There's been a lot of talk lately about supervisory control and data acquisition (SCADA) systems, most of it having to do with cybersecurity. Originally, and still to some extent, people not very familiar with control system architectures thought all control systems were SCADA, and so it became a buzzword for all types of industrial control systems.
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SCADA systems were developed from the radiotelemetry stations installed by the government for military and space program use. Because of the low bandwidth and mostly analog signals, the architecture was developed, so that all of the control was done locally through a remote telemetry unit or RTU, while the data was sent to a control room called the "head end," where operation of the system was monitored. Rarely would anyone attempt to add some control features.
COTS and SCADA
In the 1970s, SCADA systems were purpose-built from the RTU to the head end (the control panel). In the 1980s and 1990s, the advent of PLCs and HMI software made it possible for the newly emerged control system integrator to produce a custom SCADA system out of components and integrate it himself. SCADA system architecture diverged. In many cases, the old line SCADA companies disappeared or were bought out, such as the purchase of Bristol Babcock by Emerson Process Management. The majority of SCADA installations in water and wastewater, for example, are now done this way. Small SCADA companies, such as Multitrode Pty., just acquired by Xylem Corp., continue to manufacture purpose-built RTUs, but provide impressive connectivity to ControlLogix, OPC, DNP3, Modbus and other protocols. The larger oil and gas-centered SCADA brands, such as Bristol Babcock and Telvent, now parts of Emerson and Schneider Electric, respectively, continue to produce purpose-built RTUs and SCADA head ends because their markets need the specific types of intellectual property built into the system architectures, such as flow calculations and custody transfer algorithms.
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The PLC or PAC-based SCADA systems are used in many other applications, such as water treatment and distribution, wastewater distribution, and the like.
Radio-Liberated SCADA—Now Come Cellular and Mesh Radios
At first, most SCADA system RTUs were interconnected with the head end using leased lines from the phone company. These were incredibly expensive and could transmit only one variable or status (alarm) setting per leased line. In the 1970s, two things changed. First, digital telemetry from the space program became simple and cost-effective because of the introduction of microprocessors. Digital telemetry could be transmitted via radio (or satellite in the case of remote oil and gas pipelines). Many digital signals could be encoded and transmitted simultaneously at a relatively high rate of throughput. This enabled real control and not just "supervisory" control from the head end.
Radio SCADA systems, especially water and wastewater systems in metropolitan areas were spectrum limited. There were only a few frequencies available in the Industrial Scientific Medical (ISM) bands, and once those were gone, they were gone. The advent of cellular radio made it somewhat easier to use radio telemetry for SCADA, but until the pricing came down, cellular radio made only moderate inroads into applications. In the early 2000s, however, cellular providers realized the potential for SCADA applications, and created pricing plans that made them competitive with leased lines and broadcast radio frequencies.
The use of cellular wireless networks made integrating RTUs relatively simple. The field devices and motor controls connect to an RTU, which connects to a cellular modem. Some SCADA companies, such as Opto22, Advantech, and Exemys have introduced wireless field devices (Exemys has standardized on Zigbee, a protocol similar to WirelessHART or ISA100.11a), so the installation and integration is even simpler. The wireless field devices form themselves into a mesh network with a gateway. The gateway connects to the RTU. Sometimes the gateway itself is wireless, using the IEEE 802.11 protocol.
Integrating wireless field devices using IEC62591-WirelessHART or ISA100.11a makes it possible for real-time diagnostic information, as well as the process values (PVs) and alarms and status indications, to be transmitted through the RTU to the SCADA head end. This means that the cost of operators driving to the remote stations to check status and evaluate and inspect the field devices, pumps and motor controls is reduced to nearly nothing.