Serial communication program based on Modbus protocol_RS485 bus system application
This paper explores the design of Modbus communication systems, focusing on both its serial transmission modes and corresponding serial communication programs. It also provides a sample VB program to illustrate practical implementation. Modbus is a widely used communication protocol that supports various physical layers such as RS-232, RS-422, RS-485, and Ethernet. It is commonly adopted in industrial automation systems, where devices like PLCs, DCS systems, and smart meters rely on it for data exchange. The protocol allows controllers to communicate with each other over a network or directly through serial connections. The Modbus protocol defines a standardized message format that enables devices to request data, respond to queries, and handle errors. This makes it highly flexible and adaptable across different communication mediums. It ensures consistency in how devices interact, regardless of the underlying network type. The protocol also includes mechanisms for error detection and logging, which are essential for reliable operation in industrial environments. RS-485 is a standard for serial communication that operates in half-duplex mode and supports multi-point data transmission. Its typical network topology is a linear bus structure, where all devices are connected in series. Unlike star or ring topologies, RS-485 does not support branching or looping configurations. One of the key advantages of RS-485 is its ability to reject common-mode noise due to its balanced differential signaling. This makes it ideal for long-distance communication, even in electrically noisy environments. Some advanced transceivers allow for up to eight times more nodes on a single bus, enhancing scalability. RS-485 is extensively used in industrial settings for communication between PLCs and other control devices. To help clarify the concept, an example using Modbus RTU mode is provided. In this scenario, a computer communicates with 10 temperature and humidity sensors installed in different rooms, forming an RS-485 network. Each sensor is assigned a unique address from 01H to 0AH. When the computer sends a query, it uses an 8-byte command frame. Upon receiving the command, the respective sensor responds with an 11-byte data frame containing the measured temperature and humidity values. This process demonstrates the basic mechanism of Modbus RTU communication over an RS-485 network. Three Phase Voltmeter,Control Device,Digital Instrument,Voltage Meter,Multifunctional digital panel voltmeter zhejiangjinyidianqiyouxiangongsi , https://www.jooeei.com
