I. Overview
HART (Highway Addressable Remote Transducer) protocol uses FSK frequency shift keying signal based on Bell202 standard to superimpose amplitude on low frequency 4-20mA analog signal
Two-way digital communication for 0.5mA audio digital signal, data transmission rate of 1200bps. 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
The compatibility with the existing simulation system. In HART protocol communication, the main variables and control information are transmitted by 4-20mA, and if necessary, additional measurement process parameters, device configuration, and control information.
Calibration and diagnostic information is accessed through HART protocol.
At the application level, HART specifies a series of orders that work in a command manner. It has three classes of commands, the first class is called the general command, which is the command that all devices understand and execute; The second category is called ordinary
By applying commands, the functionality provided can be implemented in many (though not all) field devices; The second type is called device-specific commands to facilitate the implementation of special functions in some devices, such as orders
The command is either open for use in the foundation or unique to the company that developed it.
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, the function of measuring, calculating a variety of variables, and can communicate with the host HART equipment.
3. Long Frame: The Address is a HART data frame composed of 5 bytes. All HART orders support long frame data frames. The meaning of each byte of the long frame address is as follows:
The Manufacturer ID is assigned by the HART Foundation;
In production, pass type equipment serial number is unique, Huai Fu. 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 consists of 1 byte HART data frame, short frame can only be used by the general command 0, short frame byte meaning is as follows: all
HART slave supports both 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, which can be passed by the command
To change; The use of the leading character is to make the data receiver generate CD carrier detection signal in the hardware circuit to realize the synchronization of data communication.
8, Delimiter: shows the beginning of the data frame transmission, indicates the transmission direction of the data, the physical type, and it also specifies the frame type of the data frame (long and short frames), the meaning of the content
As follows:
9, Address: Delimite specifies two types of short frame (1 byte) or long needle (5 bytes).
