The operator interfaces which enables monitoring and the issuing of process commands, like controller set point changes, are handled through the SCADA computer system. The subordinated operations, e.g. the real-time control logic or controller calculations, are performed by networked modules connected to the field sensors and actuators.

The SCADA concept was developed to be a universal means of remote access to a variety of local control modules, which could be from different manufacturers and allowing access through standard automation protocols. In practice, large SCADA systems have grown to become very similar to distributed control systems in function, while using multiple means of interfacing with the plant. They can control large-scale processes that can include multiple sites, and work over large distances as well as small distances. It is one of the most commonly used types of industrial control systems, in spite of concerns about SCADA systems being vulnerable to cyberwarfare/cyberterrorism attacks.

Both large and small systems can be built using the SCADA concept. These systems can range from just tens to thousands of control loops, depending on the application. Example processes include industrial, infrastructure, and facility-based processes, as described below:

• Industrial processes include manufacturing, process control, power generation, fabrication, and refining, and may run in continuous, batch, repetitive, or discrete modes.
• Infrastructure processes may be public or private and include water treatment and distribution, wastewater collection and treatment, oil and gas pipelines, electric power transmission and distribution, and wind farms.
• Facility processes, including buildings, airports, ships, and space stations. They monitor and control heating, ventilation, and air conditioning systems (HVAC), access, and energy consumption.

However, SCADA systems may have security vulnerabilities, so the systems should be evaluated to identify risks and solutions implemented to mitigate those risks.

A SCADA system usually consists of the following main elements:

Supervisory computers

This is the core of the SCADA system, gathering data on the process and sending control commands to the field-connected devices. It refers to the computer and software responsible for communicating with the field connection controllers, which are RTUs and PLCs, and includes the HMI software running on operator workstations. In smaller SCADA systems, the supervisory computer may be composed of a single PC, in which case the HMI is a part of this computer. In larger SCADA systems, the master station may include several HMIs hosted on client computers, multiple servers for data acquisition, distributed software applications, and disaster recovery sites. To increase the integrity of the system the multiple servers will often be configured in a dual-redundant or hot-standby formation providing continuous control and monitoring in the event of a server malfunction or breakdown.

Remote terminal units

Remote terminal units, also known as (RTUs), connect to sensors and actuators in the process and are networked to the supervisory computer system. RTUs have embedded control capabilities and often conform to the IEC 61131-3 standard for programming and support automation via ladder logic, a function block diagram or a variety of other languages. Remote locations often have little or no local infrastructure, so it is not uncommon to find RTUs running off a small solar power system, using radio, GSM or satellite for communications, and being ruggedized to survive from -20C to +70C or even -40C to +85C without external heating or cooling equipment.

Programmable logic controllers

Also known as PLCs, these are connected to sensors and actuators in the process and are networked to the supervisory system. In factory automation, PLCs typically have a high-speed connection to the SCADA system. In remote applications, such as a large water treatment plant, PLCs may connect directly to SCADA over a wireless link, or more commonly, utilize an RTU for communications management. PLCs are specifically designed for control and were the founding platform for the IEC 61131-3 programming languages. For economic reasons, PLCs are often used for remote sites where there is a large I/O count, rather than utilizing an RTU alone.

Communication infrastructure

This connects the supervisory computer system to the RTUs and PLCs and may use industry-standard or manufacturer proprietary protocols. Both RTUs and PLCs operate autonomously on the near-real-time control of the process, using the last command given from the supervisory system. Failure of the communications network does not necessarily stop the plant process controls, and on resumption of communications, the operator can continue with monitoring and control. Some critical systems will have dual redundant data highways, often cabled via diverse routes.

Human-machine interface

The human-machine interface (HMI) is the operator window of the supervisory system. It presents plant information to the operating personnel graphically in the form of mimic diagrams, which are a schematic representation of the plant being controlled, and alarm and event logging pages. The HMI is linked to the SCADA supervisory computer to provide live data to drive the mimic diagrams, alarm displays, and trending graphs. In many installations, the HMI is the graphical user interface for the operator, collects all data from external devices, creates reports, performs alarming, sends notifications, etc.

Mimic diagrams consist of line graphics and schematic symbols to represent process elements or may consist of digital photographs of the process equipment overlain with animated symbols.

Supervisory operation of the plant is by means of the HMI, with operators issuing commands using mouse pointers, keyboards, and touch screens. For example, a symbol of a pump can show the operator that the pump is running, and a flow meter symbol can show how much fluid it is pumping through the pipe. The operator can switch the pump off from the mimic by a mouse click or screen touch. The HMI will show the flow rate of the fluid in the pipe decrease in real-time.

The HMI package for a SCADA system typically includes a drawing program that the operators or system maintenance personnel use to change the way these points are represented in the interface. These representations can be as simple as an on-screen traffic light, which represents the state of an actual traffic light in the field, or as complex as a multi-projector display representing the position of all the elevators in a skyscraper or all of the trains on a railway.

A “historian”, is a software service within the HMI that accumulates time-stamped data, events, and alarms in a database which can be queried or used to populate graphic trends in the HMI. The historian is a client that requests data from a data acquisition server.

Communication methods:

SCADA systems have traditionally used combinations of radio and direct-wired connections, although SONET/SDH is also frequently used for large systems such as railways and power stations. The remote management or monitoring function of a SCADA system is often referred to as telemetry. Some users want SCADA data to travel over their pre-established corporate networks or to share the network with other applications. The legacy of the early low-bandwidth protocols remains, though.

SCADA protocols are designed to be very compact. Many are designed to send information only when the master station polls the RTU. Typical legacy SCADA protocols include Modbus RTU, RP-570, Profibus and Conitel. These communication protocols, except for Modbus (Modbus has been made open by Schneider Electric), are all SCADA-vendor specific but are widely adopted and used. Standard protocols are IEC 60870-5-101 or 104, IEC 61850, and DNP3. These communication protocols are standardized and recognized by all major SCADA vendors. Many of these protocols now contain extensions to operate over TCP/IP. Although the use of conventional networking specifications, such as TCP/IP, blurs the line between traditional and industrial networking, they each fulfill fundamentally differing requirements. Network simulation can be used in conjunction with SCADA simulators to perform various ‘what-if’ analyses.