HART protocol hierarchy

We are always unfamiliar with the HART protocol. This is because the structure of this protocol is associated with some knowledge about communication principles. Here is a brief introduction. Then we will elaborate on its hierarchy. There are a variety of different hosts and Field Equipment in modern industrial production. To use them well, a complete communication protocol is necessary. The HART protocol was initially developed by Rosemount and has been applied for many years. The HART protocol uses the PSK Technology in 4 ~ 20mA a frequency signal is superimposed on the signal process, and the analog signal and the digital signal are successfully communicated simultaneously without mutual interference. According to the open Interconnection Model of the International Organization for Standardization (ISO), the HART protocol uses the OSI Standard physical layer, data link layer, and application layer. The HART Protocol specifies the physical form of transmission, message structure, data format, and a series of operation commands. It is a master-slave protocol. When the communication mode is "Q & A", a field device only responds to the request. The HART protocol allows two hosts in the system, for example, one for system control and the other for HART communication, up to 15 slave devices can be connected to a single cable line in a multi-point system.

Physical Layer

The physical layer specifies the signal transmission methods and media. The system uses the Bell202-standard PSK signal ~ 20mA the analog signal is stacked with a frequency digital signal for Bidirectional digital communication. The digital signal range is 0. 5 m A, the data transmission rate is bytes BPS, the frequency Hz represents the logic "1", and the 2200Hz represents the logic "0 ".

Data Link Layer

The data link layer specifies the format of the HART protocol frame. The addressable range is 0 ~ 15. When the value is "0", it is in the range of 4 ~ 20mA and digital signal point-to-point mode. The field meter and two digital communication Main devices, also known as communication equipment or main devices, use specific serial communication. The main devices include PCs or control room systems and handheld communication devices. In a single-site operation, the main variable process variable can be output in analog form or read in digital communication mode. When read in digital mode, the polling address is always 0. That is to say, the digital signal and 4 ~ 20mA both analog signals are valid.

"1 ~ 15 "is in full digital communication, working in point-to-point mode. The communication mode includes the Q & A and burst modes (point-to-point and Automatic Continuous sending of information ). The data update rate during Q & A operations is 2 ~ 3 times/s, the data update rate is 3 ~ 4 times/s.

With essential security requirements, only one power supply can be used to connect 15 on-site instruments. Each on-site device can have 256 variables, and each information can contain up to 4 variables. This is the so-called multi-site operation mode. This method is especially suitable for remote monitoring, such as pipeline systems and tank storage sites.

Multi-Point Mode, 4 ~ The analog output signal of 20mA is no longer valid. The output is no longer set to 4mA to minimize power consumption. It is mainly used to power the transmitter, and each field device is connected in parallel ), the System reads the measured values or other data of multiple on-site instruments connected to a pair of transmission lines in sequence through digital communication ). If a control system is formed in this way, the cost of field cabling can be significantly reduced and the input interface circuit of the main device can be reduced, which is of great value to the control system.

Based on the Redundant Error code information, the HART protocol uses an automatic repeated request sending mechanism to eliminate data errors caused by line noise or other interference.

Powered by bus, it can meet the essential safety and explosion-proof requirements.

The frame format of the HART protocol is encoded with 8 bits. Each byte is encoded with a start bit, a parity bit, and a stop bit for serial transmission. Generally, UART universal asynchronous receiving/transmitter is used to transmit bytes. The length of the HART data cannot exceed 25 bytes, because the data length and availability are not constant.