Based on the Modbus protocol, this paper expounds the design examples of Modbus's two transmission modes and serial communication programs, and gives a list of VB language programs.
Serial Communication Program Based on Modbus Protocol----Introduction to Modbus ProtocolThe MODBUS protocol supports legacy RS-232, RS-422, RS-485 and Ethernet devices. Many industrial equipment, including PLCs, DCS, smart meters, etc., use the Modbus protocol as the communication standard between them.
The Modbus protocol is a versatile language applied to electronic controllers. Through this protocol, controllers can communicate with each other and between controllers via a network (such as Ethernet) and other devices. The Modbus protocol defines a message structure that a controller can recognize, regardless of the network through which they communicate. It describes the process by which a controller requests access to other devices, if it responds to requests from other devices, and how to detect errors and record them. It develops a common format for message domain patterns and content.
Rs-485 adopts half-duplex working mode and supports multi-point data communication. The rs-485 bus network topology generally adopts a terminal-matched bus type structure. That is to say, each bus is connected in series by a bus, and a ring or star network is not supported.
The rs-485 uses balanced transmit and differential receive, so it has the ability to reject common mode interference. In addition to the high sensitivity of the bus transceiver, it can detect voltages as low as 200mv, so the transmitted signal can be recovered beyond the kilometer. Some rs-485 transceivers modify the input impedance to allow up to 8 times more nodes to be connected to the same bus. The most common application of rs-485 is communication between programmable logic controllers in an industrial environment.
Serial communication program design exampleFor ease of understanding, an application example using RTU mode communication is listed below. The hardware of this example consists of a computer and 10 temperature and humidity meters distributed in 10 rooms to form an RS485 network architecture. The address of the temperature and humidity meter is set to 01H to 0AH. The command message frame of the computer reading each temperature and humidity table data contains 8 bytes:
After receiving the above command message, the named temperature and humidity meter sends temperature and humidity data to the computer, and the message frame contains 11 bytes:
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