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"Weinberg's Corollary:
An expert is a person who
avoids small errors while
sweeping on to the
grand fallacy. "

SCADA Primer
for those who stumbled into my home pages by accident.

Components of SCADA systems

Recommendation: read this introductory page before looking at the components.

The SCADA Master
SCADA Communications

SCADA Introduction

Suppose you have a simple electrical circuit consisting of a switch and a light. Like this

This circuit allows an operator to watch the light ( ) and know whether the switch is open or closed. The switch may indicate that a motor is running or stopped, or whether a door is open or closed, or even whether there has been a fault or the equipment is working.

A simple animated example showing a tank being filled, and a pump starting and emptying it is available here. (PLC Tutor site) This example does not involve telemetry as the pump is close to the tank.

So far there is nothing special about this. But now imagine that the switch and the lamp were 100 kilometres apart. Obviously we couldn't have an electrical circuit this large, and it would now be a problem involving communications equipment.

Now complicate the problem a bit further. Imagine we had 2000 such circuits. We could not afford 2000 communications circuits. However someone found that we could use one communications circuit by sharing it. First we send the status (open/closed or 0/1) of the first circuit. Then we send the status of the second circuit, and so on. We need to indicate which circuit the status applies to when we send the data.

The poor operator at the other end still has a problem. He has to watch all 2000 circuits. To simplify his task, we could use a computer. The computer would monitor all circuits, and tell the operator when he needs to look at a particular circuit. The computer will be told what the normal state of the circuit is and what state is an "alarm". It monitors all circuits, and informs the operator when any circuit goes into alarm.

Some circuits may contain "analog" data, ie they represent a number eg level of water in a tank. In these cases the computer will be told a top and bottom level value that are to be considered normal. When the value goes outside this range, the computer will consider this an alarm, and the operator will be informed.

Here is a page that describes some of the early attempts at long distance communications by wire (PLC Tutor site)

Typical SCADA alarm page Picture from Citect site

We could also use the computer to present the information in a graphical manner ( a picture is worth a thousand words). It could show a valve as red when closed, or green when open, and so on.

Some SCADA picturesPicture from RT site"

Another SCADA picture

A real SCADA system is more complex. There are more sites than one. Some have 30,000 to 50,000 "points". They usually have "analogue" information as well as digital or status information (eg numbers such as level of fluid in a tank). They can send a status value (eg start a pump) as well as receive it (the pump is started). And the power of the computer can be used to perform complex sequencing of operations. eg OPEN a valve, then START a pump, but only if the pressure is greater than 50.

The computer can be used to summarise and display the data it is processing. Trends (graphs) of analogue values over time are very common. Collecting the data and summarising it into reports for operators, and management are normal features of a SCADA system.

Central SCADA Computer Picture from Pantek site

Typical SCADA trend

Another SCADA trend Picture from Citect site

Another SCADA trend Picture from Onspec site. (You can get a demonstration copy of the ONSPEC package at this site).

General Definition

SCADA (supervisory control and data acquisition): An industrial measurement and control system consisting of a central host or master (usually called a master station, master terminal unit or MTU); one or more field data gathering and control units or remotes (usually called remote stations, remote terminal units, or RTU's); and a collection of standard and/or custom software used to monitor and control remotely located field data elements. Contemporary SCADA systems exhibit predominantly open-loop control characteristics and utilise predominantly long distance communications, although some elements of closed-loop control and/or short distance communications may also be present.

Systems similar to SCADA systems are routinely seen in factories, treatment plants etc. These are often referred to as Distributed Control Systems (DCS). They have similar functions to SCADA systems, but the field data gathering or control units are usually located within a more confined area. Communications may be via a local area network (LAN), and will normally be reliable and high speed. A DCS system usually employs significant amounts of closed loop control.

SCADA systems on the other hand generally cover larger geographic areas, and rely on a variety of communications systems that are normally less reliable than a LAN. Closed loop control in this situation is less desirable.

Packet radio communications systemPicture from Pantek site

Telephone based systemPicture from Pantek site

So what is SCADA? It is used to monitor and control plant or equipment. The control may be automatic, or initiated by operator commands. The data acquisition is accomplished firstly by the RTU's scanning the field inputs connected to the RTU (it may be also called a PLC - programmable logic controller). This is usually at a fast rate. The central host will scan the RTU's (usually at a slower rate.) The data is processed to detect alarm conditions, and if an alarm is present, it will be displayed on special alarm lists. Data can be of three main types. Analogue data (ie real numbers) will be trended (ie placed in graphs). Digital data (on/off) may have alarms attached to one state or the other. Pulse data (eg counting revolutions of a meter) is normally accumulated or counted.

The primary interface to the operator is a graphical display (mimic) which shows a representation of the plant or equipment in graphical form. Live data is shown as graphical shapes (foreground) over a static background. As the data changes in the field, the foreground is updated. Eg a valve may be shown as open or closed. Analog data can be shown either as a number, or graphically. The system may have many such displays, and the operator can select from the relevant ones at any time.

If you want to learn more here is a web page that will teach you how to program PLC's (PLC=programmable logic controller) PLC Tutor

Here is a web page that explains what an RTU is.

To see a simulated water supply system being controlled by SCADA, click here (preferably if you have a 800x600 screen and a relatively recent version of IE4 or Netscape (4.07 say)).

Here is a site that gives a more technical introduction to SCADA. Miri Telemetry

Another introduction to SCADA.The Telematix Institute

Some projects on the web

What is involved in a SCADA project - see this project methodology

The Tek Soft Methodology


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© 1997 Tek Soft Consulting.