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Article: The HART Communication Protocol





Communication with machinery and control and measuring instruments is the basis of industrial automation. The simplest, most robust and oldest communication technique implemented is a current loop with a normal range of 4 mA to 20 mA. By employing a transmitter, a receiver and a power supply, it is possible to transmit process variable data. The current loop is reliable and highly immune to environmental interference during communication over long distances. No wonder, then, that it is still in widespread use.


However, the obvious disadvantage is that a single loop allows only one-way communication from a sensor to an actuator.


Developed in the mid-1980s, the HART ™ (Highway Addressable Remote Transducer) protocol allows two-way communication of additional information - in addition to the normal process variables - between intelligent field devices.


4-20 mA loops are very popular worldwide and so became HART protocol. The fact that HART operates over 4-20 mA loops allowed the already huge installed base of these loops to be reused with the benefits of having a digital communication protocol for sending and receiving information between the end devices and the control or monitoring systems. (source:


How does it work?


The HART protocol uses FSK (Frequency Shift Keying) to overlay digital communication signals to the analog 4-20 mA signal from the transducer. This way, both signals (analog and digital) live together without interfering one another. HART protocol communication happens at a 1200 bps (bits per second) baud rate, making possible for a host application (master) to obtain two or more digital updates per second from a device.(source:


Therefore the HART protocol provides at least two simultaneous communication channels: the 4-20 mA analog signal and a digital signal. The 4-20 mA signal communicates the primary measured  value (the temperature of a process, for example). Additional information (such as device status, diagnostics, additional measured or calculated values, etc.) is communicated by using a digital signal that is superimposed on the analog signal. Together, the two communication channels provide a complete, low-cost, very robust and easy-to-use and configure solution.


Figure 1: HART signal added to a 4-20 mA analog signal


HART communication occurs between two HART-enabled devices, usually an intelligent sensor or actuator device, and a control or monitoring system.


The HART protocol is a master-slave communication protocol, which means that during normal operation each slave data sending (HART device) is initiated by a request (or command) from the master communication device (host). The master or host is usually a distributed control system, PLC or PC-based, for example. The slave device is typically a device for measuring some physical variable such as pressure, level, temperature, flow or even an electromechanical actuator, such as a proportional valve.


The HART protocol can be used in modes point-to-point or multidrop. In point-to-point mode, a single loop of current is established communicating a HART device and the monitoring system. The primary variable travels using the analog signal 4 to 20 mA, while additional information travels digitally over the loop.

Figure 2: Point-to-point mode


In multidrop mode, all HART devices are in parallel and sharing a single transmission line. By operating in this way, each device remains fixed at 4 mA, with only data traffic occurring through the digital channel. Since simultaneous communication with multiple instruments is not possible, each device will have its own unique HART address. Currently the HART v07 protocol allows connecting up to 64 devices in multidrop mode.



Figure 3: Multipoint mode


In the HART protocol, one can have up to two masters. A primary master is typically a control or monitoring system that is continuously interacting with the slave device, reading or writing information. A secondary master is typically a temporary device that is connected to the loop, as a handheld device for configuration. HART protocol allows both masters to be used simultaneously.

As an option for achieving higher update rates, there is a "burst" communication mode. Its use is usually restricted to point-to-point configuration where a single end device (slave) keeps continuously broadcasting a standard HART reply message. (source:


To ensure that any HART-enabled device from any supplier can communicate properly and respond to a command with the correct information, the set and command types are defined in the HART specifications and implemented in all registered HART devices.


The user does not have to worry about these commands because they are included in the host functions. Device specific features (device specific commands) are available to the host through a Device Description file (DD), which functions as a kind of driver.


An important point is that the device status indications are included in each communication response to the host. The host interprets these status indicators and can provide basic diagnostic information for the device.


The HART Command Set provides a standardized communication interface for all end devices. The host control or monitoring system can choose to implement any of the available commands, selecting the required ones to be used in a specific application. The command set includes three classes:


Universal: These commands must be recognized and supported by all devices that use the HART protocol. Universal commands provide access to information that is used in normal operations such as read primary variable and units.


Common Practice: This set of commands provides functions implemented by many - but not necessarily all - HART communication devices.


Device Specific: Device specific commands represent functions unique to each device. These commands access configuration and calibration information, as well as information about device construction. Information about Device Specific commands is available from device manufacturers.


The table below shows some examples of HART commands.



Device Comissioning


There are several features provided by HART to reduce the time required to fully commission a HART network or loop, making this task easier. Among these features, we can point out the easy identification and verification of each HART device, easy verification of loop integrity and maintenance of device records as installed. Less time spent in device commissioning results in cost savings.


Device identification and verification


Before installation, a tag is usually inserted, along with other identification data and the configuration of each instrument going to the field. This information is usually provided when the device is requested by the master. After installation, the instrument identification, tag and descriptor can be easy verified by using a device configurator, which can be a handheld communicator or a PC using a modem connected to the device or the 4- 20 mA loop.


