HART protocol physical layer Specification Based on Software Design

A series of design standards based on the HART protocol, as well as the software platform and design are described in the following device application. The main reference objects in this article are explained based on a software design. Next we will give a brief introduction to this. To develop a system according to a set of standard protocols, the biggest advantage is that both hardware equipment providers and software developers can independently design according to standard specifications, standard interfaces are provided separately to greatly improve work efficiency. The final joint debugging of software and hardware will bring the greatest convenience. The HART protocol is the theoretical basis for the software design of the data platform and the entire control system, in order to achieve the communication between the upper computer software and the field instrument based on the HART protocol, data exchange can be achieved. At present, there are few documents that summarize and digest the HART protocol in China. Therefore, in this chapter, zhengzhou port Electronics Co., Ltd. will summarize the HART protocol specifications involved in the software design based on the digestion and understanding of the HART protocol in the research work.

1. HART protocol Overview:

The HARTHighway Addressable Remote Transducer protocol is a Transitional Standard compatible with 4-20mA signal and digital communication technologies. Currently, it already has Rosemount, Smar, ABB, Fuji, Moore, E + H, Honeywell, fisher Controls, Arcom Control Systems Ltd. more than 70 companies participated in the HART Protocol Fund HCF. Due to the many unquestionable advantages of the HART protocol, it became the world's most widely used field communication protocol. In 1994, the HART transmitter accounted for 76% of the world's Smart Transmitter market, and has become a de facto industrial standard. According to estimates from the industry, the International Life of the HART protocol is 15-20 years. due to objective conditions in China, this time will be longer. Therefore, for a long period of time in the future, HART products are still widely available in the market.

The HART protocol retains the industrial standard of 4-20mA process control signals, allowing both analog and digital communication signals to exist on the same loop without mutual influence. This is achieved through the use of Bell202 communication standards, and Bell202 uses the Frequency Shift Keying FSKFrequency Shift Keying Technology. HART communication is achieved by applying a frequency signal to a 4-20mA current. There are two signal frequencies. One is 1200 HZ, which represents the logic "1", the other is 2200 HZ, which represents the logic "0", and the signal amplitude is 5mA.

In the entire communication process, both the analog signal 4-20mA), the digital signal 2200 HZ and HZ), because in a signal cycle, the average value of the communication signal is 0, this does not affect the 4-20mA analog signal, which is one of the most important features of the HART protocol.

The HART Protocol references the OSIOpen Systems Interconnection model proposed by ISO. This model provides the structures and elements required by the communication system. HART only uses a simplified OSI model, with only one, two, and seven layers.

Layer number layer name OSI level HART level

7. HART command for formatting data at the application layer

6. No data is converted in the presentation layer.

5 Session Layer Control session none

4. Ensure that the information is complete at the Transmission Layer

3 network layer route transfer unavailable

2. data link layer error handling Protocol rules

1 physical layer connection device BELL 202

Relationship between HART protocol and OSI reference model

The HART protocol includes the physical layer, data link layer, application layer, and DLL language.

2. HART protocol physical layer Specification

The physical layer specifies the signal transmission method, signal level, device impedance, and medium. Generally, the physical layer uses twisted pair wires as the media to transmit digital signals or analog signals separately on twisted pair wires. Valid communication distance is 5000 feet 1500 ). The transmission of HART digital signals is a simple extension of analog signal transmission. In the current mode, a high-frequency current is superimposed on an existing low-frequency analog current typically 4-20 mA. In the voltage mode, the HART voltage signal is superimposed on the existing DC voltage signal (typically 1-5VDC. These two signal transmission modes share the same hardware, but are separated in frequency.

HART uses the binary phase Continuous Frequency Shift Keying of 1200bit/s ).

Information Format: 1 start bit 0), 8 data bits, 1 parity check), and 1 stop bit.