How does hart protocol work

Years of success using these benefits explain why HART technology is the largest of all communication protocols, installed in more than 30 million devices worldwide. It has undergone continued development, up to and including automation products now shipping with built-in Wireless HART Communication.

Frequency Shift Keying. The digital signal is made up of two frequencies— 1, Hz and 2, Hz representing bits 1 and 0, respectively. Sine waves of these two frequencies are superimposed on the direct current dc analog signal cables to provide simultaneous analog and digital communications.

Because the average value of the FSK signal is always zero, the 4—20 mA analog signal is not affected. The digital communication signal has a response time of approximately 2—3 data updates per second without interrupting the analog signal. A minimum loop impedance of W is required for communication.

The HART Protocol communicates at bps without interrupting the mA signal and allows a host application master to get two or more digital updates per second from a smart field device. As the digital FSK signal is phase continuous, there is no interference with the mA signal. Communication occurs using standard instrumentation grade wire and using standard wiring and termination practices.

The mA signal communicates the primary measured value in the case of a field instrument using the mA current loop — the fastest and most reliable industry standard. Additional device information is communicated using a digital signal that is superimposed on the analog signal. The digital signal contains information from the device including device status, diagnostics, additional measured or calculated values, etc.

Together, the two communication channels provide a low-cost and very robust complete field communication solution that is easy to use and configure. HART devices can operate in one of two network configurations—point-topoint or multidrop.

In point-to-point mode, the traditional 4—20 mA signal is used to communicate one process variable, while additional process variables, configuration parameters, and other device data are transferred digitally using the HART protocol Figure 2.

The 4—20 mA analog signal is not affected by the HART signal and can be used for control in the normal way. The HART communication digital signal gives access to secondary variables and other data that can be used for operations, commissioning, maintenance, and diagnostic purposes. The HART Protocol permits all digital communication with field devices in either point-to-point or multidrop network configurations:. Higher update rates are possible with this optional burst communication mode and use is normally restricted to point-to-point configuration.

The multidrop mode of operation requires only a single pair of wires and, if applicable, safety barriers and an auxiliary power supply for up to 15 field devices. All process values are transmitted digitally. Communication Modes. HART is a master-slave communication protocol, which means that during normal operation, each slave field device communication is initiated by a master communication device. Unlike the traditional mA protocol, the HART technology is a bi-directional communication protocol.

How does this protocol work? Read the next section to know the answer. The 1, Hz signal represents bit 1 and the 2, Hz signal represents bit 0. The data transfer takes place when sine waves of these frequencies superimpose on DC analog signal cables. During this transfer the mA signal is not affected because the average value of an FSK signal equals to zero. This enables the host application to receive two or three digital updates in a second, from a smart device.

The HART protocol supports two communication channels at a time — mA analog signal and digital signals. The digital signals can be any information such as the operational status of the device, diagnostics, any measured value, and so on. The protocol is used in two modes, which is discussed in detail in the next section. This technology requires no connection cables, rather it makes use of a digital parameter range.

It has become the first wireless communication standard for industrial automation. HART-enabled devices can be operated in either of these two network configurations :. In this configuration, the traditional mA signal is used to define one variable, whereas other important process variables would be communicated through digital signals over the HART protocol. In this way, the HART digital signal offers an access to secondary variables as well as important data pertaining to commissioning, maintenance, and diagnostics and so on.

The HART protocol can support up to two masters — primary and secondary masters. The secondary masters can be any PC or a handheld terminal.

Transmitters, controllers, and actuators are a few slave devices normally connected to the primary or secondary master. The HART protocol has been a common method for communication in industrial automation for many years. In the late s, the telecom industry adopted the Bell standard to transmit caller ID information on the same line as voice signals. Audio frequency-shift keying FSK , which uses modulated tones to produce a digital signal, transfers the digital information containing the phone number.

The data transfers at a rate of 1, bps using 1, Hz and 2, Hz frequencies representing a binary 1 or 0, respectively, as shown in Figure 1. The caller ID information does not interfere with the voice signal so that all of the information can transfer over the same physical wiring.

The inclusion of the Bell FSK allowed the transceiver to send large amounts of data before the receiver handset was lifted and was then filtered and used to communicate caller ID information. Once the handset was lifted the FSK signal would terminate and the modulated signal carrying the voice information would then be demodulated and transmitted to the receiver ear-piece.

In the caller ID example, the voice signal represents the PV. Similarly, in industrial applications, the signal that must be monitored at all times is known as the primary measured value. The primary measured value could be the temperature or pressure level measured in the field. Digital information such as sensor or device identification information, calibration data, or other diagnostics, can now be transferred over the same two-wire loop as the mA signal.

Figure 2 shows the coupled digital and analog signal. HART has two main operational modes: point-to-point and multi-drop mode. In point-to-point mode, there is a single master and a single slave.