Application of HART protocol in communication chip

Through some previous content, we know about the HART protocol and its important role in the bus. So today, let's analyze a type of application. The following describes how to use the HART protocol for communication chips. The HART (Highway Addressable Remote Transducer) protocol can address the Remote sensor express connect open communication protocol. It is a communication protocol between on-site intelligent instruments and control room equipment launched by Rosemen in 1985 ｡

The HART protocol adopts half-duplex communication. According to the ISO/OSI Open System Interconnection model, it adopts a simplified three-layer model structure, that is, the first layer physical layer, the second layer data link layer and the seventh layer application layer ｡

The first layer: physical layer. It specifies the signal transmission method and transmission medium. In order to achieve both analog and digital communication without mutual interference, the HART protocol uses the Frequency Shift Keying Technology, that is ~ 20mA a frequency signal is superimposed on the analog signal. The frequency signal adopts the Be11202 international standard. The Transmission baud rate of the digital signal is set to 1200bps. The 1200Hz indicates the logic "1", and the 2200Hz indicates the logic "0 ", the signal amplitude is 5mA. Because the average value of the PSK signal is 0, the analog signal size transmitted to the control system is not affected, ensuring compatibility with the existing simulation system ｡

Layer 2: data link layer. It specifies the HART frame format to establish, maintain, and terminate Link communication. The HART protocol uses an automatic repeated request sending mechanism based on Redundant Error code information, eliminate data communication errors caused by line noise or other interference, so that communication data can be transmitted without errors. Layer 7: Application layer. It is a HART command set used to implement HART commands, general commands, Common commands, and dedicated commands ｡
 
Hardware Design of smart Transmitters
 
Intelligent On-site instruments require 4 ~ 20mA traditional transfer of measurement results, the HART protocol digital communication method is used to exchange information with the control device, such as the meter setting parameters, intermediate measurement data, and calibration parameters. Therefore, the HART protocol communication module must complete the digital code of the measurement result to 4 ~ 20mA analog signal DA conversion, as well as the aforementioned many other information of the HART protocol digital communication ｡
 
The sensor converts the Capacitance change signal of the metal capacitor sensor into an electrical signal related to the pressure through the shock circuit, the analog-to-digital converter is provided after the signal is conditioned. The analog-to-digital conversion chip adopts the type AD7799. It is a new type, high precision, wide dynamic range, and 24-bit AD converter of the AD Company. automatic Calibration, system calibration, and background calibration Functions, zero point error, full range error, and temperature drift can be eliminated ｡

This design uses the 16-bit digital analog converter AD421 launched by AD, Which is powered by Loop + and Loop-, and can directly convert digital signals into 4 ~ 20mA current output. Two lines of power supply are used, and the voltage regulator is provided in the chip. In addition to providing its own electricity, it can provide the voltage of 5 V, 3.3V, or 3 V for other parts of the system. At the same time, it can also provide precise reference voltage of + 1.25V and + 2.5V. In this design, AD421 uses LATCH, CLOCK, and DATA MCU for serial communication ｡

The modem used in this design is HT20C15, and adopts a phase-continuous double-duplex operation mode, the data rate is ipvbps. the device is integrated with the following components: A tuner, a modem, a timing circuit, and a detection and control circuit that complies with the BELL202 standard ｡
 
HT20C15 and MCU exchange digital signals and serve as AD421 analog signal interfaces. When HT20C15 receives signals ~ After receiving the signal on the 20mA loop, the system extracts and decrypts the PSK signal as a digital signal after band-pass filtering, amplification, and shaping, and decrypts the signal of the Hz to "1 ", the 2200Hz signal is demodulated to "0" and sent to the MCU. When sent, the MCU receives data, performs modulation and waveform shaping, and is coupled to the AD421, 1 is modulated to 1200Hz, 0 is modulated to 2200Hz, and is superimposed to the loop through AD421. HT20C15 and MCU are connected through the general serial communication interface of the latter, to implement HART communication ｡
 
Software Design for smart Transmitters

During HART communication, the host sends a command frame. After the command frame is received by the field instrument through a serial terminal, the MCU processes the corresponding data and generates a response frame, which triggers sending interruption, send a response to complete a command exchange ｡

After the system is powered on or the watchdog is reset, the main program initializes the communication part, mainly to complete the clock selection, baud rate setting, serial port working mode setting, open interrupt, and so on ｡

After the initialization is complete, the communication is always in the prepare and accept state. When a command is sent from the host computer, the OCD on the HT20C15 pin changes to a low level, triggering the interruption. The MCU starts receiving through the serial port, the program enters the receiving process. After receiving the message, the MCU reads the host command for explanation, executes the corresponding operation, and finally generates a response frame and sends it to the sending buffer. The Pin is set to low in INRTS, start sending. After sending the answer frame, wait for the host command and wait for the next interruption ｡
 
Conclusion
 
The smart products based on HART Protocol have excellent performance, good compatibility, and high cost performance, it has become an industrial standard in the field of intelligent control. For a long time, HART protocol products will still have a very broad market at home and abroad ｡