Some field devices provide information about their physical configuration. For example, devices can handle wet materials, ratings, or boundaries of different types.


Physical data and other configuration data can be verified in the control room. The verification process is important for safety and in compliance with governmental regulations and ISO quality requirements.


The commissioning process can be further simplified by connecting a PC-based configuration application to each HART loop. This can be done by integrating with the control system or by using one of the many available HART multiplexing I/O systems. With this centralized approach, there is no need to move the device from one endpoint configuration to the next during the commissioning of all devices on the network.


Loop Integrity Check


Once a field instrument has been identified and its configuration data confirmed, the integrity of the analog circuit can be verified using the loop test feature, supported by many HART-enabled devices.


The loop test allows the analog signal of a HART transmitter to be set to a specific value. This fixed value checks the integrity of the loop and ensures a proper physical connection between all network devices.


Additional integrity can be obtained if the analog value is compared to the digital value reported on a device. For example, something may have entered an offset to the analog value of 4 to 20 mA that was not counted in the control system. By comparing the digital value of the primary variable with the analog value, the integrity of the loop can be checked.


Maintenance of records as installed


Many HART configurators also keep records of device configuration data, so they can be restored for archiving or printing. In case of device replacement, this ensures configuration recovery, while facilitating record-keeping compliance. (source:




HART technology offers several ways to get intelligent information in a HART-enabled device. This can be done using a permanent or temporary physical connection.


You can access all HART data from a HART-enabled device anywhere on the 4-20 mA signal. The connection does not need to be at the device terminals. This feature can reduce the number of field trips, since the access can be made in the relay or termination panel.


Among the many reasons to communicate with a specific device we can list:


 • Device configuration

• Device diagnostics

• Device troubleshooting

• Loop Integrity Check

• Troubleshooting the process


Devices with HART protocol can be configured using software and hardware tools provided by the FieldComm Group member companies.


Some control and monitoring systems provide configuration resources for permanently connected devices. In order to configure a HART-enabled device using a temporary connection, you can use either a universal handheld configuration tool or a PC running a device configuration application and a HART modem. For both cases, a power supply and a load resistor are also needed. (source:


Universal Handheld Communicators


Handheld communicators are portable – usually battery-operated – and can be operated in a wide set of environments. They are available from suppliers around the world and are supported by FieldComm Group. These configurators are able to fully configure any HART device by previously installing the proper Device Description (DD) file for that device.


Even not having the DD for a specific device, the configurator will still be able to communicate and configure it by using HART Universal and Common Practice commands. However, on this case, extra or device specific commands will not be accessed, which can lead to a limited experience.


There are between 35 and 40 standard data items on all registered HART devices. Data can be accessed by any communicator or approved configuration tool. These items do not require a DD and usually include the basic functionality of the device. Accessing device specific data requires a current DD for that particular device as it provides the communicator with the information needed to fully access all of the device's specific features.


If equipped, a HART handheld communicator can also make it easier to maintain device configuration records. The configuration data of the installed device can be stored in memory or on a disk for later archiving or printing. Handheld connections can be made anywhere in the control loop, they do not need to be physically located near the device.


Figure 4: Primary master and secondary master


Configuring PC-Based Devices and Management Tools


You can also configure a HART-enabled device with a desktop or laptop PC or other equivalent portable devices. To do this, use a PC-based software application and a HART interface modem. Compared to a handheld communicator, a PC can offer an improved display. It can also support more DDs and device settings because of the additional memory storage capacity. Due to the critical nature of the device settings in the factory environment, PCs can also be used as backup storage for handheld data communicators.


Software applications are available from many suppliers. It is important to review its features to determine ease of use, the ability to add or download native FieldComm Group DDs, and general functionality.


HART protocol advantages and limitations


Devices equipped with the HART protocol are nowadays widely accepted throughout the industry. The combination of digital communication with the 4-20 mA signals provides flexibility to transmitters, valve positioners, flow meters and other intelligent devices. In addition to the primary variable monitored, the protocol brings additional information of other variables measured or calculated by the transmitter.


Although the digital signal is relatively slow and the loss of functionality of the analog signal in multidrop mode, there are still many advantages to using the HART protocol. The use of HART multiplexers and the integration with other technologies and protocols can overcome the speed and the restricted number of devices (up to 64 devices in multidrop mode).


Self-diagnosis is a valuable resource to detect any problems in the plant. The protocol also ensures interoperability between registered HART devices regardless of manufacturer. This is due to the stringent technical requirements that must be met in the certification process, both the device hardware and the DD.


The cabling structure cost is another attraction. In many cases the reuse of the previously installed 4-20 mA structure takes place. Another outstanding feature of the protocol is the ability to accurately encode and decode HART communication signals in noisy and hostile industrial environments.


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