Modbus RTU (Remote Terminal Unit) is a serial communication protocol that is widely used in industrial automation systems. It is a binary-based protocol that allows communication between a master device (such as a supervisory control and data acquisition – SCADA system or a programmable logic controller – PLC) and multiple slave devices (such as sensors, actuators, or other field devices).
Below is how Modbus RTU communication works:
- Physical Layer: Modbus RTU typically uses RS-485 or RS-232 serial communication standards as the physical layer. RS-485 is commonly used due to its ability to support multi-drop communication, where multiple devices can be connected on the same bus.
- Data Frame Format: Modbus RTU uses a simple and compact data frame format for communication. Each data frame consists of the following components:
- Slave Address: Each slave device on the Modbus RTU network is assigned a unique address from 1 to 247. The master device includes the slave address in the data frame to specify which slave it wants to communicate with.
- Function Code: The function code indicates the type of operation the master device wants to perform on the slave device. For example, it could be a read operation (reading input registers) or a write operation (writing to coils or holding registers). The function code helps define the format and purpose of the data being transmitted.
- Data: The data field contains the actual payload of the message. It could be a request for data or a response containing the requested data. The data field can vary in size depending on the specific function code and the type of data being transmitted.
- Error Checking: Modbus RTU uses a cyclic redundancy check (CRC) to ensure data integrity during transmission. The CRC is calculated based on the contents of the data frame and is included at the end of the frame. The receiving device verifies the CRC to check for any transmission errors.
- Master-Slave Communication: In a Modbus RTU network, the master device initiates communication by sending a request data frame to a specific slave device. The slave device, upon receiving the request, processes the command and sends a response data frame back to the master device. The master device can then interpret the response and take further action based on the received data.
- Timing and Synchronization: Modbus RTU communication is asynchronous, meaning there is no fixed timing or synchronization between the master and slave devices. The devices communicate at their own pace, and timing is typically controlled by the master device. The timing parameters, such as baud rate, parity, and stop bits, are configured to ensure proper communication between devices.
Modbus RTU is a widely supported and relatively simple protocol, making it popular for communication in industrial automation applications. It offers efficient and reliable communication over serial connections, allowing for real-time control and monitoring of devices in industrial systems.