How HART Works
“HART” is an acronym for Highway Addressable Remote Transducer. The HART Protocol makes uses Frequency Shift Keying (FSK) standard to superimpose digital communication signals at a low level on top of the 4-20mA. This enables two-way field communication to take place and makes it possible for additional information beyond just the normal process variable to be communicated to/from a smart field instrument.
The HART Protocol communicates at 1200 bps without interrupting the 4-20mA 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 4-20mA signal. The HART Protocol provides two simultaneous communication channels: the 4-20mA analog signal and a digital signal. The 4-20mA signal communicates the primary measured value (in the case of a field instrument) using the 4-20mA 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.
Frequency Shift Keying (FSK)
HART Communication occurs between two HART-enabled devices, typically a smart field device and a control or monitoring system. Communication occurs using standard instrumentation grade wire and using standard wiring and termination practices.
Two Communication Channels
HART technology is a master/slave protocol, which means that a smart field (slave) device only speaks when spoken to by a master. The HART Protocol can be used in various modes such as point-to-point or multidrop for communicating information to/from smart field instruments and central control or monitoring systems.
The HART Protocol provides for up to two masters (primary and secondary). This allows secondary masters such as handheld communicators to be used without interfering with communications to/from the primary master, i.e. control/monitoring system.Primary and Secondary Masters
The HART Protocol permits all digital communication with field devices in either point-to-point or multidrop network configurations:Point-to-Point Configuration
There is also an optional "burst" communication mode where a single slave device can continuously broadcast a standard HART reply message. Higher update rates are possible with this optional burst communication mode and use is normally restricted to point-to-point configuration.
The HART Protocol is a master-slave communication protocol which means that during normal operation, each slave (a field device) communication is initiated by a request (or command) from the master (host) communication device. The master or host is generally a distributed control, PLC, or PC-based asset management system for example. The slave device is typically a field measurement device such as pressure, level, temperature, flow or other transmitters.
In order to make certain any HART-enabled device from any supplier can communicate properly and respond to a command with the correct information, the set and types of commands are defined in the HART Specifications and implemented in all HART registered devices.
Users need not worry about these commands because they are included in the functions of the host. The specific capabilities of a device (device specific commands) are available to the host when the host is given the instructions included in the Device Description (DD) of a specific device.
An important point is that defined device status indications are included with each communication response to the host. The host then interprets these status indicators and may provide basic device diagnostic information.
The HART Command Set provides uniform and consistent communication for all field devices. Host applications may implement any of the necessary commands for a particular application. The command set includes three classes:
All devices using the HART Protocol must recognize and support the universal commands. Universal commands provide access to information useful in normal operations (e.g., read primary variable and units).
Common Practice commands provide functions implemented by many, but not necessarily all, HART Communication devices.
Device Specific commands represent functions that are unique to each field device. These commands access setup and calibration information, as well as information about the construction of the device. Information on Device Specific commands is available from device manufacturers.
A Partial List of HART Commands:
Common Practice Commands
Device Specific Commands
Device Commissioning HART-based instruments have several features that significantly reduce the time required to fully commission a HART network or loop. Less time spent in device commissioning can result in substantial cost savings.
Device commissioning includes the time and cost-saving features:
After commissioning, devices need to undergo configuration and calibration prior to use.
Device Verification Before installation, manufacturers usually enter device tags and other identification and configuration data into each field instrument. This information is usually provided when the device is ordered. After installation, the instrument identification, the tag and descriptor, can be verified either on the workbench or in the control room using a device configurator, which can be a handheld communicator or PC using a modem connected to the device or to the 4-20mA loop.
Some field devices provide information on their physical configuration. For example the devices may deal with wetted materials, ratings or limits of different types.
Physical and other configuration data can be verified in the control room. The verification process is important for safety and in conforming to governmental regulations and ISO quality requirements.
The commissioning process can be further streamlined by connecting a PC-based configuration application to each HART loop. This can be done by integration 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 configuration device from one termination point to the next while commissioning all devices on the network.
Loop Integrity Check Once a field instrument has been identified and its configuration data confirmed, the analog loop integrity can be checked using the loop test feature, which is supported by many HART-enabled devices. The loop test enables the analog signal from a HART transmitter to be fixed at a specific value.
This fixed value verifies loop integrity and ensures a proper connection to support devices such as indicators, recorders, and DCS displays. This approach ensures a proper physical connection among all network devices.
Additional integrity can be achieved if the analog value is compared to the digital value being reported in a device. For example, someone might have provided an offset to the 4-20mA analog value that has not been accounted for in the control system. By comparing the digital value of the Primary Variable to the analog value, loop integrity can be verified.
As-Installed Record Keeping Many HART configurators also facilitate record keeping. As-installed device configuration data can be stored in memory or on a disk for archiving or printing. This facilitates record keeping compliance and provides a documented trail of the configuration to simply device replacement.
HART technology offers several ways to get to the intelligent information in a HART-enabled device. This can be done using either a permanent or temporary physical connection.
