HART protocol design principles and parameter standards

The HART protocol is an open reference model based on ISO standards. This Protocol also belongs to the scope of communication protocols. Various communication applications and standards have been applied in many fields. HART, or Highway Addressable Remote Transducer, is short for Addressable Remote Sensor highways. It was first developed by Rosemount and supported by more than 80 famous instrumentation companies, it features no interference 4 ~ 20mA analog signals allow two-way digital communication at the same time. The smart electromagnetic flowmeter compliant with the HART protocol can not only detect various traffic and display it locally, but also remotely set it through the upper computer, changing the zero point, range, and self-diagnosis of a flow meter greatly facilitates the use and maintenance of the flow meter, and thus has a strong market competitiveness ｡

HART Protocol Introduction

The HART Protocol follows the OSI Open System Interconnection Reference model developed by ISO and adopts the first, second, and seventh layers of the OSI model, namely the physical layer, data link layer, and application layer ｡

1 physical layer Specification

The HART protocol adopts the American telephone communication system Bell202 Frequency Shift Keying (PSK) standard, ranging from 4 ~ 20mA simulation ± 0. 5mA of the sine wave, the baud rate is percentile BPS. Because the average value of the superimposed sine signal is 0, in addition, the phase Continuous Frequency Shift Keying Technology requires that the phase angle at the boundary of Data bit 1 and 0 at the baud rate is continuous, so the digital communication signal will not affect 4 ~ Analog Signal of 20mA ｡

2 Data Link Layer specifications

This part of the Protocol specifies the HART frame format to implement the function of establishing, maintaining, and communicating data links. The HART protocol checks the error code based on redundancy, communication data is transmitted without errors through the error detection mechanism and the automatic resending request protocol (ARQ ｡

Protocol-related data transmission is implemented in the form of frames. frames are the encapsulation of user data by user control and addressing information, only the byte count in the frame ends and the error is verified to receive the correct message or the physical layer notifies the signal transmission to terminate (if the carrier is not detected) before frame identification. The HART frame format is 1 ｡

PREAMBLE leading signal ｡

DELIM delimiters: A unique or easiest character to recognize ｡

ADDR address byte: contains the source address and destination address. The maximum bit is used to indicate the address of the master device associated with the frame ｡

COM command byte: indicates the function to be executed by the field meter ｡

Total byte cornt Data Length: This value indicates the number of bytes from the next BYTE of byte cornt to the last BYTE (excluding the validation bytes ｡

DATA bytes ｡

CHK parity: vertical parity ｡

The HART protocol in the data link layer is a "Master/Slave" protocol. In the Response Message of the slave device, besides the received validation command, many Messages also contain data asked by the master device. The HART protocol allows two master devices to run simultaneously in the system, one basic master device, A sub-master device. The sub-master devices distribute their respective response command messages to the sub-master devices based on their addresses ｡

3 Application Layer specifications

Three types of commands in the HART message package are defined. The first type is General commands, which are applicable to all products that comply with the HART Protocol and provide functional descriptions for devices that comply with the HART protocol; the second type is common operation commands, applicable to most products that comply with the HART protocol; the third type is special commands for devices, applicable to special products that comply with the HART protocol ｡