuit having an output signal terminal and a variable impedance connected between the output signal terminal and the neutral terminal and having a control terminal receiving the logic level output signal from the logic output terminal of the logic circuit, the impedance of said variable impedance following in an inverse manner the voltage difference between the neutral terminal and the control terminal;
• a data receiver circuit having an input signal terminal for providing a voltage to the output signal terminal of the similar second interface circuit, and receiving at the input signal terminal an output data signal comprising a varying voltage whose level is controlled by the impedance of the variable impedance in the data transmitter circuit of the similar second interface circuit, and an output terminal providing to the logic input terminal a logic level input signal having a voltage whose level corresponds to voltage levels in the output data signal, wherein the first interface circuit comprises
• a) a pull-up resistor in the data receiver circuit connected between the power terminal and the input signal terminal;
• and
• b) a bias resistor connecting the output data terminal to the variable impedance's control terminal,
• whereby loss of voltage at the second power terminal allows current flow from the pull-up resistor to flow through the bias resistor in the second interface circuit's transmitter circuit to bias the control terminal of the variable impedance therein and pull its voltage away from the voltage at the neutral terminals, reducing the impedance of the variable impedance in the similar second interface circuit, and reducing the voltage at the input signal terminal to a level creating a constant logic level input signal to the logic input terminal.