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Background to SCADA

1.1 Introduction and brief history of SCADA This manual is designed to provide a thorough understanding of the fundamental concepts and the practical issues of SCADA systems. Particular emphasis has been placed on the practical aspects of SCADA systems with a view to the future. Formulae and details that can be found in specialized manufacturer manuals have been purposely omitted in favor of concepts and definitions.

This chapter provides an introduction to the fundamental principles and terminology used in the field of SCADA. It is a summary of the main subjects to be covered throughout the manual.

SCADA (supervisory control and data acquisition) has been around as long as there have been control systems. The first ‘SCADA’ systems utilized data acquisition by means of panels of meters, lights and strip chart recorders. The operator manually operating various control knobs exercised supervisory control. These devices were and still are used to do supervisory control and data acquisition on plants, factories and power generating facilities. The following figure shows a sensor to panel system.

Sensors

Figure 1.1 Sensors to panel using 4–20 mA or voltage

2 Practical SCADA for Industry

The sensor to panel type of SCADA system has the following advantages: • It is simple, no CPUs, RAM, ROM or software programming needed • The sensors are connected directly to the meters, switches and lights on the

panel • It could be (in most circumstances) easy and cheap to add a simple device like

a switch or indicator

The disadvantages of a direct panel to sensor system are: • The amount of wire becomes unmanageable after the installation of hundreds

of sensors • The quantity and type of data are minimal and rudimentary • Installation of additional sensors becomes progressively harder as the system

grows • Re-configuration of the system becomes extremely difficult • Simulation using real data is not possible • Storage of data is minimal and difficult to manage • No off site monitoring of data or alarms • Someone has to watch the dials and meters 24 hours a day

1.2 Fundamental principles of modern SCADA systems In modern manufacturing and industrial processes, mining industries, public and private utilities, leisure and security industries telemetry is often needed to connect equipment and systems separated by large distances. This can range from a few meters to thousands of kilometers. Telemetry is used to send commands, programs and receives monitoring information from these remote locations.

SCADA refers to the combination of telemetry and data acquisition. SCADA encompasses the collecting of the information, transferring it back to the central site, carrying out any necessary analysis and control and then displaying that information on a number of operator screens or displays. The required control actions are then conveyed back to the process.

In the early days of data acquisition, relay logic was used to control production and plant systems. With the advent of the CPU and other electronic devices, manufacturers incorporated digital electronics into relay logic equipment. The PLC or programmable logic controller is still one of the most widely used control systems in industry. As need to monitor and control more devices in the plant grew, the PLCs were distributed and the systems became more intelligent and smaller in size. PLCs and DCS (distributed control systems) are used as shown below.

Background to SCADA 3

SensorsA fieldbus

PLCor

DCS

PC

Figure 1.2 PC to PLC or DCS with a fieldbus and sensor

The advantages of the PLC / DCS SCADA system are: • The computer can record and store a very large amount of data • The data can be displayed in any way the user requires • Thousands of sensors over a wide area can be connected to the system • The operator can incorporate real data simulations into the system • Many types of data can be collected from the RTUs • The data can be viewed from anywhere, not just on site

The disadvantages are:

• The system is more complicated than the sensor to panel type • Different operating skills are required, such as system analysts and

programmer • With thousands of sensors there is still a lot of wire to deal with • The operator can see only as far as the PLC

As the requirement for smaller and smarter systems grew, sensors were designed with

the intelligence of PLCs and DCSs. These devices are known as IEDs (intelligent electronic devices). The IEDs are connected on a fieldbus, such as Profibus, Devicenet or Foundation Fieldbus to the PC. They include enough intelligence to acquire data, communicate to other devices, and hold their part of the overall program. Each of these super smart sensors can have more than one sensor on-board. Typically, an IED could combine an analog input sensor, analog output, PID control, communication system and program memory in one device.

4 Practical SCADA for Industry

A fieldbusPC

Ethernet

IED's

Figure 1.3 PC to IED using a fieldbus

The advantages of the PC to IED fieldbus system are: • Minimal wiring is needed • The operator can see down to the sensor level • The data received from the device can include information such as serial

numbers, model numbers, when it was installed and by whom • All devices are plug and play, so installation and replacement is easy • Smaller devices means less physical space for the data acquisition system

The disadvantages of a PC to IED system are:

• More sophisticated system requires better trained employees • Sensor prices are higher (but this is offset somewhat by the lack of PLCs) • The IEDs rely more on the communication system

1.3 SCADA hardware A SCADA system consists of a number of remote terminal units (RTUs) collecting field data and sending that data back to a master station, via a communication system. The master station displays the acquired data and allows the operator to perform remote control tasks.

