Abstract: Gas detection and monitoring are relatively common applications at present, and are widely used in industrial production, medical diagnosis, environmental monitoring, national defense and other fields. This article will introduce the RS-485 networking scheme in the gas detection industry in detail.

In recent years, with the rapid development of China's economy, the advancement of science and technology has provided conditions for the development of the gas detection instrument industry. The promotion of market and government policies, the improvement of people's safety awareness, and the improvement of relevant laws and regulations are the development of the gas detection industry. core power. In recent years, the global gas detection instrument has maintained a growth rate of about 8%. With the rapid development of my country's economy, it will also usher in greater prosperity. The broad prospects of the gas detection instrumentation industry have attracted the attention of relevant industrial instrumentation giants.

1. Function and classification of gas detection sensors

Gas detector is a kind of instrumentation tool for gas leakage concentration detection, including: portable gas detector, hand-held gas detector, fixed gas detector, online gas detector, etc. The gas sensor is mainly used to detect the type of gas existing in the environment. The gas sensor is a sensor used to detect the composition and content of the gas. Widely used in industrial, medical, petrochemical and power industries.

Gas detector application diagram

The output signal of the gas detector is mainly divided into two types: 4-20mA and RS-485.

l4-20mA: This method is usually adopted for civilian use;

lRS-485: mainly used in industrial sites, and some application scenarios will use isolation;

The characteristics of 4~20mA analog transmission: the processing speed is fast, the trouble of software development can be saved in application, and it is very convenient to integrate into the system. But in implementation, from the aspect of wiring, the acquisition of analog quantity can only be wired one-to-one, that is, an analog quantity signal must be wired with one (pair) line. In addition, the anti-interference ability of the analog quantity is weak, and the transmission distance and the data that can be represented are also very limited.

Features of RS-485 transmission: Compared with analog signals, RS485 has a slower response speed. But RS485 communication can transmit data with larger bit width and more data content. In addition, in the wiring, the collection of RS485 signals can use a one-to-many bus method (only two lines can collect multiple signals), the transmission distance of RS485 signals is also far, and the anti-interference ability is also strong. Industry products will gradually replace 4~20mA with RS-485 bus.

Table 1 Comparison of communication methods of gas detectors

2. Network characteristics of gas detection instruments

The gas alarm control system consists of a gas detection and detection unit, a signal transmission and acquisition unit, a display unit (centralized display on the alarm control host), a monitor, a control unit, an alarm unit, and a controlled equipment unit (such as a fan and a solenoid valve). The gas detection probe converts the gas concentration detected on site into standard signals, such as 4-20mA or RS485 signals, and then transmits the signals to the alarm control host for unified display. The network block diagrams of the two communication forms are shown in Figure 2 and Figure 3 below.

4~20mA communication application box

RS-485 communication application block diagram

Three, gas detector industry analysis

1. Upgrading of industry communication solutions

The gas detector used 4~20mA analog transmission in the early days, and will be replaced by RS-485 due to two reasons.

The reason for the characteristics of the communication form: the analog transmission has problems such as the unnetworked system, poor anti-interference and small number of transmissions;

Secondly, for gas detection manufacturers, 4~20mA detection instruments are too general and have been replaced. RS-485 will be used as an independent protocol to establish replacement barriers.

2. Development of industry standards

The release of industry standards brings opportunities to isolate RS-485 demand. Gas detectors are widely used in metallurgy, petroleum, chemical, municipal, underground pipe gallery and other industries. According to the national standard "GB15322.2-2003 Combustible Gas Detector Part 2 : Independent Combustible Gas Detector" section 5.1.16 requires that in the municipal and urban underground pipe gallery projects, the gas detector equipment is subject to the tests in the following table, and the detector function is normal after the test. Include the following:

During the test, the detector shall not send out an alarm signal or an irreversible fault signal;

After the test, the difference between the alarm action value of the detector and the alarm set value should not exceed ±5% LEL.

Specifically as follows:

Gas detector industry standard

Based on the self-developed and designed power supply chip ZLG1001, ZLG Zhiyuan Electronics launched the Priority series of optimal power supply for all working conditions (referred to as P series) to meet the needs of all working conditions and provide users with stable and high-quality power supply solutions.

The parameters of P series power supply products are as follows:

Isolation voltage: 3000VDC;

Product packaging: SIP is compatible with domestic and international product packaging;

Support continuous short circuit, self-recovery;

RoHS compliant production process;

Perfect aging test, EMC test.

In addition, the fully isolated SK series open-board isolated RS-485 module SK4400S launched by ZLG Zhiyuan Electronics has been widely recognized by the industry due to its simpler application and smaller display area than discrete construction. The specific parameters of SK4400S are as follows:

Set isolation and ESD bus protection functions in one;

Isolation voltage 3500VDC/2500VAC;

A maximum of 128 nodes can be connected to the same network;

Working temperature: -25℃～+85℃;

With short circuit protection, over temperature protection.