Fieldbus is one of the hotspots in the field of automation in the field of automation today, and is known as the computer LAN in the field of automation. Its appearance provides powerful technical support for distributed control systems to realize real-time and reliable data communication between various nodes. The bus protocol is widely used between field smart meters and control rooms and is a communication protocol. For example: CAN bus protocol, Profibus bus protocol etc. Is HART protocol a bus protocol?
Because the HART communication protocol uses a half-duplex communication method, according to the definition of the field bus, the HART communication protocol does not fully comply with the requirements of the field bus technology. It can be said that it is not a true fieldbus technology, but a transitional measure between the analog system and the digital bus. Therefore, in the current transition period, it has strong market competitiveness and has achieved rapid development.
The HART protocol uses the FSK frequency-shift keying signal based on the Bell 202 standard to superimpose a 0.5-mA audio digital signal on a low-frequency 4-20 mA analog signal for bi-directional digital communication, and the data transmission rate is 1.2 kbps. Since the average value of the FSK signal is 0, it does not affect the size of the analog signal transmitted to the control system, ensuring compatibility with the existing analog system. In HART protocol communication, the main variables and control information are transmitted by 4-20mA. If necessary, additional measurements, process parameters, device configuration, calibration, and diagnostic information are accessed through the HART protocol.
HART stipulates a series of commands and works according to the commands. It has three types of commands. The first type is called common command. This is the command that all devices understand and execute. The second type is called general behavior command. The provided function can be implemented in many (though not all) field devices. This type of command includes the most commonly used function library for field devices; the third type is called special device commands to facilitate special functions in certain devices. Such commands can be used both in the foundation and in the Foundation. Unique to the company developing this order. These three types of commands can usually be found in a field device.
HART uses a unified device description language, DDL. Field device developers use this standard language to describe device characteristics. The HART Foundation is responsible for registering and managing these device descriptions and compiling them into a device description dictionary. The main device uses DDL technology to understand the characteristics of these devices without having to use these devices. Develop a dedicated interface. However, due to this analog digital mixed signal system, it has been difficult to develop a communication interface chip that can meet the requirements of various companies. HART can use bus power to meet intrinsically safe explosion-proof requirements, and can be composed of a dual-master system consisting of a handheld programmer and a host of the management system as the master device.
The difference between HART protocol and other bus protocols:
First, the fieldbus adopts true all-digital communication, and HART is superimposed on the original 4-20mA analog signal in the form of FSK, so it can be directly connected to the existing DCS system without reconfiguration.
Second, the fieldbus uses multiple connections, HART protocol will generally use multi-point connection only when doing monitoring.
Third, with the control system composed of fieldbus, devices can communicate directly without the intervention of the host.
Fourth, field bus devices can provide more diagnostic information than HART devices.
In short, fieldbus devices are suitable for high-speed network control systems, while the advantages of HART devices are reflected in compatibility with existing analog systems.
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