The accurate and timely data allows for optimization of the plant operation and process. Other benefits include more efficient, reliable and most importantly, safer operations. This results in a lower cost of operation compared to earlier non-automated systems.

On a more complex SCADA system there are essentially five levels or hierarchies: • Field level instrumentation and control devices • Marshalling terminals and RTUs • Communications system • The master station(s) • The commercial data processing department computer system

Background to SCADA 5

The RTU provides an interface to the field analog and digital sensors situated at each remote site.

The communications system provides the pathway for communication between the master station and the remote sites. This communication system can be wire, fiber optic, radio, telephone line, microwave and possibly even satellite. Specific protocols and error detection philosophies are used for efficient and optimum transfer of data.

The master station (or sub-masters) gather data from the various RTUs and generally provide an operator interface for display of information and control of the remote sites. In large telemetry systems, sub-master sites gather information from remote sites and act as a relay back to the control master station.

1.4 SCADA software SCADA software can be divided into two types, proprietary or open. Companies develop proprietary software to communicate to their hardware. These systems are sold as ‘turn key’ solutions. The main problem with this system is the overwhelming reliance on the supplier of the system. Open software systems have gained popularity because of the interoperability they bring to the system. Interoperability is the ability to mix different manufacturers’ equipment on the same system.

Citect and WonderWare are just two of the open software packages available in the market for SCADA systems. Some packages are now including asset management integrated within the SCADA system. The typical components of a SCADA system are indicated in the next diagram.

I/ODatabase

RS-232 Trend Server TaskReport Server Task

Input / Output Server Task

In InOut

Analog Digital

OutInstrumentation

& Control

DisplayServer #1

DisplayServer #2 Printer

RadioModem

RadioModem

PC PC

PC

Figure 1.4 Typical SCADA system

Key features of SCADA software are: • User interface • Graphics displays • Alarms • Trends • RTU (and PLC) interface • Scalability

6 Practical SCADA for Industry

• Access to data • Database • Networking • Fault tolerance and redundancy • Client/server distributed processing

1.5 Landlines for SCADA Even with the reduced amount of wire when using a PC to IED system, there is usually a lot of wire in the typical SCADA system. This wire brings its own problems, with the main problem being electrical noise and interference.

Interference and noise are important factors to consider when designing and installing a data communication system, with particular considerations required to avoid electrical interference. Noise can be defined as the random generated undesired signal that corrupts (or interferes with) the original (or desired) signal. This noise can get into the cable or wire in many ways. It is up to the designer to develop a system that will have a minimum of noise from the beginning. Because SCADA systems typically use small voltage they are inherently susceptible to noise.

The use of twisted pair shielded cat5 wire is a requirement on most systems. Using good wire coupled with correct installation techniques ensures the system will be as noise free as possible.

Fiber optic cable is gaining popularity because of its noise immunity. At the moment most installations use glass fibers, but in some industrial areas plastic fibers are increasingly used.

CoreLight Rays

Light Ray

Cladding

Multimode

Monomode

Sheath

Figure 1.5 Glass fiber optic cables

Future data communications will be divided up between radio, fiber optic and some infrared systems. Wire will be relegated to supplying power and as power requirements of electronics become minimal, even the need for power will be reduced.

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3.0 SCADA Architectures SCADA systems have evolved in parallel with the growth and sophistication of modern computing technology. The following sections will provide a description of the following three generations of SCADA systems:

• First Generation – Monolithic

• Second Generation – Distributed

• Third Generation – Networked

3.1 Monolithic SCADA Systems When SCADA systems were first developed, the concept of computing in general centered on “mainframe” systems. Networks were generally non-existent, and each centralized system stood alone. As a result, SCADA systems were standalone systems with virtually no connectivity to other systems.

The Wide Area Networks (WANs) that were implemented to communicate with remote terminal units (RTUs) were designed with a single purpose in mind–that of communicating with RTUs in the field and nothing else. In addition, WAN protocols in use today were largely unknown at the time.

The communication protocols in use on SCADA networks were developed by vendors of RTU equipment and were often proprietary. In addition, these protocols were generally very “lean”, supporting virtually no functionality beyond that required scanning and controlling points within the remote device. Also, it was generally not feasible to intermingle other types of data traffic with RTU communications on the network.

Connectivity to the SCADA master station itself was very limited by the system vendor. Connections to the master typically were done at the bus level via a proprietary adapter or controller plugged into the Central Processing Unit (CPU) backplane.

Redundancy in these first generation systems was accomplished by the use of two identically equipped mainframe systems, a primary and a backup, connected at the bus level. The standby system’s primary function was to monitor the primary and take over in the event of a detected failure. This type of standby operation meant that little or no processing was done on the standby system. Figure 3.1 shows a typical first generation SCADA architecture.

