| SCADA (Supervisory Control and Data Acquisition) systems are used in industrial and civil engineering applications to
control distributed systems from a master location. SCADA is a very broad umbrella that describes solutions across a large
variety of industries, including but not limited to the following:
The broad architecture of a SCADA solution involves physical equipment such as switches, pumps, and other devices able to be
controlled by a Remote Telemetry Unit RTU. The dual roles of the master computers are to
provide the information such as meter readings and equipment status to human operators in a digestible form and to allow the
operators to control the field equipment in predefined ways. Most SCADA deployments choose to restrict access to the master
computers, and interface with the system using operator consoles which communicate with the master computers over a network.
While the SCADA human-machine interface usually
allows operators to view the state of any part of the plant equipment, most operator interaction with the system is driven by
alarms. Alarms are automatically detected abnormal conditions in the plant equipment that require operator attention, and may
require operator intervention to keep things running smoothly.
SCADA master computers typically run on top of a third party operating system. Nearly all SCADA products run on either a
UNIX variant or HP OpenVMS, although many
vendors are beginning to provide Microsoft Windows as a host operating system option. More "open" platforms such as Linux are not
as widely used due to the highly dynamic development environment and because a SCADA customer that is able to afford the field
hardware and devices to be controlled is usually able to also purchase UNIX or OpenVMS licenses.
SCADA systems typically implement a distributed database which contains data called points. A point represents a single input
or output value monitored or controlled by the system. Points can be either "hard" or "soft". A hard point is representative of
an actual input or output connected to the system, while a soft represents the result of logic and math operations applied to
other hard and soft points.
The human-machine interface package for the SCADA
system typically includes a drawing program which 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 a 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 of the
elevators in a skyscraper or all of the trains on a railway. The interface is usually 2D and is displayed using the X11 Protocol, although some vendors provide immersive 3D interfaces and
support for other display APIs such as Win32 GDI/DirectDraw.
Since the early 1990s the role of SCADA systems in large civil engineering solutions has changed, requiring them to perform
more operations automatically. Solutions sold as SCADA also often have Distributed Control System (DCS) components. Use of "smart" RTUs or PLCs (programmable logic
controllers), which are capable of autonomously executing simple logic processes without involving the master computer, is
increasing. A functional block programming language, IEC 61131-3, is frequently used to create programs which run on these RTUs and PLCs. Unlike a procedural
language such as the C programming language or FORTRAN, IEC 61131-3 has minimal training requirements. This allows SCADA system engineers to
perform both the design and implementation of a program to be executed on a RTU or PLC.
For example, instead of relying on operator intervention, or master station automation, RTUs may now be required to operate on
their own to control tunnel fires or perform other safety-related tasks. The master station software is required to do more
analysis of data before presenting it to operators including historical analysis and analysis associated with particular industry
requirements. Safety requirements are now being applied to the system as a whole, and even master station software must meet
stringent safety standards for some markets.
For some installations the costs that would result from the control system failing is extremely high. Possibly even lives
could be lost. Hardware for SCADA systems is generally ruggedized to withstand temperature, vibration, and voltage extremes, but
in these installations reliability is enhanced by having redundant hardware and communications channels. A failing part can be
quickly identified and its functionally automatically taken over by backup hardware. A failed part can often be replaced without
interrupting the process. The reliabillity of such systems can be calculated statistically and is stated as the mean time to
failure. The calculated mean time to failure of such high reliability systems can be in the centuries.
SCADA systems have traditionally used combinations of radio and direct serial or modem connections to meet communication
requirements, although Ethernet and IP over SONET is also frequently used at large sites
such as railways and power stations.
This has also come under threat with some customers wanting 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 and many are designed to send information to the master station only when the master
station polls the RTU. Current standard SCADA products include Modbus, Conitel,
DNP3, IEC 60870-5-101 and RP-570. Many of these protocols now contain extensions
to operate over TCP/IP, although it is good security engineering practice to avoid connecting SCADA systems to the Internet so the attack surface is reduced.
See also:
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