Instrumentation & Automation
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Alexander Bratsuk, Kiev.

HART protocol


HART protocol is one of the communication protocols widely used in industrial automation. Each industrial protocol is dedicated to its tasks, and HART is the best solution for obtaining information on measured values of devices, diagnosing and tuning these devices using a standard current loop 4-20 mA (modern standard analogue current loop signal). The most common HART usage is for sensors, but it can also be used for actuators, valves with a current control signal, etc.

HART ("Highway Addressable Remote Transducer") is specifically designed for industrial automation and measurement technology. It is also called hybrid because it combines both analog and digital signals. In other words, the digital signal is transmitted over the analog output of the sensor. The digital component of the signal does not affect the analog output of the sensor in any way - it is simply filtered out.

The analog component of the signal is usually transmitted in one direction, for example from the sensor output to the input of the secondary device, while the digital signal can be transmitted along the line in two directions, from the HART device to the sensor, requesting its state and from the sensor to the HART communicator, transmitting measured value.

Physical layer

For transmitting / receiving data on a physical level, the HART protocol uses relatively low-speed method of frequency shift keying, which to encode binary data:

HART communicators

HART communicator is a microprocessor device connected to a current loop, often with a transformer input, for receiving, processing and transmitting digital information in this circuit. There are also HART modems - devices that are connected to the computer via USB or RS232 port.

HART to the current loop when measuring the sensor signal

The figure on the left shows a typical circuit of the HART communicator in the current loop between the sensor and the secondary device. On the resistor R, usually 250 Ohm, when a current passes through it, according to Ohm's law, a voltage is generated, which is fed to the HART input of the communicator.

The HART communicator can read the physical value measured by the sensor, or it can be used to diagnose and adjust the sensor. Typically, a HART communicator is a portable device for calibrating sensors and similar devices, and there is no need to use it continuously according to the scheme given. Most often it is connected to the analog circuit temporarily.

It would be very inconvenient if you had to break the current loop every time to connect the HART communicator, especially in control systems. Therefore, most HART compatible sensors and devices have terminals for connecting it, without breaking the analog output signal.

Usually analog or digital secondary devices can only measure / display the input signal from the sensor and there is little that can tell about its state. HART communicator can not only display the measured parameter, but also

How does a HART communicator know with what type of sensor it is working and how to configure this sensor? To do this, the communicator has a database of compatible device drivers. When connected to a HART device, the communicator recognizes and downloads the required device driver from its database.

And what if the device is newer than the communicator itself, it was released recently, and the necessary device driver in the database is not found? To do this, you need to look for the device description driver on the device manufacturer's website or on the HART Communication Foundation's website, which is commonly used by well-known device manufacturers for instrumentation and automation. All this applies to universal HART communicators.

Although the HART protocol is open and standard, sometimes hardware manufacturers based on or for commercial purposes, to increase the number of sales, or from the goals of simplifying devices to the detriment of their versatility, prefer to manufacture their own HART devices to customize their equipment. In this case, nothing else remains, except how to purchase the corresponding device from the manufacturer.