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Understanding the Remote Terminal Unit in SCADA Systems: In the world of supervisory control and data acquisition (SCADA) systems, the term “RTU” is frequently encountered. But what exactly is a Remote Terminal Unit, or RTU for short? In this comprehensive guide, we’ll delve into the intricacies of RTUs, their history, components, and their crucial role in SCADA systems.

Unveiling the RTU

Remote Terminal Unit (RTU), as the acronym suggests, is a control device stationed in remote locations as an integral part of expansive systems. The primary function of an RTU is to oversee and manage an array of field devices, encompassing valves, actuators, sensors, and more. In the SCADA landscape, RTUs play a pivotal role by bridging the gap between SCADA control and the physical processes they govern.

ACE 3600 Remote Terminal Unit

ACE 3600 Remote Terminal Unit (Image Courtesy of Ebipax)

Distinguishing RTUs from PLCs

It’s not uncommon for RTUs to be mistaken for programmable logic controllers (PLCs) due to their resemblances and the latter’s prominence in industrial automation. However, they serve distinct purposes and possess technical disparities that render them suitable for different applications.

The Historical Evolution of RTUs

To comprehend the evolution of RTUs, we must delve into the history of SCADA itself. The earliest SCADA systems sprouted in power generation plants during the 1950s. The challenge of that era lay in collecting data from interconnected substations and transmitting it to the central control room. This challenge was met through the inception of telemetry applications—the progenitors of RTUs.

Advancements in computer science during the 1960s and 1970s ushered in a new era for SCADA and RTU systems. Instrumentation expanded to cover greater distances, and RTUs enabled the acquisition of larger data volumes while performing rudimentary control functions.

The modern RTU, as we know it today, emerged in the 1980s and matured throughout the 1990s. These microprocessor-based RTUs boasted advanced logic capabilities akin to PLCs. Furthermore, new communication protocols such as RS-232, RS-482, and Ethernet were introduced, facilitating the exchange of even more information. Contemporary RTUs are packed with enhanced features and functionalities.

Deconstructing the RTU: Components in Detail

Let’s dissect the components that constitute an RTU:

RTU hardware structure.

RTU hardware structure. (Image from ResearchGate)

Central Processing Unit (CPU)

The CPU, often comprised of intricate circuit cards housing 32-bit microprocessors, is the heart of the RTU. It employs a watchdog timer to validate the proper execution of cycles. Some RTUs even feature ethernet communication ports, and redundancy is assured in devices equipped with dual CPUs.

Power Supply

While RTUs draw continuous power from a primary source, remote locations where they’re installed typically employ backup battery modules. These modules ensure uninterrupted operation, with lead-acid and lithium batteries being the most common choices.

Communication Ports

Communication with the client or master SCADA station necessitates at least one communication port. RTUs support a plethora of communication protocols, including Ethernet, RS-232, and Modbus, and can be incorporated into various network topologies like ring, series, and star configurations.

Physical I/O

The four fundamental I/O modules supported by RTUs are digital input, digital output, analog input, and analog output. Digital inputs capture status and alarm signals from field devices, while digital outputs transmit signals and commands. Analog inputs and outputs work within specific current or voltage ranges, typically 0-1 mA or 0-10 V, respectively.

RTUs in the Context of SCADA Systems

Example of a flow meter and a wireless sensor connected to an RTU. Image used courtesy of Bentek Systems

In a comprehensive SCADA architecture, various field devices are employed, and one or more RTUs connect these devices, facilitating the transmission of data concerning physical events. Field devices encompass a wide array of sensors, including proximity sensors, temperature sensors, flow meters, level meters for liquids, and wireless sensors, to name a few. The RTU’s pivotal role lies in compiling data from these field devices and transmitting it to the central SCADA control.