Modbus 通訊協定是什麼？Modbus RTU一次搞懂

A communication protocol, also referred to as a protocol, is a set of rules that define how data is exchanged between different devices in the industrial automation field. In essence, it acts as the "language" through which electronic devices communicate.

To illustrate, think about how human communication works: if two people don’t share a common language, they cannot understand each other. Similarly, industrial electronic devices require a shared communication protocol to exchange information effectively. When their protocols differ, the devices are unable to transmit or interpret data.

This concept is crucial in automation, where diverse systems such as programmable logic controllers (PLCs), sensors, and human-machine interfaces (HMIs) must collaborate seamlessly. Protocols ensure that these devices "speak" a common language, enabling smooth integration and operation.

Modbus is a widely used communication protocol in industrial automation, serving as a common language for devices to communicate.

Like human communication requiring both language and a medium, devices need a communication protocol to define data exchange rules and a communication standard (e.g., RS-232, RS-485, Ethernet) for signal transmission.

Modbus works across different physical layers, adapting to various scenarios for efficient data exchange.

The Modbus communication protocol is categorized into two main types based on the communication standard used: serial communication and Ethernet communication.

1. Serial Communication

In serial communication, data is transmitted bit by bit, and it includes two protocols: Modbus RTU and Modbus ASCII.

• Modbus RTU:
  • Uses binary encoding for data transmission.
  • Commonly paired with RS-232, RS-485, or RS-422 serial interfaces.
  • Ideal for short distances and multi-device applications.
  • Widely used in the industry due to its compact and efficient data format.
• Modbus ASCII:
  • Uses the American Standard Code for Information Interchange (ASCII) for data encoding.
  • Also paired with RS-232, RS-485, or RS-422 interfaces.
  • ASCII encoding is easier to manually verify but consumes more storage space.

2. Ethernet Communication

• Modbus TCP:
  • Operates over Ethernet, based on the TCP/IP protocol.
  • Uses network cables or wireless connections for data transmission.
  • Commonly applied in networked automation systems or remote monitoring setups.

Key Differences Between Modbus RTU and Modbus ASCII

While Modbus RTU and Modbus ASCII differ in encoding formats, their actual transmission speed depends on the baud rate of the serial interface (e.g., RS-232, RS-485).

• Modbus RTU: Binary format, compact data structure, suitable for bandwidth-limited environments.
• Modbus ASCII: ASCII format, each byte is represented by two ASCII characters, resulting in larger data size and slower transmission.

Many manufacturers choose the Modbus communication protocol for their PLCs (Programmable Logic Controllers) due to its simplicity and cost-effectiveness. The protocol's straightforward structure and the fact that it is free to use without licensing fees make it an attractive option.

Key Advantages of Modbus for PLCs:

1. Simple Structure & Free Licensing:

   Modbus is easy to implement, reducing development complexity and costs.

2. Support for Multiple Interfaces:
   • Compatible with RS-232, RS-485, and TCP/IP interfaces.
   • Adapts to various physical transmission media, such as twisted-pair cables, fiber optics, and wireless networks.
3. Flexibility Across Different Protocol Types:

   As discussed in the third section, Modbus protocols can be tailored for specific networks:

   • Fieldbus Networks: Protocols like Modbus RTU, Profibus, CC-Link, and CAN often use RS-485 for communication, supporting master-slave architectures or connections between controllers, devices, and nodes.
   • Ethernet Networks: Protocols such as Modbus TCP, PROFINET, EtherCAT, and EtherNet/IP enable efficient communication between clients and servers or controllers and devices.
4. High Compatibility with Industrial Networks:

   Modbus is highly compatible with both Ethernet and RS-485 networks, meeting the demands of industrial automation systems.

Due to its simplicity, versatility, and excellent compatibility with industrial communication standards, Modbus has become the preferred communication protocol for many PLC manufacturers. It offers reliable and efficient solutions for both fieldbus and Ethernet-based applications.

The Modbus RTU protocol uses a master-slave architecture where the master controls and initiates all communications, while slaves receive and respond to commands.

A master request includes a specific slave address, identifying the target device. Although all slaves receive the request, only the addressed device responds.

For example, consider a scenario with three slave devices assigned addresses 1, 2, and 3:
• If the master device sends a request to the slave device with address 2, only the slave with address 2 will respond, while the other devices will remain silent.
• If the master device uses broadcast mode (where the slave address in the request is set to 0), all slave devices (1, 2, and 3) will execute the command, but none will respond to the master device.
  
  This architecture ensures efficient communication and precise control in systems where multiple slave devices are connected to a single master.

In the Modbus protocol, each data field has a specific meaning and position. Users need to input a sequence of codes, which represent: "slave address, function code (for reading input registers), register address, number of registers to read, and CRC check code." Upon receiving these data, the Modbus master parses each field according to the Modbus format to determine the operation on the target slave, such as reading or writing a register. Here’s an example implementation of flow monitoring between a PLC and a flow meter using RS485 and Modbus RTU:
-Hardware Setup: Ensure that both the flow meter and PLC have RS485 interfaces and support Modbus RTU communication. Connect the PLC and flow meter through RS485 communication wiring.
-Modbus RTU Configuration and Program Design: Set the Modbus address of the flow meter (e.g., the slave address) and parameters like baud rate. Configure the PLC with the appropriate Modbus master functions and define the data to be read by the master. Ensure that the communication parameters (baud rate, data bits, stop bits, and parity bits) for both the PLC and flow meter match. 

The flow meter transmits data to the PLC via the RS485 interface, where the PLC reads the data using the Modbus RTU protocol. The data from the flow meter is then displayed in real-time on a computer monitoring interface, showing a curve of the flow readings.