HART Communication Protocol - Process Instrumentation

Process instrument with HART protocol (Foxboro)

The Highway Addressable Remote Transducer Protocol, also known as HART, is a communications protocol which ranks high in popularity among industry standards for process measurement and control connectivity. HART combines analog and digital technology to function as an automation protocol. A primary reason for the primacy of HART in the process control industry is the fact that it functions in tandem with the long standing and ubiquitous process industry standard 4-20 mA current loops. The 4-20 mA loops are simple in both construction and functionality, and the HART protocol couples with their technology to maintain communication between controllers and industry devices. PID controllers, SCADA systems, and programmable logic controllers all utilize HART in conjunction with 4-20 mA loops.

HART instruments have the capacity to perform in two main modes of operation: point to point, also known as analog/digital mode, and multi-drop mode. The point to point mode joins digital signals with the aforementioned 4-20 mA current loop in order to serve as signal protocols between the controller and a specific measuring instrument. The polling address of the instrument in question is designated with the number "0". A signal specified by the user is designated as the 4-20 mA signal, and then other signals are overlaid on the 4-20 mA signal. A common example is an indication of pressure being sent as a 4-20 mA signal to represent a range of pressures; temperature, another common process control variable, can also be sent digitally using the same wires. In point to point, HART’s digital instrumentation functions as a sort of digital current loop interface, allowing for use over moderate distances.

HART in multi-drop mode differs from point to point. In multi-drop mode, the analog loop current is given a fixed designation of 4 mA and multiple instruments can participate in a single signal loop. Each one of the instruments participating in the signal loop need to have their own unique address.

Since the HART protocol is a standardized process control industry technology, each specific manufacturer using HART is assigned a unique identification number. This allows for devices participating in the HART protocol to be easily identified upon first interaction with the protocol. Thanks to the open protocol nature, HART has experienced successive revisions in order to enhance the performance and capabilities of the system relating to process control. The standardization of “smart” implementation, along with the ability to function with the legacy 4-20 mA technology and consistent development, has made HART a useful and popular component of the process measurement and control industry framework.

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The Basics of a Fieldbus Control Network

Computer systems used within the industrial sector are connected by networks known generally as Fieldbus. Fieldbus systems are a way to connect computers and instruments to a single network in a manufacturing plant and allow for real-time control and monitoring. Fieldbus industrial networks can be broken down into four levels each with increasing levels of complexity.

The most basic level is the sensor bus networks. Sensor bus networks are the least complex of networks developed for industrial application. In these networks, multiple basic field devices like limits witches or level optical sensors are connected to one network cable. The sensor bus network is also capable of transmitting output signals from the controller over one cable to indicator lamps, alarms, or other actuator devices.

The next increasingly complex level of industrial Fieldbus networking is the device bus network. The device bus network is similar in function to the sensor bus network but works on a larger scale connecting many sensors and actuators together. The device bus network also connects equipment to variable speed drives and motor control centers that allow for control of individual elements in the network.

Moving up the pyramid, the next increasing complex level of Fieldbus networking is the control bus network. Control bus networks are the most advanced networks used on the factory floor and data communication happens at a high level. PLC's, or programmable logic controllers, are connected to each other alongside HMI's or human machine interface panels to allow for complete configuration and control of every instrument on the network. Smart instruments, capable of performing complex operations, can also be connected at this network level. For instance, there might be a smart instrument that measures wear and tear on a valve. When the wear reaches a dangerous level it will signal the controller that the valve needs to be replaced.

The enterprise or information level network in a company connects all computers and departments together it is the most overarching and complex of all the various network levels. This level of networking is primarily computer driven which allows for data collection, data monitoring, file transfers, and email exchange on a large scale. The various levels of interconnected Fieldbus networking help to keep industry functioning smoothly and successfully.