Definition and Basic Concepts of Remote Terminal Units

A Remote Terminal Unit (RTU) is a microprocessor-controlled electronic device that serves as an equipment interface, bringing data into distributed control systems or Supervisory Control and Data Acquisition (SCADA) systems. RTUs transmit telemetry data to the master system and use data from the main monitoring system to control connected devices. In industrial automation and remote monitoring fields, RTUs play a crucial role, especially in applications requiring remote data acquisition and control.

RTUs can also be described as remote telemetry units or remote telecontrol units, with their core function being to connect field devices with central control systems, enabling bidirectional data transmission and control command execution.

RTU Architecture

Remote terminal units can monitor analog and digital data in the field and transmit this data back to the central monitoring station. A typical RTU system includes the following key components:

Hardware Components

• Central Processing Unit Card: Responsible for data processing and command execution
• Communication Interface Card: Enables communication with central control systems
• Analog Input (AI) Card: Collects continuously changing physical quantities such as temperature, pressure, and flow
• Digital Input (DI) Card: Monitors switch states such as valve positions and circuit breaker status
• Digital or Control Output (DO/CO) Card: Executes control commands, such as switching equipment
• Analog Output (AO) Card: Provides variable control signals

Power Supply

RTUs are equipped with power supply units that convert AC mains power into different voltages required by CPUs and interface cards. In critical applications, RTUs typically include backup batteries and charging circuits to ensure normal operation during mains power interruptions.

Key Functions of RTUs

Data Acquisition Capabilities

• Digital Inputs: Obtain real-world information that can be represented by two states, such as equipment on/off status and alarm conditions
• Analog Inputs: Monitor various analog signals, such as 4-20mA current loops and 0-10V voltage, converting physical quantities into appropriate units

Control Output Functions

• Digital Outputs: Control field equipment switching through relays
• Analog Outputs: Provide variable inputs for controlling devices that require continuous adjustment

System and Logic Control

Modern RTUs can execute simple programs without requiring intervention from the main system, which not only simplifies system deployment but also provides redundancy for safety. For example, an RTU in a water treatment plant management system can suspend automatic control in maintenance mode to avoid equipment damage or personnel injury caused by accidental operations.

Communication Capabilities

RTUs need to communicate with multiple systems, including master workstations and Intelligent Electronic Devices (IEDs). Communication can take place through various networks:

• Serial Communication: Such as RS232, RS485, RS422
• Ethernet Communication: Suitable for scenarios requiring higher bandwidth
• Wireless Communication: Appropriate for long-distance or situations where wired installation is difficult

RTUs support multiple communication protocols, such as Modbus, IEC 60870 series protocols, DNP3, and IEC 61850, to achieve interoperability with third-party devices.

Communication Modes

• Polling Mode: The master station periodically queries data from slave devices
• Active Reporting: When data changes exceed preset thresholds, slave devices report proactively
• Periodic Synchronization: Complete data exchange at regular intervals to ensure information synchronization

SeRTU200 Telemetry Terminal Product Features

Taking Key-IoT company’s SeRTU200 series as an example, this is a multi-functional telemetry terminal that integrates data collection with various communication capabilities:

Product Overview

The SeRTU200 series is a multi-functional telemetry terminal that integrates 4G/NB-IoT, LoRa, and GPS communication functions, equipped with rich acquisition control interfaces including RS232, RS485, Ethernet, analog input, digital input, and relay functions. The product features user-friendly design and rich functionality, capable of achieving long-term continuous automatic data collection in various environments with real-time automatic reporting.

Core Features

• Wire-free installation, flexible network configuration, plug-and-play
• Multi-channel image acquisition capability
• Support for remote querying of local data
• Support for local serial port configuration and upgrades
• Remote querying of current rainfall, water level, water flow, water quality, and other data
• Support for three-in-one functionality: image monitoring + water conservancy data collection + wireless communication transmission
• Support for rainfall collection, warning triggers, local storage, scheduled reporting, and other functions
• Support for remote management, upgrades, and configuration through cloud platforms

Hardware Specifications

• SPI Flash: 16MB
• Industrial Serial Ports: 2 RS232, 2 RS485 ports
• Support for RTC and hardware watchdog functions
• Grounding Protection: M4 screw grounding protection terminal
• Gigabit Ethernet: 1 gigabit Ethernet port
• 4G Module: PCIE interface, USB2.0 signal

Interface Configuration

• 4-channel analog input interface (12-bit AD, supports 4-20mA current signal input, optional 0-5V voltage signal input)
• 2-channel digital input interface (opto-isolated)
• 2-channel relay output interface
  • Maximum switching voltage: 30VDC/220VAC
  • Maximum switching current: 4A
  • Maximum switching power: 120W/880VA

Comparison Between RTUs and Other Control Systems

The functionality of RTUs increasingly overlaps with Programmable Logic Controllers (PLCs) and Distributed Control Systems (DCS), but there remain some distinctions:

• Application Scope: RTUs are more suitable for long-distance systems, especially scenarios requiring wireless communication; PLCs are more applicable to shorter-distance systems such as factories and production lines
• Programming Tools: PLCs primarily use programming tools based on the IEC 61131 standard, while RTUs often employ proprietary programming tools
• Communication Capabilities: RTUs typically possess stronger remote communication capabilities and protocol support

It’s worth noting that with technological developments, many manufacturers now offer RTUs with simple graphical user interfaces, allowing users to easily plan their installations, while some applications have also integrated data logger functionality.

RTU Application Scenarios

Remote Terminal Units have wide applications across multiple domains:

• Water Conservancy and Hydrological Monitoring: Such as rainfall, water level, water flow monitoring, and early warning
• Environmental Monitoring: Air quality, noise, radiation, and other environmental parameter monitoring
• Power Systems: Substation automation and distribution network monitoring
• Oil and Gas Pipelines: Pressure and flow monitoring, leak detection for long-distance pipelines
• Agricultural Irrigation: Remote monitoring and control of automated irrigation systems
• Municipal Facilities: Street light control, drainage pump station monitoring, and other urban infrastructure management

Taking the SeRTU200 as an example, it is particularly suitable for ecological environment monitoring and pollution source monitoring scenarios, achieving comprehensive functions such as data acquisition, storage, display, control, alarm, and transmission.