Introduction to the HART Protocol

HART protocol is irreplaceable in bus applications. We all know that bus is one of the most popular technologies today. We must also understand the knowledge of this Protocol. Let's talk about this HART protocol.

The field bus technology is one of the hot spots in the current automatic detection technology. From the perspective of the formation of the field bus technology, it is the inevitable result of the development of control, computer, communication, network and other technologies, and intelligent instruments lay the foundation for the emergence and application of the field bus. Since Honeywell launched its intelligent instrument-Smar transmitter in 1983, various manufacturers in the world have successively launched their distinctive intelligent instruments. In order to solve the issue of sharing open resources, users and vendors all strongly require the formation of unified standards to promote the formation of field bus technology. Currently, several influential fieldbus technologies include: Foundation Fieldbus, LonWorks, PROFIBUS, CAN, and HART, all of which are fully digital fieldbus protocols except HART.

Full digitalization means that the traditional analog signal transmission mode will be abolished, and each field device is required to have intelligent and digital communication capabilities, so that the operator or other equipment sensors, actuators, etc) send commands to the site, such as set value, range, and alarm value ), at the same time, it can also obtain information about all aspects of the field equipment in real time, such as measured values, environmental parameters, equipment operation status and equipment calibration, self-diagnosis, alarm information, and fault data ). In addition, the original Control Operations completed by the main controller are also distributed to various field equipment, greatly improving the reliability and flexibility of the system. The key of the Field Bus Technology lies in the openness of the system, emphasizing consensus and compliance with standards, breaking the independent standards of traditional manufacturers, this ensures that products from different manufacturers can be integrated into the same fieldbus system, and resources can be shared with other systems through gateways.

At present, on the one hand, the fieldbus standard is in the stage of perfection and development, on the other hand, the traditional ~ 20mA of simulation devices are also widely used in various fields of industrial control information. Therefore, it is unrealistic to achieve full digitalization right away. To meet the transition from analog to full digital, the HART Protocol emerged. HART uses the Frequency Shift Keying (PSK) technology. It is based on the Bell202 communication standard, in 4 ~ 20mA different frequency signals are superimposed on the analog signal. 2200Hz indicates "0", and 1200Hz indicates "1") to transmit digital signals. The data transmission rate of the HART protocol is bytes BPS bit/s ). HART fieldbus (HF) system adopts the master-slave mode: the host is a IBM-PC machine; the slave is one or more HF smart transmitters that comply with the HART protocol.

When the slave has only one HF Smart Transmitter, that is, the Smart Transmitter works in the dot-dot mode, you can continue to use the traditional 4 ~ 20mA signals are transmitted in analog mode, while measurement, adjustment, and test data are transmitted in digital mode. When the slave machine uses multiple HF smart transmitters, that is, when the smart transmitters work in the multi-station mode, 4 ~ 20mA signal is voided, and the working current of each transmitter is about 4mA. All measurement, adjustment, and test data are transmitted digitally. Each HF transmitter has a unique ID, so the host can operate on each transmitter. HART provides device Description Language DDL) to ensure interoperability. It should be pointed out that HART is regarded as a de facto industrial standard, but it is not a mixture of fieldbus simulation and digital. It can only be said to be a prototype of fieldbus and a transitional protocol. Because ~ 20mA analog signal standards will exist for a long time in the future, so studying the HART protocol is still of great significance.