What is SCADA ?

SCADA Definition

SCADA (Supervisory Control And Data Acquisition) systems are mainframe, workstation, or PC computers interfacing with field devices accessed through remote terminal units (RTU's) or programmable logic controllers (PLC's). These systems collect data and/or perform control actions on field devices. Operators and programmers interface with the computer system via a human machine interface (HMI). SCADA systems minimise the responsibilities of the operator and in many cases eliminate the need for a full time operator. SCADA systems allow remote sites to communicate with a control facility and provide the necessary data to control processes. For many of its uses, SCADA provides an economic advantage. As the distance and inaccessibility to remote sites increases, SCADA becomes a better alternative than an operator or repairman's visiting the site for adjustments and inspections.

SCADA Standards

The American National Standards Institute (ANSI) developed ANSI C37.1 (also known as IEEE C37.1), "IEEE Standard Definition, Specification, and Analysis of Systems Used for Supervisory Control, Data Acquisition, and Automatic Control." ANSI C37.1 applies to systems used for monitoring and controlling substations and power plants. This standard contains useful definitions and features for SCADA systems. In 1992 a standard was published, which established recommended practices for master station equipment communications protocols to remote equipment. This standard is IEEE Std999-1992, "IEEE Recommended Practice for Master/Remote Supervisory Control and Data Acquisition.

History of SCADA

SCADA can be traced to the development of telemetry from the first half of the century. Telemetry is the transmission and collection of data obtained by sensing real-time conditions. The technology of rockets and aircraft afforded man with the opportunity to investigate weather and planetary data. This required a simple way to get data from space that observers could not normally achieve. Manned stations on the surface of the Earth such as lighthouses, post offices, weather stations, etc., were able to collect and monitor data on weather. However, for accurate weather prediction, more detailed information was needed from the atmosphere. There were two questions to be answered. How could accurate data be gathered from the atmosphere and communicated back to a facility on the Earth's surface? And, how might data be gathered from a number of sites in one centralised location to record, analyse, and then predict the weather. Typically, there are three major elements that make up a SCADA system:
1. The master terminal unit (MTU)
2. The remote terminal unit (RTU)
3. The communications equipment The MTU monitors information from remote sites and displays information for the operator. The relationship between MTU and RTU is typically defined as a "master and slave" and refers to the communications protocol. The simplest form of a SCADA system is where a single MTU and RTU reside in the same building such as a small water treatment plant. The three major elements are further defined in the following sections.

Master Terminal Unit (MTU)

At the heart of the system is the master terminal unit (MTU). The master terminal unit initiates all communication, gathers data, stores information, sends information to other systems, and Interfaces with operators. The major difference between the MTU and RTU is that the MTU initiates virtually all communications between the two. The MTU also communicates with other peripheral devices in the facility like monitors, printers, and other information systems. The primary interface to the operator is the monitor or CRT that portrays a representation of valves, pumps, etc. As incoming data changes, the screen is updated.

Remote Terminal Unit (RTU)

Remote terminal units gather information from their remote site from various input devices, like valves, pumps, alarms, meters, etc. Essentially, data is either analog (real numbers), digital (on/off), or pulse data (e.g., counting the revolutions of a meter). Many remote terminal units hold the information gathered in their memory and wait for a request from the MTU to transmit the data. Other more sophisticated remote terminal units have microcomputers and programmable logic controllers (PLC) that can perform direct control over a remote site without the direction of the MTU. Protocols can be open, like Transmission Control Protocol and Internet Protocol (TCP/IP) or proprietary. The RTU receives its information because it sees its node address embedded in the protocol. The data is then interpreted, and the CPU directs the appropriate action at the site.

Communications Equipment

Communication equipment is required for bi-directional communications between an RTU and the MTU. This can be done through public transmission media or atmospheric means. SCADA systems are capable of communicating using a wide variety of media such as fibre optics, dial-up, or dedicated voice grade telephone lines, or radio. Recently, some utilities have employed Integrated Services Digital Network (ISDN). Since the amount of information transmitted is relatively small (less than 50K), voice grade phone lines, and radio work well.

The topology of a SCADA system is the way a network is physically structured. For example: a ring, bus, or star configuration. It is not possible to define the typical SCADA system topology because it can vary with each system. Some topologies provide redundant operation and others do not. A redundant topology is highly recommended for water treatment plants and other critical control functions.

