Hart (highway addressable remote transducer) is a field communications protocol that is widely accepted as a standard for digital communications with smart field equipment. The Hart protocol enables two-way communication using a conventional 4 to 20 mA analog output signal. This makes it possible to examine and manipulate instrument parameters from remote locations.
The overall benefits are:
* Significant improvement in plant information management.
* Greatly reduced initial installation and commissioning costs.
* Annual maintenance and operations savings.
The Hart protocol is supported by major instrument suppliers and the Hart Communication Foundation (HCF). Its wide industry use (over four million existing installations, projected to be 15 million by 2003) indicates the popularity of the Hart technology. The HCF supports the industry and its members as the official source of information regarding Hart technology. This includes maintaining specifications, training and support for applications, directing quality assurance programme and educating the industry on the capabilities and value of this important technology.
The Hart communication protocol is a very useful tool to GLI International because it enables its customers to use the two-way communication capability of its analysers as well as that of other Hart-equipped devices. With a conventional 4 to 20 mA configuration, its customers have the ability to monitor one variable from a GLI analyser. Using the Hart protocol, numerous items can be monitored on a single communication line including process variables, temperature, relay status, configuration changes and other functions. Additionally, any GLI analyser equipped with the Hart protocol option can be connected to an existing Hart field communication system. The flexibility of Hart communications permits any transmitter or analyser with Hart capabilities to be connected in a network and communicate within the system.
Method of operation
Analytical instruments have used the conventional 4 to 20 mA analog output signal technology as the field communications standard for many years. The Hart protocol uses an instrument's existing 4 to 20 mA output signal as the means to communicate. Hart makes use of the Bell 202 frequency shift keying (FSK) standard to superimpose digital communication signals onto the conventional 4 to 20 mA signal (Figure 1).
The digital signal consists of two frequencies:
1200 Hz, represents a '1' bit.
2200 Hz, represents a '0' bit.
This digital information travels at a rate of 1200 bps. Because the average value of the FSK signal is always zero, the conventional 4 to 20 mA signal is unaffected. Hart is a master/slave protocol, meaning that a field device (slave) only speaks when spoken to by a master and never initiates communication. Up to two master devices may be connected to each Hart system loop. The primary master is typically a personal computer (PC) while a handheld device is often a secondary master. The PC is used as a data acquisition system to record and manipulate transmitted information. The handheld device is often used throughout the plant to interrogate information without interfering with communications from the primary master.
Hart network configurations
Two network configurations of field devices are available:
* Point-to-point network.
* Multidrop network.
The point-to-point network arrangement (Figure 2) simply consists of a single field device (slave), Hart modem and a PC or handheld device. In this configuration, the conventional 4 to 20 mA analog output signal remains unaffected by the Hart communications and can be used in the normal way to communicate one process variable.
The multidrop network arrangement (Figure 3) requires only a single pair of wires to connect up to 15 field devices using their conventional 4 to 20 mA analog output signals. By installing a multiplexer into the configuration, additional field devices can be added. In the multidrop network configuration, the conventional 4 to 20 mA analog output signal is always fixed at 4 mA and cannot be used for normal purposes. It becomes dedicated to only Hart communications. A multidrop network arrangement is useful for supervising large installations so that information can be monitored at a central location (master unit).
Hart software structure
Hart follows the basic open systems interconnection (OSI) reference model developed by the International Standards Organization. This model for program structure was made popular and proven effective by the Internet.
The Hart command set provides uniform and consistent communication for all field devices and includes three classes of commands:
* Universal commands: all devices using the Hart protocol recognise and support these commands. Universal commands provide access to information useful in normal operation. Universal commands enable monitoring of four 'process variables'. For example, the GLI model D53 can have two sensor inputs. In this case, it would be 'sensor # 1', 'sensor # 2', 'temperature # 1' and 'temperature # 2'. Additional commands include manufacturer, device type, current output and so on.
* Commmon practice commands: many but not all field transmitters recognise and respond to these commands. Some of these commands do not apply to GLI products. The pertinent commands were configured into the GLI common practice command set. For example, command #38 sets a flag when the unit configuration has been changed. This is especially useful when there are multiple shifts working on the same equipment. If a third-shift employee changed the analyser configuration, a flag would be set to notify the other shifts that the configuration has been modified.
* Device-specific commands: these commands are unique to each field device. They access items such as set-up and calibration information. GLI International has written a device-specific command set for each of its analysers with Hart capability. These are called 'transmitter-specific command specifications'.
Hart system requirements
There are minimal requirements for implementing Hart communications protocol into a system.
* Point-to-point system: this system set-up requires the Hart-equipped field device, Hart modem, personal computer or handheld device and a load resistor. (GLI offers an 'H-link' interface module consisting of a power supply and load resistor in a NEMA 4X enclosure, along with a Hart modem and Hart signal extension cable.)
* Multidrop system: a network of up to 15 Hart-compatible field devices requires a suitable power supply, Hart modem, load resistor and personal computer or handheld device. A multidrop system network can have a cable distance of up to 4000 ft.
For basic network capability, GLI offers an 'H-link' interface module consisting of a power supply and load resistor in a NEMA 4X enclosure, along with a Hart modem and Hart signal extension cable. For mid-level network capability using a PLC or personal computer, a GLI 'smart-link' interface module is available. It consists of an HtNode gateway, power supply and load resistor in a NEMA 4X enclosure.
For maximum network capability/capacity, GLI offers the 'Max-link' interface module (Figure 4). It enables up to 240 field devices (expandable to 960 devices) to communicate with the PC master or handheld device. The 'Max-link' interface is housed in a NEMA 4X enclosure and consists of a power supply, one multiplexer module and a high wattage load resistor. The 'Max-link' module has space to add three more multiplexers, enabling communication with 960 devices.
Summary
Users can achieve many benefits by using Hart field communications protocol:
* Efficient remote communication.
* Field device diagnostics.
* Cost-effective control of field devices.
* Installation savings with multidrop networking.
* Backward compatible data transmission.
This is one of several documents pertaining to the Hart communications protocol. To obtain Application Note AN-H1, consult PREI Instrumentation.
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