Abstract

Arrangement for dc signaling between a transmitter or transceiver over a single pair of conductors to another transceiver or receiver of similar design. The conductors carry a dc communications voltage at a level Vcomm (e.g., 24 to 35 volts dc) and power for the tranceivers, receivers and/or transmitters of the system. The transceiver comprises a microprocessor having a signal input terminal Vin, a signal output terminal Vout, a sensor input circuit coupled to a sensor transducer, e.g., a temperature sensor, and a control output circuit coupled to control an external device, e.g., a furnace relay. A voltage limiting circuit device coupled to the pair of conductors supplies the microprocessor with a supply voltage Vdd at a predetermined level (e.g., 5 volts dc) less than the communications voltage level Vcomm. A signal input/output circuit coupled to the pair of conductors and to the microprocessor signal input and output terminals applies incoming pulse signals from the conductors to the input terminal, and applies onto the dc conductors output pulse signals. The output signals have high and low values respectively at the dc communications voltage level Vcomm and a predetermined voltage level VZ2 (e.g., 10 to 12 volts) between the communications voltage Vcomm and the supply voltage Vdd. The signal input/output circuit can comprise a zener and a transistor.