The Platform

A SCADA system typically is an application that runs on a mainframe, workstation, or PC computers. In the past UNIX was considered the most reliable OS (operating system), however SCADA applications are increasingly moving over to Microsoft Windows NT, 2000 & XP as the preferred OS. However the platform used is of secondary importance (unless you are in the business of writing your own application). What should be considered is the ability of the SCADA application to interact with databases, reporting packages and connectivity to other hardware that your site uses.

Interfacing Graphics with Control Systems

Fundamentally the main purpose of using a SCADA system is a means of providing an easy to use (graphical or text) interface to allow plant operators (or authorised personal) to carry out adjustments with out the need of any special knowledge of the systems hardware or have any programming skills. Typically a system will consist of a personal computer connected to a PLC via a serial network.
The SCADA application would be required to carry out some if not all the processes detailed in the diagram below;

A SCADA system would be useless if it could not receive real time data (this should be considered as data automatically collected and not that entered manually) from the process being monitored. Many SCADA manufactures now provide a range of drivers (IO servers) that will allow interfacing to a wide range of PLC's or provide the means to interface via OPC. This gives the end user the advantage of 'mix n matching' the PLC supplier's with the SCADA supplier's, or even obtaining the IO server from a third party who independent of both the PLC and SCADA supplier.

SCADA System Trends

Proprietary RTU History

In the 1960s, many manufacturers started developing proprietary printed circuit boards that combined the RTU and communication functions in one small package. These circuit boards were engineered to perform a very specific function for a moderate cost. Various municipalities around the United States readily accepted these "all-in-one" proprietary RTU circuit boards. These RTUs have a fixed amount of inputs and outputs in which to monitor and control digital and analog field devices. These RTUs require constant communications with the MTU in order to function and they have a limited amount of on-board memory. They generally used a wide variety of programming languages that were not well known or supported. These proprietary "all-in-one" RTUs dominated the US market until the late 1980s, at which time the first "micro" PLCs were introduced.

Non-Proprietary PLC History

In the mid 1970s, driven by requirements in the automotive, steel, and nuclear power industries, many electrical equipment manufacturers like Allen-Bradley, General Electric, Square D, and Modicon developed large PLCs for automated systems. These PLCs were engineered to perform a very wide variety of functions and were industrially-hardened to survive extremely harsh environments. However, at that time, the PLCs were large, heavy and expensive. This changed in the late 1980s with the introduction of very small and costcompetitive "micro" PLCs. These new non-proprietary PLCs used a relatively common "ladder" programming language that was already well supported and understood in many industries. The PLCs are generally modular in nature and can be expanded to monitor and control additional field devices. As such, they are ideal for RTUs in SCADAs. For instance, they can be programmed to function even if communication with the MTU is lost.

SCADA Software

A SCADA system often includes a Human Machine Interface (HMI) to visualize the state of system variables, change setpoints, alert operators of critical conditions, and archive and present data trends. The trend is to use software packages developed originally for industry and develop applications specific to water systems. Some commonly used packages are RSView (Rockwell Automation), IFIX (Intellution), InTouch (Wonderware), and Cimplicity (GE-Fanuc). With the advent of Windows NT, the HMI software can be reliably installed on common PC hardware in an office environment.
Screens are specifically designed for ease of use for the operator. The trend is to provide much of this capability using Web-based technology. In this case, a browser such as Internet Explorer or Netscape displays HTML pages from a web server that dynamically creates the web page using realtime data collected by the SCADA. These pages would then be published on the LAN of the water system operator, or if desired, on the Internet.

Future Trends

The overall trend in the last decade has been to migrate toward a non-proprietary, open-architecture, PLCbased SCADA system. Some manufacturers like Allen-Bradley now make "micro" PLCs specifically for SCADA applications. A PLC has more intelligence than an "all-in-one" RTU circuit board. Unlike an RTU, a PLC is able to control sites without the direction of a master. An electrical controls magazine editor recently wrote, "the trend towards [PLCs as RTUs] becoming more functional and powerful will continue with advancements in the SCADA software." This transfer of control away from master stations to increasingly "intelligent" remote PLCs will hasten the extinction of proprietary RTUs as users replace older proprietary systems with open architecture control systems based on industry standard components.

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