Application of HART communication protocol to converter analysis (1)

Next, we will introduce the specific concepts and some applications of the HART communication protocol. Then we will analyze the products related to the RS232 converter. Help you understand the concept of the HART communication protocol. First, let's take a look at the specific content of HART. HART (Highway Addressable Remote Transducer), an open communication protocol that can address Remote sensor express connect, it is a communication protocol between on-site intelligent instruments and control room equipment launched by Rosement in 1985. The HART device provides communications with relatively low bandwidth and moderate response time. After more than 10 years of development, the HART communication protocol technology has become very mature abroad, and has become the industrial standard of global intelligent instruments.

The HART protocol uses a base-on-the-Bell202-Standard Frequency Shift Keying Signal. It overlays an audio digital signal with a amplitude of 20mA on a low-frequency 4-5mA analog signal for two-way digital communication. The data transmission rate is 1.2 Mbps. Because the average value of the 2's key-value pairs is 0, the analog signal sent to the control system is not affected, ensuring compatibility with the existing simulation system. The main variables and control information in the HART protocol communication are transmitted from 4-20mA, in addition, measurement, process parameters, device configurations, calibration, and diagnosis information are accessed through the HART protocol.

HART communication adopts a half-duplex communication method. It is a transitional product that implements digital signal communication over the existing analog signal transmission line, therefore, the current transitional period has a strong market competitiveness and achieved rapid development. HART specifies a series of commands that work in command mode. It has three types of commands. The first type is generic commands, which are understood and executed by all devices. The second type is general behavior commands, the functions provided can be implemented in many field devices, though not all). Such Commands include the function libraries of the most commonly used field devices. The third class is the special device commands, to facilitate work to implement special functions in some devices, such commands can be made available in the foundation, and can be exclusive to the companies that develop this command. In a field device, these three types of commands are usually found at the same time.

The HART communication protocol adopts the unified device Description Language DDL. Field Equipment developers use this standard language to describe device features. The HART Foundation is responsible for registering and managing these device descriptions and compiling them into a device description dictionary, the master device uses DDL technology to understand the characteristic parameters of these devices without having to develop special interfaces for these devices. However, due to this analog digital hybrid signal system, it is difficult to develop a communication interface chip that can meet the requirements of various companies. HART can use bus power supply to meet the essential safety and explosion-proof requirements. It can form a dual-master device system consisting of a handheld programmer and a management system host.

For example:

HART/RS232 converter song Mao electronic SM100-A) and pressure transmitter communication instructions

Step 1: the two-wire connection between the HART/RS232 converter and the pressure transmitter is performed in accordance with the instructions, and the connection is confirmed to be correct.

Step 2: Open the test software of the HART/RS232 converter, open the serial port and connect to it, and test the serial port. The HART/RS232 converter can communicate with the PC normally.

Step 3: Press Start detection. The test software starts to automatically detect the test instrument. The test software displays the instrument ID and other parameters,

Command analysis:

The command format is described as follows:

Start position sequent separator address command Data Length checkpoint Terminator

To ensure the complete sending of the commands to be sent, we specifically add a start bit "23" before the standard Command Format prefix and an end character "40" after the check bit ".

1) For example: Send Read Device serial number command 23 FF 02 80 00 00 82 40

The command annotation is shown in the following table:

Start bit 23 overhead FF delimiter 02 address 80 command 00 Data Length 00 check bit 82 Terminator 40

The returned command format is as follows:

Sequent delimiter address command data length data check bit

Returned data: FF 06 80 00 0E 00 00 FE 11 0E 05 05 01 0C 08 00 5E EE 09 D5