HART Communication

HART - Highway Addressable Remote TransducerProtocol

is an early implementation of Fieldbus, a digital industrial automation protocol. Its most notable advantage is that it can communicate over legacy 4-20 mA analog instrumentation wiring, sharing the pair of wires used by the older system.

Important Points to Consider

1. Regardless of the age or capability of your control, safety or asset management system, HART technology can provide additional information to help extend time between shutdowns, lower maintenance cost and improve process performance.
2. The two most common tools used to temporarily connect to a HART-enabled device are: a handheld communicator or a PC running a configuration software application and connected via an external modem.
3. A handheld communicator or a properly equipped PC can be connected at any point on the loop to access HART Communication data – eliminating the need to be located at or connected to the terminals on the specific device.
4. In order to access all of the intelligent information in a specific device, the handheld communicator or the PC-based software application need to contain the current device description (DD) that matches the model and revision level of your device.
5. There are 35-40 standard data items in every registered HART device that can be accessed without using the DD.
6. Once a device has been configured, the configuration can be saved to facilitate device replacement or maintenance / regulatory record keeping.
7. HART technology can be used on a Point-to-Point basis – monitoring the information from a single device – or on a Multipoint basis – simultaneously monitoring the information from many devices.
8. The information from intelligent HART-enabled devices can be monitored for device status alerts on a temporary basis (daily scan of all devices on the network) or can be monitored full-time and the information integrated into the control or monitoring system.
9. The translation of the digital HART Protocol signal can be done by an external (or stand alone) device or can be done using a HART-enabled I/O system that allows the termination of the 4-20mA signal to an I/O card. (The Primary Variable is converted to a digital signal and the HART data is translated into digital information, which can be used by the system.)
10. The actual conversion or translation of HART Protocol information can be done in the field – close to the HART-enabled device – or in the control or relay room.
11. Many products and tools are available to retrofit existing systems to access HART data. With these tools, you can easily replace wire termination boards (in some cases pin-for-pin replacements), which allows the 4-20mA signal to be passed into the control system – unchanged.  The HART data is passed to the HART-enabled asset management system.
12. Since HART data is read using a modem, it is important to know the ratio of modems to input signals in the HART-enabled I/O. This item is one of the main factors used to determine the scan rate of each signal connected to the I/O card.
13. Gateways are used to convert HART Protocol information to other standardized protocols such as HART to Modbus, PROFIBUS, Ethernet, TCP/IP, etc.
14. HART multiplexers can be used as the primary I/O front end for a HART-based control or monitoring system
15. Not all suppliers implement HART Protocol the same way.  It is important to understand how your suppliers implement this enabling technology so that you will be better able to predict performance and results.
16. Gaining access and using HART data can be done one loop at a time.  Migrating to larger applications will provide full-time monitoring of more devices, therefore increasing the benefit. This scalability offers the User the ability to determine the size and scope of the use of valuable HART Protocol information.

Notes-

• HART technology enables two-way digital communication to occur between intelligent devices and connected host or control systems.
• That HART is an enabling technology meaning that it can be applied in many different application including control, monitoring, safety, asset management, etc.
• HART provides two simultaneous communication channels:  the 4-20mA analog signal and a digital signal.
• The digital signal contains information from the device including device status, diagnostics, additional measured or calculated values, etc.
• The 4-20mA signal communicates the primary measured value (in the case of a field instrument) using the 4-20mA current loop.
• The digital communication part of the communication is a request-response communication protocol, which means that during normal operation, each device communication is initiated by a request from a host device.
• The digital signal is made up of two frequencies - 1,200 Hz and 2,200 Hz representing bits 1 and 0.  Sine waves of these two frequencies are superimposed on the direct current (dc) analog signal wire to provide simultaneous analog and digital communications.
• A minimum loop impedance of 230 ohms is required for communication.
• HART devices can operate in one of two network configurations-point-to-point or multidrop.
• The HART Command Set includes three classes: universal, common practice, and device specific.
•  There are 35-40 data items that are standard in every HART registered device.  Host applications may implement any of the necessary commands for a particular application.
•  The HART device suppliers provide a DD file for each HART device and the DD file combines all of the information needed by the host application into a single structured file.
• The DD file includes all of the information needed by a host application to fully communicate with the field device and access all of the information available from the device.
• The new HART enhanced Device Description Language extends the capabilities of DDL to provide for advanced visualization of intelligent device information.
• HART Protocol is backward compatible with the installed base of instrumentation and control systems in use today. A new HART-enabled device can replace an existing 4-20mA analog-only device of similar measurement capability without change to the host system or wire.
• Most HART devices provide multiple process variables, which can be used for monitoring or control to gain a better insight into the process.
• Device diagnostics and status information are returned with every communication message.
• The installation practice for a HART communicating device is similar for a conventional 4-20mA instrument.
• HART communicating devices can be used in applications that require IS operation.