Modbus, developed in 1979 by Modicon (now part of Schneider Electric), is the world's first industrial communication protocol. It serves as a standard for connecting programmable logic controllers (PLCs), smart terminals, and computers through either a public line or a dedicated local network. The Modbus system comprises both hardware and software components, enabling it to support various data acquisition and process monitoring applications. In this system, there is only one master device, and all communication originates from it. A single Modbus network can theoretically support up to 247 remote slave devices, though the actual number depends on the communication hardware used. Each PC connected to the network can exchange information with the central host without affecting its own control functions.
**Modbus RTU Communication Protocol**
When a command is sent from the master to a device, the device with the matching address code receives the message, strips off the address, processes the request, and if no errors occur, performs the corresponding action. It then sends back the result, including the address code, function code, data, and a CRC-16 error-checking code. If an error occurs, no response is sent.
**1. Information Frame Structure**
The Modbus RTU frame consists of four main parts: Address Code, Function Code, Data Area, and Error Check Code.
- **Address Code**: An 8-bit value ranging from 0 to 255, identifying which slave device should respond.
- **Function Code**: Indicates the specific operation the slave should perform.
- **Data Area**: Contains the data being read or written, such as register values or parameters.
- **Error Check Code**: A 16-bit CRC checksum used to verify the integrity of the transmitted data.
Each frame follows the same structure: address, function, data, and error check.
**2. Error Checking**
The CRC-16 error-checking mechanism uses a 16-bit binary value calculated at the sending end and appended to the message. Upon receiving the message, the recipient recalculates the CRC and compares it with the received value. If they do not match, an error is detected, ensuring reliable communication.
**Modbus RTU Data Frame Format**
Setting the communication speed involves configuring specific registers. For example:
- Address 0x0002 can be set to different baud rates like 2400, 4800, 9600, or 19200.
Commands such as reading or writing internal registers follow specific formats. For instance, reading the communication rate might involve a frame with address 01, function code 03, and appropriate data fields.
Time settings, sampling frequencies, sensor data, temperature readings, and acceleration data are also managed through distinct register addresses and data structures. Each command has a unique frame format that ensures accurate and reliable communication across the Modbus network.
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