Basics of the HART Communication Protocol -Working principle ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

Basics of the HART Communication Protocol -Working principle

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The worldwide accepted standard for analog signals in the measurement industry is the 4 to 20mA signal. One key drawback of this signal standard is that it can only transmit one parameter or measured value.

A two-way or bidirectional communication protocol, which addresses this drawback, has been developed with which additional information can be transmitted using an alternating current signal superimposed on the 4 – 20mA analog signal. This system is called the HART communication protocol. HART stands for: Highway Addressable Remote Transducer

The HART communication protocol has become a widespread solution, allowing for convenient and efficient parameterization of smart (intelligent) measuring devices. Additionally, device-specific diagnostic data can be read which provides information about the device's physical health and allow for predictive maintenance. Monitoring various device parameters is also possibility with the HART protocol.

How the HART Protocol Works:
The FSK (Frequency Shift Keying) procedure is the basis for the HART communication.
The HART digital signal is made up of two frequencies — 1,200 Hz and 2,200 Hz representing bits 1 and 0, respectively as shown below:

Sine waves of these two frequencies (shown in the diagram above) are superimposed on the direct current (dc) analog signal to provide simultaneous analog and digital communications. Because the average value of the FSK signal is always zero, the 4‐20mA analog signal is not affected. The HART protocol is often called a hybrid protocol because it combines analog and digital communication. A typical HART setup in a 4 -20mA system is shown below:
In this setup, the HART devices (PC or handheld communicator) requests information from the field device(HART compatible). The field device supplies the information which can then be used for condition monitoring, diagnostics, predictive maintenance and any other use that is required of the digital information supplied to the HART device. Meanwhile, the analog 4 -20mA signal is active and undistorted and can be used for control purposes while digital communication with HART is on-going.

Wireless HART
The HART standard has evolved over the years since its first introduction and is now offering a new stage with completely new possibilities for wireless transmission of HART information through a technology called WirelessHART.

WirelessHART is the first standardized wireless communication in the field of process automation. However, as this solution does not include the connection cable, only the digital parameter range is available. The analog measuring signal is not provided. Digital wireless communication does away with cabling thus, allowing for easy and cost-saving installation of additional measuring points for diagnostic purposes.

Basically, there are two different WirelessHART solutions:
(a) WirelessHART Adapter for enhancing existing HART devices
(b) Self-powered Wireless HART transmitter.

These wireless solutions use the central element of wirelessHART transmission - the WirelessHART gateway - for communication. Besides the protocol, the standard also defines various security mechanisms which ensure availability and tap-proof wireless signal transmission.