3.2 Distributed SCADA Systems

The next generation of SCADA systems took advantage of developments and improvement in system miniaturization and Local Area Networking (LAN) technology to

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Figure 3.1: First Generation SCADA Architecture [5] distribute the processing across multiple systems. Multiple stations, each with a specific function, were connected to a LAN and shared information with each other in real-time. These stations were typically of the mini-computer class, smaller and less expensive than their first generation processors.

Some of these distributed stations served as communications processors, primarily communicating with field devices such as RTUs. Some served as operator interfaces, providing the human-machine interface (HMI) for system operators. Still others served as calculation processors or database servers. The distribution of individual SCADA system functions across multiple systems provided more processing power for the system as a whole than would have been available in a single processor. The networks that connected these individual systems were generally based on LAN protocols and were not capable of reaching beyond the limits of the local environment.

Some of the LAN protocols that were used were of a proprietary nature, where the vendor created its own network protocol or version thereof rather than pulling an existing one off the shelf. This allowed a vendor to optimize its LAN protocol for real-time traffic, but it limited (or effectively eliminated) the connection of network from other vendors to the SCADA LAN. Figure 3.2 depicts typical second generation SCADA architecture.

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Figure 3.2: Second Generation SCADA Architecture [5] Distribution of system functionality across network-connected systems served not only to increase processing power, but also to improve the redundancy and reliability of the system as a whole. Rather than the simple primary/standby failover scheme that was utilized in many first generation systems, the distributed architecture often kept all stations on the LAN in an online state all of the time. For example, if an HMI station were to fail, another HMI station could be used to operate the system, without waiting for failover from the primary system to the secondary.

The WAN used to communicate with devices in the field were largely unchanged by the development of LAN connectivity between local stations at the SCADA master. These external communications networks were still limited to RTU protocols and were not available for other types of network traffic.

As was the case with the first generation of systems, the second generation of SCADA systems was also limited to hardware, software, and peripheral devices that were provided or at least selected by the vendor. 3.3 Networked SCADA Systems The current generation of SCADA master station architecture is closely related to that of the second generation, with the primary difference being that of an open system architecture rather than a vendor controlled, proprietary environment. There are still multiple networked systems, sharing master station functions. There are still RTUs utilizing protocols that are vendor-proprietary. The major improvement in the third generation is that of opening the system architecture, utilizing open standards and

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protocols and making it possible to distribute SCADA functionality across a WAN and not just a LAN.

Open standards eliminate a number of the limitations of previous generations of SCADA systems. The utilization of off-the-shelf systems makes it easier for the user to connect third party peripheral devices (such as monitors, printers, disk drives, tape drives, etc.) to the system and/or the network.

As they have moved to “open” or “off-the-shelf” systems, SCADA vendors have gradually gotten out of the hardware development business. These vendors have looked to system vendors such as Compaq, Hewlett-Packard, and Sun Microsystems for their expertise in developing the basic computer platforms and operating system software. This allows SCADA vendors to concentrate their development in an area where they can add specific value to the system–that of SCADA master station software.

The major improvement in third generation SCADA systems comes from the use of WAN protocols such as the Internet Protocol (IP) for communication between the master station and communications equipment. This allows the portion of the master station that is responsible for communications with the field devices to be separated from the master station “proper” across a WAN. Vendors are now producing RTUs that can communicate with the master station using an Ethernet connection. Figure 3.3 represents a networked SCADA system.

Wide Area Network (WAN)

SCADA Master

CommunicationsServer

Legacy RemoteTerminal Unit

Networked RemoteTerminal Unit

Figure 3.3: Third Generation SCADA System [5]

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Another advantage brought about by the distribution of SCADA functionality over a WAN is that of disaster survivability. The distribution of SCADA processing across a LAN in second-generation systems improves reliability, but in the event of a total loss of the facility housing the SCADA master, the entire system could be lost as well. By distributing the processing across physically separate locations, it becomes possible to build a SCADA system that can survive a total loss of any one location. For some organizations that see SCADA as a super-critical function, this is a real benefit.

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2.2 Comparison of the terms SCADA, DCS, PLC and smart instrument

2.2.1 SCADA system A SCADA (or supervisory control and data acquisition) system means a system consisting of a number of remote terminal units (or RTUs) collecting field data connected back to a master station via a communications system. The master station displays the acquired data and also allows the operator to perform remote control tasks.

The accurate and timely data (normally real-time) allows for optimization of the operation of the plant and process. A further benefit is more efficient, reliable and most importantly, safer operations. This all results in a lower cost of operation compared to earlier non-automated systems.

There is a fair degree of confusion between the definition of SCADA systems and process control system. SCADA has the connotation of remote or distant operation. The inevitable question is how far ‘remote’ is – typically this means over a distance such that the distance between the controlling location and the controlled location is such that direct-wire control is impractical (i.e. a communication link is a critical component of the system).

