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Temperature and humidity sensor RS-485 communication and Modbus communication protocol

by:Rika Sensors     2021-12-03
Temperature and humidity sensor RS-485 communication and Modbus communication protocol
For a long time, human beings have been accustomed to obtain information directly from their own sensory organs, but this way of obtaining information is very limited. In order to overcome the limitations of human organs and natural conditions, people invented sensors and sensor communication technology. With the rapid development of various sensing technologies, the means of obtaining and uploading information have undergone changes again and again, greatly enhancing people's ability to obtain information.

1. The origin of RS-485 and Modbus

RS-485: Normally, data upload uses serial communication for data exchange. The method used at first was RS232 communication. Due to the complexity of the industrial site, various sensor equipment will generate a lot of electromagnetic interference in the environment, and transmission errors often occur. In addition, RS232 can only realize point-to-point communication, without networking function, and the transmission distance can only reach more than ten meters, which cannot meet the requirements of long-distance communication. These problems were not resolved until the emergence of RS-485.

RS-485 signal adopts differential transmission mode, which can effectively solve the problem of common mode interference, the communication distance can reach 1200 meters, and it allows multiple transceivers to be connected to the same bus.

Modbus protocol: With more and more industrial application communications, Schneider Electric formulated a bus protocol Modbus protocol for industrial field in 1979, and now many RS-485 communication occasions in the industry use Modbus protocol. Next we will talk about RS-485 communication and Modbus communication protocol.

Second, the relationship between RS-485 and Modbus protocol

First, RS-485 is a hardware layer protocol, and Modbus is a software layer protocol above this hardware layer, and is an application layer message transmission protocol. In layman's terms, ModBus stipulates the master and slave, what commands the master should send to the slave. ModBus stipulates the data interaction between the master and the slave, what format needs to be followed, and how to ensure that the data does not conflict during the transmission. As long as they all follow this protocol, the master and slave machines of different manufacturers can be shared. Modbus protocol includes RTU, ASCII, TCP. Among them, MODBUS-RTU is very commonly used and relatively simple.

1. RS-485 communication

RS-485 is a standard that defines the electrical characteristics of drivers and receivers in a balanced digital multipoint system. The standard is defined by the Telecommunications Industry Association and the Electronics Industry Alliance. Digital communication networks using this standard can effectively transmit signals under long-distance conditions and in environments with high electronic noise. RS-485 makes it possible to connect to the local network and the configuration of multi-branch communication links.

RS-485 has two wirings, two-wire and four-wire. The four-wire system can only achieve point-to-point communication. It is rarely used now, and the two-wire wiring method is mostly used. This wiring method is Bus topology structure, up to 32 nodes can be connected on the same bus.

2, Modbus communication protocol

Modbus protocol is a universal language applied to electronic controllers. Through this protocol, the controllers can communicate with each other, and between the controllers and other devices via a network (such as Ethernet). It has become a general industry standard. With it, control equipment produced by different manufacturers can be connected to an industrial network for centralized monitoring. This protocol defines the message structure that a controller can recognize and use, regardless of what kind of network they communicate through. It describes the process of a controller requesting access to other devices, how to respond to requests from other devices, and how to detect and record errors. It has developed a common format for the structure and content of the message domain.

Modbus has the following characteristics:

(1) Standard and open, users can use the Modbus protocol for free and with confidence, without paying license fees, and will not infringe on intellectual property rights.

(2) Modbus can support a variety of electrical interfaces, such as RS-232, RS-485, etc., and can also be transmitted on various media, such as twisted pair, optical fiber, wireless, etc.

(3) Modbus frame format is simple, compact and easy to understand. It is easy for users to use and easy for manufacturers to develop.

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