At present in industrial automation, different types of smart field devices are utilized but monitoring every device is very difficult within the industry by authorities or field engineers. So generally, this kind of monitoring is achieved with smart devices, which allow data transfer between different connected devices inside and outside of the industry to the major monitoring system. So, the HART protocol was introduced in 1980 and builds on the Bell 202 standards. This protocol has become an industry standard, so used in industrial automation. So, this article discusses an overview of the HART protocol – working with applications.
The term HART in HART protocol stands for “Highway Addressable Remote Transducer” which is an open standard protocol used worldwide to transmit & receive digital data with analog wiring in between smart devices & control systems. This protocol is very popular, so above 30 million devices across the globe are powered with the HART protocol. This protocol is used to establish communication among host systems as well as smart field devices in industries. HART protocol gained more popularity due to its capability to support the older 4-20 mA based analog protocol, while including the major advantages of digital smart instrumentation.
This protocol describes physical connection technology and also commands utilized by different applications. Hart commands are three types Universal, Common Practice & Device Specific.
Universal-type commands are implemented through all HART devices. These commands are mainly used by a controller for identifying a field device as well as reading process data.
Common Practice type commands are used to describe different functions that are normally applicable to field devices only. These devices include commands for changing the range, choosing engineering units & execute self-tests. Device-specific type commands are not the same for every device. These commands execute unique configuration & modification functions. So, it is very significant to note that when devices from various manufacturers implement similar functionality externally. For instance, differential pressure measurement may include different hardware completely and also different device-specific command sets.
HART protocol operates in two network configurations like point to point and multi-point which are discussed below.
In the point-to-point network configuration, for communicating a single process variable, the fixed 4–20 mA signal is utilized whereas extra process variables, and design parameters are digitally transmitted with the HART protocol. So, the 4–20 mA analog signal is not changed by the HART signal & can be utilized in a normal way. The HART communication digital signal provides the right of entry to secondary variables & other data can be utilized for maintenance, commissioning, operations & diagnostic purposes.
This network configuration allows various devices to be connected with a single pair of wires. The communication within this configuration is completely digital since communication throughout the analog loop current is disabled as the current throughout every device is fixed at a minimum sufficient value for operation of the device normally 4mA.
The HART communication protocol uses the Bell 202 FSK (Frequency Shift Keying) standard to superimpose digital signals which are represented with two different frequencies like 1,200 Hz & 2,200 Hz. Here, 1,200 Hz frequency represents bit 1 whereas 2,200 Hz frequency represents bit 0 correspondingly.
When sine waves with these frequencies place over on DC analog signal cables then the data transfer occurs. So, during this transfer of data, the 4-20 mA signal is not affected due to the standard value of a frequency shift keying signal equivalents to zero. This protocol supports at a time two communication channels like 4-20 mA analog signal & digital signals. The analog signal communicates the primary measured value with the 4-20mA current loop whereas the additional device data is communicated through a digital signal that is overlaid on the analog signal.
The digital signal includes the information of the device like the condition of the device, diagnostics, calculated values, etc. So jointly, the two communication channels offer a very robust & low-cost communication solution that is very simple to utilize & configure. This protocol is frequently called a hybrid protocol because it combines both analog & digital communication. The HART technology is differentiated as a master/slave protocol because the slave device only works once a master device is connected to it. Here, the slave device is a smart device, and the master device is a computer.
Generally, for communication within the HART Protocol, the device used within the network is PLC or distributed control system that is chosen as the Master whereas other field devices are considered as slaves like sensors or actuators. But here the communication between master and slave mainly depends on the mode of communication to which the system is arranged. HART protocol network communicates in two modes like master/slave mode and burst mode.
This mode is also known as the request-response mode. In this type of mode, slave devices simply transmit data once a request from the Master device is issued. For each HART loop, two masters can be connected. So the primary master is normally a DCS (distributed control system), PC (personal computer), or PLC (programmable logic controller) whereas the secondary master is another PC or a handheld terminal. Slave devices are actuators, controllers, and transmitters that react to commands from the master devices.
Some HART protocol-enabled devices simply support this communication mode. This mode allows faster communication like three to four data updates for each second. The master device in this mode instructs the slave device to transmit a typical HART reply message continuously. The master gets the message at high speed until it orders the slave to stop bursting. This mode is applicable where the above one HART device is necessary to communicate from the HART Loop.
| HART protocol | Modbus |
| HART is a hybrid protocol. | Modbus is a data communication protocol. |
| HART is extensively used in process & instrumentation systems which range from small automation to highly complicated industrial applications. | Modbus is normally used for transmitting signals from instrumentation & control devices to a data gathering system or main controller. |
| This protocol operates in two operational modes like point to point and multi-drop. | Modbus operates in two transmission modes like ASCII Mode or RTU Mode. |
The disadvantages of the HART protocol include the following.
The applications of the HART protocol include the following.
Thus, this is an operation of the HART protocol. This protocol is the global standard used for transmitting & receiving digital data across analog wires in between smart field devices as well as monitoring or control systems such as DCS & PLC systems. This bidirectional communication protocol simply provides the right of entry to additional data between the field & the host controller ranging from a handheld device to an asset management system or plant controller. Here is a question: What is the full form of the HART protocol?