You can access all HART data from a HART-enabled device anywhere on the 4-20mA signal. The connection does not have to be at the terminals on the device. This feature may reduce the number of trips to the field since access can be made in the relay or termination panel. Please follow all local plant connection guidelines.
There are many reasons to communicate with a specific device. They include:
A HART-enabled device can be configured using software and hardware tools provided by member companies of the FieldComm Group.
Some control, asset management or safety systems provide permanently connected device configuration capabilities. In that case, follow supplier instructions.
To configure a single device using a temporary connection, you need a universal handheld configuration tool, a power supply, a load resister and a HART-enabled device. Configuration can also be accomplished using a PC running a device configuration application and using a HART modem.
Universal Handheld Communicators The advantages of using a handheld communicator this approach include portability and, potentially, the ability to operate in harsher environments.
HART Handheld Communicators
HART handheld communicators are available from major instrumentation suppliers around the globe and are supported by FieldComm Group member companies. Using Device Description (DD) files, the communicator can fully configure any HART device for which it has a DD installed. If the communicator does not have the DD for a specific device, it will still communicate and configure the device using the HART Universal and Common Practice commands but may not access the extra or device specific commands available in the device.
There are 35-40 standard data items in every registered HART device. The data can be accessed by any approved communicator / configuration tool. These items do not demand a DD and typically include basic device functionality. Accessing device specific data requires a current DD for that particular device, as this provides the communicator with the information needed to fully access all the device specific capabilities.
A HART handheld communicator, if equipped, can also facilitate device configuration record keeping. The installed device configuration data can be stored in memory or on a disk for later archiving or printing. There are many types of handheld communicators available. Be sure to review features and ability to meet your specific requirements.
When connecting to a device, follow wiring instructions provided by the device supplier and observe all plant area safety requirements. Remember, the connections to a device can be made anywhere on the control loop. The connections do not have to be physically located at the device.
PC-Based Device Configuration and Management Tools You can also configure a HART-enabled device with a desktop or laptop PC, or other equivalent portable devices. For this, use a PC-based software application and a HART interface modem. Compared to a handheld communicator, a PC may offer an improved screen display. It also may support more DDs and device configurations because of additional memory storage capacity. Due to the critical nature of device configurations in the plant environment, PCs can also be used as backup storage for data from handheld communicators.
Connecting a PC to a HART device
Software applications are available from many suppliers. It is important to review their features to determine ease of use, ability to add or download native DDs from the FieldComm Group, and general functionality.
Once your device is configured, the device retains the configuration allowing installation on the process application.
It is critical to check the calibration of all field devices periodically. By design, the HART Protocol provides commands to easily enable this function. It is one reason why HART technology is so popular. Calibration ensures that a field device provides a control system with the correct value of a process variable. The controller can then use this information to take action.
The calibration process consists of three major steps, as shown in the diagram below. They involve transforming and scaling the process variable, followed by production of the signal.
These steps are necessary because of sensor physics, measurement techniques and process variable dynamics. The HART Protocol provides standardized trim and related commands to facilitate the process.
Transforming the Process Variable The first calibration step is the transducer block (see diagram above left). It transforms a variable – such as pressure, temperature or flow – into a digital representation of that parameter.
In this step, the digital value is generated from properties like the transducer upper and lower limits, trim points, and characterization data. In general, the field device is calibrated by providing a value from a traceable reference near the upper and lower limits. Trimming the resulting internal mathematical calculations compensates for any error. Process calibrators are available commercially to perform this function using the HART Protocol.
Scaling the Process Variable The second step is referred to as the range block or "zeroing and spanning the device." This provides values for the upper and lower limits, as well as points in-between.
Here, the upper and lower range values are used to produce digital values. These will eventually correspond to a 4mA signal for the lower range and a 20mA signal for the upper range. Percent range, which covers everything between these two extremes, is developed in this step.
In addition, an appropriate transfer function – such as linear, square root, quadratic, cubic spline, and so on – may be applied as part of this scaling. The transfer function corrects for non-linear relationships between a process variable and a transducer.
In some cases, this is needed because the transducer is measuring a related variable. Square root functions, for example, are used to approximate a flow measurement from a differential pressure measurement. Note that this ranging of a field device is not the same as calibrating it.
Producing the Signal The third step creates the final signal. Performed by the DAC (digital to analog conversion) block, this step produces the 4-20mA output. It ensures that 0% equals exactly 4mA and 100% equals 20mA. The signal thus generated corresponds to the process value. From a system perspective, this signal – and the fact that it corresponds to a particular process value – is the most critical function of the field device.
For additional information, refer to the calibration references technical paper "Calibrating HART Transmitters” by Kenneth L. Holiday, Southwest Research Institute. Mr. Holiday is a member of the ISA Field Calibration Technology Committee.
Be sure to follow the recommended calibration procedures provided by your device supplier. This information may also identify additional capabilities provided in your device.