Signal selection circuit

A signal selection circuit, including a scan bus, for normally maintaining the scan bus at essentially the same voltage as one of two limiting signals of a plurality of first direct voltage signals which are normally variable within an operating voltage range between a maximum operating voltage and a minimum operating voltage, the two limiting signals being a maximum, i.e., most positive, signal of the plurality of first signals and a minimum, i.e., most negative, signal of the plurality of first signals, the signal selection circuit comprising:

• a first direct voltage source for providing a first constant reference voltage disposed outside the operating voltage range of said first signals and displaced from the one limiting signal in the same direction as the other limiting signal is displaced from the one limiting signal;
• a second direct voltage source for providing a second constant reference voltage disposed outside the operating voltage range of said first signals such that the operating voltage range of said first signals is disposed between said first and second reference voltages;
• alarm indicating means for providing indication whenever the scan bus voltage is disposed between the operating voltage range of said first signals and the second reference voltage;
• a load resistor connected between the first voltage source and the scan bus;
• a plurality of detector circuits having first inputs respectively connected to receive said first signals, second inputs connected to receive said second reference voltage, and first outputs connected to scan bus, wherein each detector circuit includes
• an operational amplifier with high voltage gain, having an output, a non-inverting input connected to the detector circuit first input to receive one of said first signals, and an inverting input connected to the detector circuit first output to receive the scan bus voltage,
• a diode, connected between the amplifier output and the detector circuit first output to allow current to flow therethrough in one direction so that, when the first signal being received at the amplifier non-inverting input becomes said one limiting signal, the amplifier output voltage is abruptly switched to a voltage which forward-biases the diode and allows sufficient current flow through the load resistor to maintain the scan bus voltage at the amplifier inverting input essentially equal to the one limiting signal at the amplifier non-inverting input,
• a first resistor connected between the amplifier output and the detector circuit second input receiving the second reference voltage, for forward-biasing the diode in the absence of an amplifier output voltage and allowing sufficient current flow through the load resistor to maintain the scan bus voltage at a voltage intermediate the operating voltage range of said first signals and the second reference voltage;
• whereby the alarm indicating means provides indication that the one limiting signal of said first signals is an abnormal signal disposed outside the normal operating voltage range of the first signals, or that no output voltage is being generated by one of the operational amplifiers of the detector circuits.