What protocol does a Remote Terminal Unit use?

Industrial Automation
1

What protocol does a Remote Terminal Unit use?

image
image1766×1055 246 KB

2

A Remote Terminal Unit (RTU) is a crucial component in SCADA (Supervisory Control and Data Acquisition) systems, interfacing with multiple master stations and Intelligent Electronic Devices (IEDs) using different communication protocols. These protocols are typically implemented over serial (RS-232, RS-485, RS-422) or Ethernet connections.

RTU Supported Protocols

An RTU may support a variety of standard protocols to ensure interoperability with third-party software and devices, including:

  • Modbus (Modbus RTU, Modbus TCP) Commonly used in industrial automation.
  • DNP3 (Distributed Network Protocol 3) Used in utilities for reliable data transmission.
  • IEC 60870-5-101/103/104 Standard in power system monitoring.
  • IEC 60870-6-ICCP Used for inter-control center communication.
  • IEC 61850 Applied in substation automation.
  • Profibus/Profinet Used in process and factory automation.
  • HART (Highway Addressable Remote Transducer) For smart field devices.
  • BACnet – Common in building automation.
  • MQTT (Message Queuing Telemetry Transport) Used for IIoT (Industrial Internet of Things) applications.

RTU Data Transfer and Synchronization

Data transfer between RTUs, master stations, and IEDs can be initiated from either end, using various techniques to ensure synchronization while minimizing data traffic:

  1. Polling Mechanism: The master station periodically polls the RTU or RTU polls the IED for data updates.
  2. Analog value reporting: Reported only if changes exceed a defined threshold.
  3. Digital (status) value reporting: Transmitted in groups (bytes) only when at least one bit changes.
  4. Event-Driven Updates: A subordinate unit initiates an update when a predetermined change occurs in analog or digital data.
  5. Periodic Full Data Transmission: Ensures full synchronization and eliminates stale data, irrespective of individual updates.

Most communication protocols support both polling and event-driven updates, configurable by the installer.

Multi-Drop Communication

Multiple RTUs or IEDs can share a communications line using a multi-drop scheme. Each unit is assigned a unique address and only responds to its designated polls and commands.

IED Communications with RTU

Communication between the RTU and IEDs facilitates data transfer, reducing the need for extensive hardware inputs and outputs. This exchange can occur via copper or fiber optic lines, improving efficiency and reducing wiring complexity.

Master Communications with RTU

Communication between the RTU and a master station typically involves a larger control system or data collection system. Data transfer can take place over various mediums, including:

  • Copper cables
  • Fiber optics
  • Radio frequency communication systems