I. Overview

HART (Highway Addressable Remote Transducer) protocol uses FSK frequency shift keying signal based on Bell202 standard to superposition 0.5mA audio digital signal on low frequency 4-20mA analog signal for two-way digital communication. The data transfer rate is 1200bps. Since the average value of the FSK signal is 0, the size of the analog signal transmitted to the control system is not affected, and the compatibility with the existing analog system is guaranteed. In HART communication, the main variables and control information are transmitted by 4-20mA, and additional measurement, process parameters, equipment configuration, calibration, and diagnostic information are accessed through HART if needed.

At the application level, HART specifies a series of commands that work in a command manner. It has three types of commands. The first type is called general command, which is the command that all devices understand and execute. The second category, called common application commands, provides functionality that can be implemented in many (though not all) field devices; The third category is called device-specific commands to make it easier to work on implementing special features in certain devices, which are either open to use in the foundation or unique to the company that developed the command.

Ii. Meaning

1, the host (Master) : divided into the first host and the second host, it can be connected to the HART network, issue commands and slave communication.

2, slave machine (Field Device) : connected in the process control site, with the function of measuring, calculating a variety of variables, and can communicate with the host HART equipment.

3, Long Frame (Long Frame) : Address consists of 5 bytes HART data frame, all HART commands support long frame data frame. The meaning of each byte of the long frame address is shown in the following figure:

The Manufacturer ID is assigned by the HART Foundation;

In production, the serial number of the pass-type equipment is unique and never repeated. The broadcast address uses a long frame structure, and the lower 38 bits are all 0.

The manufacturer specified device Type (Decive Type) and manufacturer ID meet the following requirements:

4, Short Frame (Short Frame) : The Address is composed of 1 byte HART data frame, short frame only the general command 0 can be used, short frame byte meaning is as follows: all HART slave machine support long and short frames.

5, request frame (STX) : by the host (such as hand copy) to the slave machine (field equipment) issued a request data command.

6, Reply frame (ACK) : the slave machine replies to the host data.

7. Preamble: The 0xFF character sent by the device that initiates the command (data) transmission before the transmission of the data frame, the number of sent characters is generally 5 to 20, and the number can be changed by command; The use of the leading character is to enable the data receiver to generate the CD carrier detection signal on the hardware circuit to realize the synchronization of data communication.

8, Delimiter: indicates the beginning of the data frame transmission, indicating the transmission direction of the data, the physical layer type, and it also specifies the frame type of the data frame (long and short frames), its meaning is as follows:

9, Address (Address) : by Delimite delimite specified as a short frame (1 byte) or long needle (5 bytes) two types.

10. Expansion Bytes: The company retains the current HART protocol version, which is 0.

11, Command (Command) : function code, indicating the specific function of a data frame, the command has three categories of general command, ordinary application command, device special command.

12. Bytes Count: indicates the actual Data data.

13, Data: Set or read the parameter data of the specified slave machine (the final result of communication).

14, Check Byte (Check Byte) : from the delimiter to the data of all bytes of the “xOR” value, that is, vertical check.