1 Introduction
HART, the Highway Addressable Remote Transducer, is the abbreviation of the Addressable Remote Sensor Highway. It was first developed by Rosemount Company of the United States and supported by more than 80 famous instrument companies. It is characterized by not disturbing the 4-20mA analog signal. Allow two-way digital communication.
The intelligent electromagnetic flowmeter conforming to the HART protocol can not only realize various flow detection and local display, but also remotely set by the upper position meter, change the zero point and range of the flow meter, and complete self-diagnosis and other functions, to a large extent. It facilitates the use and maintenance of the flowmeter and thus has a strong market competitiveness. [1] This paper discusses the technical problems of the realization of intelligent electromagnetic flowmeter based on HART protocol. One is to solve the design problem of hardware circuit, and the other is to discuss the realization of HART protocol command set programming and the implementation of PC software.
2 Introduction to HART Protocol
The HART protocol follows the ISO-based OSI Open System Interconnection Reference Model and uses the first, second, and seventh layers of the OSI model, namely the physical layer, the data link layer, and the application layer.
2.1 Physical layer specification
The HART protocol uses the American Telephone Communication System Bell202 Frequency Shift Keying (FSK) standard, which simulates a ±0.5 mA sine wave at 4 to 20 mA with a baud rate of 1200 bps. Because the average value of the superimposed sinusoidal signal is 0, and the phase continuous frequency shift keying technique requires that the phase angle of the boundary of data bits 1 and 0 with a baud rate of 1200 Hz is continuous, the digital communication signal does not affect 4~ 20mA analog signal.
2.2 Data Link Layer Specification [2][3]
This part of the protocol specifies the format of the HART frame to implement the establishment, maintenance, and data link communication functions. The HART protocol implements communication data based on the error detection code information and the automatic retransmission request protocol (ARQ). Error transmission.
Protocol-related data transmission is implemented in the form of frames. The frame is the encapsulation of the user data by the user control and addressing information. Only after the byte count in the frame ends and the error check is received correctly or the physical layer notification signal transmission is terminated (if the carrier is not detected), the frame can be performed. Frame recognition. The format of the HART frame is shown in Figure 1.
PREAMBLE preamble signal.
DELIM delimiter: A unique or most recognizable character.
ADDR address byte: contains the source address and destination address, the highest bit is used to indicate the address of the master device associated with the frame.
COM command byte: Indicates the function to be performed by the field instrument.
BYTE CORNT Total Data Length: This value represents the number of bytes from the next byte to the last (excluding the check byte) of BYTE CORNT.
DATA data byte.
CHK Parity: Vertical parity.
The HART protocol in the data link layer is a "master/slave" protocol. In addition to the acknowledgment command that has been received in the response message of the slave device, many messages also contain data that the master device interrogates. The HART protocol allows two master devices to run simultaneously, one basic master device and one slave master device. The slave device distributes its respective response command messages to the two master devices based on their different addresses.
The 7-inch tablet can be used as the golden size of a tablet computer. It is small and portable. It can be used at home and outdoors. You can browse the web, watch videos and play games. It is a household artifact. Although the size of the 7-inch tablet is inclined to the tablet, the function is more inclined to the mobile phone, so it can also be used as a substitute for the mobile phone. Compared with other sized tablets, the 7-inch tablet has obvious advantages in appearance and weight. Both the body size and the body weight have reached a very reasonable amount.
1.In appearance, the 7 inch tablet computer looks like a large-screen mobile phone, or more like a separate LCD screen.
2.In terms of hardware configuration, the 7 inch tablet computer has all the hardware devices of a traditional computer, and has its own unique operating system, compatible with a variety of applications, and has a complete set of computer functions.
3.The 7 inch tablet computer is a miniaturized computer. Compared with traditional desktop computers, tablet computers are mobile and flexible. Compared with Laptops, tablets are smaller and more portable
4.The 7 inch tablet is a digital notebook with digital ink function. In daily use, you can use the tablet computer like an ordinary notebook, take notes anytime and anywhere, and leave your own notes in electronic texts and documents.
7 Inches Tablet Pc,Quad Core Tablet 7 Inch,7 Inch Gaming Tablet,Supersonic Tablet 7 Inch
Jingjiang Gisen Technology Co.,Ltd , https://www.gisentech.com