a single pair of polarity independent wires conveys electrical power and two - way communication between a remote thermostat and a temperature - conditioning unit . the system is functional even if the wires are crossed , so mis - wiring is nearly impossible . the system is selectively operable in three independently distinct modes : a power mode for conveying electrical power from the temperature - conditioning unit to the thermostat , an output mode for transmitting a communication signal from the temperature - conditioning unit to the thermostat , and a feedback mode for conveying a communication signal from the thermostat to the temperature - conditioning unit . when the power mode is inactive during the output mode and feedback mode , the thermostat relies on electrical power that the thermostat stored during a previous power mode . communication is provided by a current loop circuit that is generally immune to electrical noise and tolerant of wire impedance .