How Hart Protocol Carries AC and DC Signals Simultaneously Check out the full blog post over at https://realpars.com/hart-protocol/ With HART, the analog 4-20 milliamp signal and the digital HART protocol are both available to the control system. The AC HART sine wave oscillates at either 1200 Hertz for a 1 value or at 2200 Hertz for a 0 value. The amplitude of the AC sine wave remains the same, and for every oscillation, the amplitude of the first half of each sine wave above the DC current cur