Abstract: A refrigeration system for temperature conditioning several comfort zones includes several VAV (variable air volume) valves each having a hot water coil conveying water whose flow rate is regulated by a PWM (pulse-width modulated) solenoid valve. Each VAV valve is connected to a supply air duct conveying cool supply air. When a zone's temperature is above a set point temperature, the opening of the VAV valve is modulated to meet the cooling demand, and the water coil is shut off. When a zone's temperature is below the set point, the VAV valve is opened to provide a predetermined constant airflow rate and the hot water coil's solenoid valve is cycled open and closed in a pulse-width modulated manner to meet the heating demand.

Abstract: The variable opening of a pressure independent supply air valve for a building ventilation system is controlled in response to a standard airflow transducer whose output is modified to emulate a close tolerance transducer. In a calibration mode, a microcomputer based control closes the valve and a summing amplifier reduces the transducer output by an amount equal to a compensation signal provided by the microcomputer. Once the output is reduced to a predetermined desired level, the value of the compensation signal is stored in memory for later use during a run mode. In the run mode, the actual supply airflow rate is determined based upon the transducer output less the compensation signal. And the control adjusts the position of the valve until the actual airflow rate meets a desired airflow rate as determined by a room thermostat.

Abstract: A microprocessor-based frequency shift keying (FSK) data communications module. The microprocessor selectively modulates and demodulates data directly in terms of a square wave pulse train in which a first frequency is used to convey a binary "1" and a second frequency to convey a binary "0". An input and an output port on the microprocessor are selectively coupled to a transmission line through a transformer, by means of electronic switches. Also included are a bandpass filter for shaping the square waves into sine waves and for filtering noise, and means including a notch filter for generating a carrier detect signal input to the microprocessor, when incoming data is detected on the transmission line. Many of the components in the module are used for both transmitting and receiving data.