“Basic's, Working and Applications of HART Protocol”

Communications in industrial automation systems enable the transfer of data, commands and other information to streamline process control and factory automation. It would be unrealistic for an engineer to monitor every piece of equipment in the factory (and that certainly would decrease productivity).

What is HART Communication Protocol?

The HART protocol has been a common method for communication in industrial automation for many years. In this post, I’ll look at what HART is and how it works. But first, let’s look at how the method originated.

In the late 1990s, the telecom industry adopted the Bell 202 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 1,200 bps using 1,200 Hz and 2,200 Hz frequencies representing a binary 1 or 0

The HART modem modulates and demodulates the signal using FSK, just like the Bell 202 system. 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 4–20 mA signal. This system is typically referred to as a “hybrid” system, as it combines both digital and analog signals.

One way to monitor field equipment automatically is through 4–20 mA loop sensor transmitters. The primary variable (PV) is communicated as a current level between 4 mA and 20 mA on a two-wire loop-powered system. One downfall of this method is that you can only monitor one variable at a time, so the Highway Addressable Remote Transducer (HART) protocol enables the transfer of more information on the same two-wire system.

The Address may have 1 to 5 bytes. It contains both a master and a slave address. The indication for the master address is a single bit, where 1 indicates a primary master and 0 indicates a secondary master. The indication of a slave address is a 4-bit polling address in a short frame and a 38-bit unique identifier in the long frame. One bit is used to indicate the burst mode from the slave.

Expansion may contain 0 to 3 bytes. The number of bytes in an expansion field includes bits 5 and 6 of the start character.

HART commands:.

1.The universal HART commands range from 0 to 30
2.The practice commands range from 32 to 126
3.The device-specific commands range from 128 to 253

The messages transmitted from a slave to the master. It contains two bytes indicating communication errors, the status of a received command, or the status of the receiver field device.

Data may contain 0 to 255 bytes. The presence of the data field depends on a particular command. All universal and common practice commands have 33-byte data fields.

What are the Modes of HART Communication Protocol?

Master-slave mode:

Where the host system initiates and controls the entire data communication. There can be two masters, including the primary and secondary ones in the HART loop.The primary master can be a distributed control system (DCS), a programmable logic controller (PLC), or a workstation. The secondary master can include handheld communicators or a laptop/PC. The slave devices can be HART-enabled sensors, actuators, controllers, or transmitters.

Burst mode:

Only available in a point-to-point configuration. In this mode, the slave device continuously broadcasts the standard HART reply messages to the master(s).Once a master instructs the slave device to broadcast the messages, the slave keeps sending responses without the need for polling. It stops only when it receives further instructions from the host system. This is why this mode allows for faster data communication with up to 3 or 4 updates per second.

HART-enabled devices can be operated in either of these two network configurations :

Point-to-Point:

In this configuration, the traditional 4–20 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 primary master can be programmable logic controllers (PLC), distributed control systems (DCS), and a personal computer (PC). 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 primary and secondary master devices do not interfere in each other’s functioning.

Multi-drop:

It is a burst communication mode, where only one slave device is used to broadcast a HART reply message. This mode of HART communication uses only a pair of wires. If possible an auxiliary power supply and safety barriers may also be added. All process variables are digitally transmitted. All field devices in this mode have polling addresses >0. The current is fixed at 4–20 mA.

Hart Communication in IOT .

WirelessHART:

WirelessHART is the first standardized wireless communication in the filed of process automation .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 allowing easy and cost saving installation of additional measuring points for diagnostic purposes.
There are two different WirelessHART solutions:
1.WirelessHART Adapter for enhancing existing HART devices.
2.Self powered WirelessHART Transmitter.

Wireless is already an established solution in many different industries, we can find lots of cases where end users are already using it to solve various applications.

Wireless is the easiest way to collect the data or IDM data from field devices. But we can also add the WirelessHART adapter into field devices and transfer the data to WirelessHART gateway.The WirelessHART gateway such as Fieldgate SWG70 , can be integrated into the devices and provide all the data to the IIoT cloud Computing.

HART gateways are one of the ways to use the HART communication from 4–20 mA loop and integrate it into an IIoT platform such as Netilion. Depending on the quantity and complexity , a HART gateway is good solution for IIoT platform.

The Fieldgate SFG250 ,an Ethernet HART gateway can be implemented in IIoT platform.

HART/ WirelessHART gateway can be connected to devices and will communicate securely with cloud solution and provide all the health information .

HART is a versatile method of communication for a variety of industrial automation applications. The method itself offers many benefits that reduce cost, simplify designs and provide results, including:Transmission of digital information without interrupting the primary analog signal.

What are the Benefits of Using HART Protocol ?

HART-enabled devices allow users to:

1.Make the best use of device data to optimize their operational capabilities.

2.Minimize the downtime's due to equipment failure by identifying the potential problems before they occur.

3.Cut down on device maintenance and inventory costs.

4.Reduce wait times between the problem identification and problem resolution.

5.Improve safety integrity levels through advanced diagnostics.

6.Automate record keeping for compliance purposes.

“Owing to its increasing popularity, the protocol has been updated several times since the introduction. It is often updated such that the backward compatibility is ensured and are configured to HART communicator for achieving desired performance”