Modbus RTU

Modbus RTU is a widely used communication protocol in industrial automation systems. However, many people are still unsure about what Modbus is, what RTU means, and how RTU Modbus works. This article by SCMA will explain everything from the basic operation, functions, data transmission, to its application in industry, so that you can understand and use it efficiently.

What is Modbus RTU?

Modbus RTU is the use of the Modbus protocol in a binary format, typically used through serial communication and known for its compact data representation, making it highly efficient and fast in data transmission. RTU stands for Remote Terminal Unit, which is a communication standard developed by Modicon (currently Schneider Electric) since 1979. This Modbus RTU format uses a more compact binary data encoding than Modbus ASCII, allowing more data to be transmitted within the same bandwidth.

How does Modbus RTU work?

Modbus RTU uses a Master-Slave architecture for communication, where RTU is a system with a Master device controlling and initiating all communications while Slave devices wait for commands and respond as requested.

  • Addressing - RTU Modbus assigns each Slave a specific address (1-247). When the Master sends a command, it specifies the address of the Slave to communicate with.
  • Response - Although every Slave receives the transmitted data, only the Slave with the matching address responds, preventing data collisions.
  • Broadcast Mode - Modbus can send broadcast commands using Address 0, which all Slaves will follow simultaneously but without a response.
  • Error Checking - Modbus RTU uses CRC-16 (Cyclic Redundancy Check) to verify the accuracy of transmitted data.
  • Communication Speed - Supports speeds from 1200 bps to 115200 bps, depending on distance and signal quality.

What is Modbus RTU Used For?

Modbus RTU is a widely used protocol in industrial control systems. RTU technology enables various devices to efficiently exchange data through serial communication via RS-232 or RS-485.

  • Output Coils - Stores 1-bit (0 or 1) data, used for controlling ON/OFF devices such as relays, valves, motors in Modbus RTU can read and write values
  • Input Contacts - Stores 1-bit data from Input devices like limit switches, detection sensors. Modbus allows reading values only.
  • Input Registers - Stores 16-bit data from analog measuring devices such as temperature, pressure, voltage, current. Modbus RTU uses Read-Only to read these values
  • Holding Registers - Stores 16-bit data for values that need to be both read and written, such as Setpoint, various control parameters.
  • Applications - Used in SCADA systems, PLCs, HMIs, energy measurement devices, temperature control systems, and other automation systems.

Functionality of Modbus RTU

Modbus RTU has a set of standard functions defined by Function Codes for reading and writing various types of data. RTU is the system that uses these Function Codes to command Slave devices to perform as required. Modbus RTU divides main functions into two groups: Reading (Read) and Writing (Write). Modbus defines Function Code 01-06 for reading/writing Coil and Register data, Function Code 15-16 for writing multiple values simultaneously, and other Function Codes for diagnostics and special operations.

Modbus RTU Data Frame Transmission and Reception

Data transmission in Modbus RTU has a clear frame structure. RTU stands for the binary encoding format, which makes data compact and fast to transmit.

Request Frame Structure:

  • Slave Address - Specifies the destination device address in RTU Modbus (1 byte)
  • Function Code - Indicates the type of operation required in Modbus (1 byte)
  • Data Address - The starting position of the data to be read or written (2 bytes)
  • Quantity/Value - Number of data items or values to send (2 bytes or more)
  • CRC Check - Error-checking value for Modbus RTU (2 bytes)

Response Frame Structure:

  • Slave Address - Device address that is responding (1 byte)
  • Function Code - Confirms the function being performed (1 byte)
  • Byte Count - Number of bytes in the response data
  • Data Values - Readable data or write status
  • CRC Check - Error-checking value for reliability

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Summary

Modbus RTU is an important communication protocol in industrial applications. With its simplicity, reliability, and widespread open standard usage, understanding the basic operation, functions, and data structure of Modbus RTU can help you design and maintain your system more effectively. SCMA is ready to be your partner for Modbus systems with high-quality products and comprehensive services, from consultation, sales, installation, to after-sales service, ensuring that your automation system runs smoothly and efficiently.

Frequently Asked Questions

What are RS485 and MODbus?

RS485 is a physical layer (Physical Layer) communication standard that defines electrical signal characteristics, connections, and data transmission distance. Modbus is a protocol (Protocol) that specifies the format and method of data exchange. Modbus can operate on RS485, RS232, or Ethernet. RS485 is commonly used with Modbus RTU because it has long-range transmission (1200m) and supports multiple devices (32 nodes).

How do BACnet and Modbus differ?

BACnet was specifically designed for smart building systems, featuring high complexity, support for various types of data, and a detailed Object Model. In contrast, Modbus is a simple protocol that is easy to use and suitable for general industrial equipment. BACnet has higher costs but offers more features such as automatic device discovery and scheduling, while Modbus focuses on simplicity and communication speed.

Which data layer does Modbus RTU use in communication?

Modbus RTU uses a Master-Slave communication method at the Data Link Layer, employing binary encoding and CRC-16 for error checking. At the Physical Layer, RS-485 or RS-232 is commonly used. The data frame consists of Address, Function Code, Data, and CRC, with a silent interval (Silent Interval) of 3.5 character times separating frames.

How do Profibus and Modbus differ?

Profibus is more complex than Modbus, offering high speeds up to 12 Mbps and supporting deterministic communication with profiles for specific devices. In contrast, Modbus is simpler, has lower speed (up to 115.2 kbps for RTU), but is easy to use and an open standard. Profibus is suitable for systems requiring high speed and accuracy, while Modbus is better suited for general systems that require simplicity.