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modbus rtu vs modbus tcp/ip

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Modbus RTU and Modbus TCP/IP are both communication protocols used to transmit data between devices in industrial automation systems. Here's a breakdown of their key differences:

Physical Layer:

  • Modbus RTU (Remote Terminal Unit): Utilizes a serial communication interface, typically RS-485. This allows multiple devices (slaves) to be connected on a single twisted-pair cable in a daisy-chain fashion. Each device requires a unique address on the network.
  • Modbus TCP/IP (Transmission Control Protocol/Internet Protocol): Leverages the widely used Ethernet network infrastructure. Each device connects directly to the network using a unique IP address, enabling a star topology.

Data Framing:

  • Modbus RTU: Employs a binary or ASCII format for data transmission. The binary format is more compact but requires additional processing, while ASCII offers better readability but is less efficient. It includes a Cyclic Redundancy Check (CRC) for error detection.
  • Modbus TCP/IP: Encapsulates the Modbus RTU message within a TCP/IP packet. This allows for reliable data transmission with error checking and retransmission capabilities inherent to TCP. It does not use CRC as TCP handles data integrity.

Network Scalability:

  • Modbus RTU: Limited by the cable length and number of devices supported by the RS-485 protocol (typically around 32 devices). It's suited for smaller, localized networks.
  • Modbus TCP/IP: Highly scalable due to the vast addressing capabilities of Ethernet networks. It can connect a large number of devices spread across a wider geographical area.

Applications:

  • Modbus RTU: Commonly used in point-to-point communication or small-scale networks where simplicity and cost-effectiveness are priorities. It's well-suited for connecting sensors, actuators, and PLCs (Programmable Logic Controllers) within a limited range.
  • Modbus TCP/IP: Ideal for integrating various devices across larger industrial networks or facilities. It allows for easier communication with SCADA (Supervisory Control and Data Acquisition) systems and HMI (Human-Machine Interface) applications running on computers within the Ethernet network.

Summary Table:

Feature

Modbus RTU

Modbus TCP/IP

Physical Layer

Serial (RS-485)

Ethernet

Topology

Daisy Chain

Star

Data Framing

Binary or ASCII

TCP/IP encapsulated Modbus RTU message

Error Checking

CRC

TCP (reliable data transfer)

Scalability

Limited (几十台设备 / ji shi tai she bei / dozens of devices)

High (Large networks)

Applications

Point-to-point, small networks

Large-scale industrial networks, SCADA, HMI

 

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