The DD identifies which common practice commands are supported as well as the format and structure of all device-specific commands. HART Device Description Language (DDL) is used to write the DD, that combines all of the information needed by the host application into a single structured file. The DD includes all of the information needed by a host application to fully communicate with the field device. Some HART host applications use device descriptions (DD) to obtain information about the variables and functions contained in a HART field device. Communication occurs using standard instrumentation grade wire and using standard wiring and termination practices. HART Communication occurs between two HART-enabled devices, typically a smart field device and a control or monitoring system. The HART Protocol can be used in various modes such as point-to-point or multidrop for communicating information to/from smart field instruments and central control or monitoring systems. HART technology is a master/slave protocol, which means that a smart field (slave) device only speaks when spoken to by a master. As the digital FSK signal is phase continuous, there is no interference with the 4-20mA signal. The HART Protocol communicates at 1200 bps without interrupting the 4-20mA signal and allows a host application (master) to get two or more digital updates per second from a smart field device. This enables two-way field communication to take place and makes it possible for additional information beyond just the normal process variable to be communicated to/from a smart field instrument. A minimum loop impedance of 230 W is required for communication.

The digital communication signal has a response time of approximately 2–3 data updates per second without interrupting the analog signal. Because the average value of the FSK signal is always zero, the 4–20 mA analog signal is not affected. Sine waves of these two frequencies are superimposed on the direct current (dc) analog signal cables to provide simultaneous analog and digital communications. The digital signal is made up of two frequencies- 1,200 Hz and 2,200 Hz representing bits 1 and 0, respectively. The HART communication protocol is based on the Bell 202 telephone communication standard and operates using the frequency shift keying (FSK) principle.

Hart multi tool