Modbus is a communication protocol widely used in industrial automation systems for connecting various devices, such as sensors, actuators, and programmable logic controllers (PLCs). It follows a client-server architecture, where a master device (client) initiates communication with slave devices.
Here is how Modbus communication works:
- Protocol Types: Modbus supports different protocol types, including Modbus RTU (Remote Terminal Unit) and Modbus TCP/IP (Transmission Control Protocol/Internet Protocol). Modbus RTU is a serial communication protocol that uses binary encoding, while Modbus TCP/IP is based on Ethernet and uses a TCP/IP stack for communication.
- Master-Slave Communication: In a Modbus network, there is typically one master device that initiates communication and one or more slave devices that respond to the master’s requests. The master device can read data from or write data to the slave devices.
- Addressing: Each slave device in a Modbus network is assigned a unique address. The master device uses this address to identify and communicate with specific slave devices. In Modbus RTU, the slave address is a number between 1 and 247, while Modbus TCP/IP uses IP addresses to identify the slave devices.
- Request-Response Model: The master device sends requests to the slave devices using specific Modbus function codes. These function codes define the type of operation to be performed, such as reading holding registers or writing to coils. The request message includes the slave address, function code, and any additional parameters required for the operation.
- Data Format: Modbus uses a simple data format, with data organized into coils, discrete inputs, input registers, and holding registers. Coils and discrete inputs are binary data, while input registers and holding registers store 16-bit or 32-bit integer values.
- Communication Protocol: In Modbus RTU, the data is transmitted in binary format over serial communication links, such as RS-232 or RS-485. It uses a master-slave communication model with a predefined baud rate, parity, and stop bits. Modbus TCP/IP, on the other hand, uses Ethernet as the underlying communication medium and communicates using TCP/IP packets.
- Response and Error Handling: When a slave device receives a valid Modbus request, it processes the request and sends a response back to the master device. The response message contains the requested data or an error code if the request was unsuccessful. The master device can then interpret and utilize the received data or handle any errors accordingly.
Modbus is a widely adopted protocol due to its simplicity, flexibility, and compatibility with a wide range of devices. It allows for easy integration and communication between devices from different manufacturers, making it a popular choice in industrial automation applications.