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HART Communication Protocol
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HART or not HART ?

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Relay, 4-20mA signal, Modbus RTU/TCP, Wireless or HART ... These are communication techniques between machines and control instruments. These terms are very common in industry and especially in fixed gas detection.

In this article, we will review the different communication protocols available in the installation of fixed gas detectors. We will also focus on one of these protocols: HART.

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In a fixed gas detection installation, there are three types of possible wiring architectures with different data transmission modes, regardless of the type of industrial transmitter (pressure, temperature, humidity, gas, etc.):

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Transmitter only diagram
Transmitter only diagram

The transmitter alone :

Locally powered, it manages safety and/or process actions when equipped with relays.

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The central unit/controller dedicated and wired to a transmitter
The central unit/controller dedicated and wired to a transmitter

The central unit/controller dedicated and wired to a transmitter :

In this case, the purpose of the transmitter is only to transmit status data while the configuration, modification or control actions are carried out via the central unit/controller.

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The transmitter connected to a Programmable Logic Controller (PLC)
The transmitter connected to a Programmable Logic Controller (PLC)

The transmitter connected to a Programmable Logic Controller (PLC) :

It is possible to manage a multitude of process and/or safety information.
In this last case, the PLC can receive the information from different sources coming from the transmitter:

  • relay
  • 4-20mA signal
  • Modbus communication protocol
  • HART
  • Wireless

The different types of communication protocols

1. The relays

The relays send a binary information all or nothing according to their state (alarm level, fault...)

2. The 4-20mA current loop

This is the simplest, most robust and oldest communication technique. By using a transmitter, a receiver and a power source, it is possible to transmit the data of the process variables. The information is sent continuously by wire and corresponds to the scale of the sensor.

For example: Sensor operates 0-100% LEL means that the information transmitted to the PLC will be converted as follows: 0% LEL = 4mA and 100% LEL = 20mA

The current loop is reliable and highly immune to environmental interference over long communication distances. It is not surprising, therefore, that it is still widely used. The main disadvantage, however, is that a single loop only allows one-way communication from transmitter to PLC.

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The 4-20mA current loop

3. Modbus RTU/TCP

It is based on the notion of master/slave, the transmitter sends a data exchange frame encoded in 8 bits which informs the PLC of the operating status of the transmitter (normal, alarm, degraded, fault) and/or its coding.

4. Le wireless

Wireless is the technical ability of the transmitter to send status information without cables via a protocol based on communication frequencies that transmit the data to PLCs that themselves have the ability to receive this wireless data via routers or other...

5. The HART protocol

The Highway Addressable Remote Transducer (HART™) protocol was developed in the mid-1980s to enable bi-directional communication of additional information - in addition to normal process variables - between intelligent field devices. It is based on a dual-stream (transmit-receive) standard of digital information via an analog wire cable.

In contrast to the other communication methods mentioned above, the information flow is only sent on request of the PLC which queries the transmitter about its status. In addition to remote polling by the PLC, an operator can go to the site with a HART pocket to poll the transmitter at any time.

WirelessHART is a wireless version of this protocol.

Qu’est-ce que le protocole HART ?

Le protocole HART utilise la norme FSK (Bell 202 Frequency Shift Keying – modulation par déplacement de fréquence) pour superposer en simultanée des signaux de communication numérique (HART) à un signal analogique 4-20mA issu du transducteur :

  • Le signal 4-20 mA communique la valeur primaire mesurée (la température d’un process, par exemple).
  • Le signal numérique superposé fourni des informations supplémentaires (telles que l’état de l’appareil, les diagnostics, les valeurs supplémentaires mesurées ou calculées, etc.…)

C’est cette superposition qui permet une communication bidirectionnelle et ainsi de pouvoir, via l’API ou une pocket : consulter, modifier et/ou configurer le transmetteur. Pour cela le transmetteur doit donc être équipé d’une interface HART qui permettra de moduler et démoduler les données pour les échanges avec l’API ou la pocket.

La valeur des données est encodée par différentes fréquences pour chaque valeur de bit envoyé, le transfert des données vers l’API se fait via une modulation de fréquence de 1mA superposée au signal analogique 4-20mA ce qui permet d’envoyer, par le même câble, la mesure et les données de transfert. Ensemble, les deux canaux de communication fournissent une solution complète, économique et très robuste, facile à utiliser et à configurer.

Operation of the HART communication protocol

The HART protocol is a master/slave communication protocol, i.e., during normal operation, each data transmission from the slave (HART device) starts with a request (or command) from the master communication device (host). The master is typically a programmable logic controller (PLC). The slave device is typically a device that measures a physical variable, such as a gas sensor, pressure, level, temperature, or even an electromechanical actuator, such as a proportional valve.

The HART protocol can be used in a point-to-point or multi-point manner:

  • In the point-to-point manner, a single current loop is established to communicate the HART device with the monitoring system. The primary variable flows using the analog 4-20 mA signal while the additional information flows digitally on the loop.
  • In multidrop mode, all HART devices are in parallel and share a single transmission line. When operating in this manner, each device remains fixed at 4 mA, and data traffic operates only through the digital channel. Since simultaneous communication with multiple devices is not possible, each device has its own exclusive HART address. Currently, the HART v07 protocol allows up to 64 devices to be connected in multidrop mode.

The HART protocol allows up to two masters (one primary and one secondary), allowing secondary masters, such as configurators or handhelds, to be used without interfering with the communication between the slave devices and the primary master, i.e. the control/monitoring system.

There is also the optional "burst" communication mode, where a slave device can transmit a standard HART response message continuously. This optional communication mode makes higher update rates possible and its use is generally restricted to point-to-point configuration.

Why choose HART technology?

For 30 years, the HART communication protocol has been an integral part of the process automation industry worldwide. Most manufacturers equip their products with HART interfaces (over 30 million devices according to the HART Communication Foundation).
Choosing this protocol secures investments and guarantees freedom of choice thanks to the interoperability of the equipment.

This is why this type of protocol has been democratized in many companies and industrial sites. It allows :

  1. Easy access to sensor status data.
  2. Easy configuration of the transmitter (remotely or locally).
  3. A proactive maintenance which allows to reduce the maintenance costs and thus to improve the process and/or the safety.
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