A successful SCADA installation depends on utilizing proven and reliable technology, with adequate and comprehensive training of all personnel in the operation of the system.

There is a history of unsuccessful SCADA systems – contributing factors to these systems includes inadequate integration of the various components of the system, unnecessary complexity in the system, unreliable hardware and unproven software. Today hardware reliability is less of a problem, but the increasing software complexity is producing new challenges. It should be noted in passing that many operators judge a SCADA system not only by the smooth performance of the RTUs, communication links and the master station (all falling under the umbrella of SCADA system) but also the field devices (both transducers and control devices). The field devices however fall outside the scope of SCADA in this manual and will not be discussed further. A diagram of a typical SCADA system is given opposite.

SCADA systems, hardware and firmware 13

Figure 2.1 Diagram of a typical SCADA system

On a more complex SCADA system there are essentially five levels or hierarchies:

• Field level instrumentation and control devices • Marshalling terminals and RTUs • Communications system • The master station(s) • The commercial data processing department computer system

The RTU provides an interface to the field analog and digital signals situated at each remote site.

The communications system provides the pathway for communications between the master station and the remote sites. This communication system can be radio, telephone

14 Practical SCADA for Industry

line, microwave and possibly even satellite. Specific protocols and error detection philosophies are used for efficient and optimum transfer of data.

The master station (and submasters) gather data from the various RTUs and generally provide an operator interface for display of information and control of the remote sites. In large telemetry systems, submaster sites gather information from remote sites and act as a relay back to the control master station.

SCADA technology has existed since the early sixties and there are now two other competing approaches possible – distributed control system (DCS) and programmable logic controller (PLC). In addition there has been a growing trend to use smart instruments as a key component in all these systems. Of course, in the real world, the designer will mix and match the four approaches to produce an effective system matching his/her application.

Figure 2.2 SCADA system

SCADA systems, hardware and firmware 15

2.2.2 Distributed control system (DCS) In a DCS, the data acquisition and control functions are performed by a number of distributed microprocessor-based units situated near to the devices being controlled or the instrument from which data is being gathered. DCS systems have evolved into systems providing very sophisticated analog (e.g. loop) control capability. A closely integrated set of operator interfaces (or man machine interfaces) is provided to allow for easy system configurations and operator control. The data highway is normally capable of fairly high speeds (typically 1 Mbps up to 10 Mbps).

Figure 2.3 Distributed control system (DCS)

2.2.3 Programmable logic controller (PLC) Since the late 1970s, PLCs have replaced hardwired relays with a combination of ladder–logic software and solid state electronic input and output modules. They are often used in the implementation of a SCADA RTU as they offer a standard hardware solution, which is very economically priced.

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PLCs

A.programmable logic controller (PLC),.also.referred.to.as.a.programmable controller,.is.the.name.given.to.a.type.of.computer.commonly.used.in.commercial.and.industrial.control.applications..PLCs.differ.from.office.computers.in.the.types.of.tasks.that.they.perform.and.the.hardware.and.software.they.require.to.perform.these.tasks..While.the.specific.applications.vary.widely,.all.PLCs.monitor.inputs.and.other.variable.values,.make.decisions.based.on.a.stored.program,.and.control.outputs.to.automate.a.process.or.machine..This.course.is.meant.to.supply.you.with.basic.information.on.the.functions.and.configurations.of.PLCs.with.emphasis.on.the.S7-200.PLC.family..

SF/DIAG

Motor

Pump

PushbuttonSensor

Indicator Light

Basic PLC Operation. The.basic.elements.of.a.PLC.include.input modules.or points,.a.central processing unit (CPU),.output modules.or.points,.and.a.programming device..The.type.of.input.modules.or.points.used.by.a.PLC.depends.upon.the.types.of.input.devices.used..Some.input.modules.or.points.respond.to.digital.inputs,.also.called.discrete.inputs,.which.are.either.on.or.off..Other.modules.or.inputs.respond.to.analog.signals..These.analog.signals.represent.machine.or.process.conditions.as.a.range.of.voltage.or.current.values..The.primary.function.of.a.PLC’s.input.circuitry.is.to.convert.the.signals.provided.by.these.various.switches.and.sensors.into.logic.signals.that.can.be.used.by.the.CPU..

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The.CPU.evaluates.the.status.of.inputs,.outputs,.and.other.variables.as.it.executes.a.stored.program..The.CPU.then.sends.signals.to.update.the.status.of.outputs..

Output.modules.convert.control.signals.from.the.CPU.into.digital.or.analog.values.that.can.be.used.to.control.various.output.devices.

The.programming.device.is.used.to.enter.or.change.the.PLC’s.program.or.to.monitor.or.change.stored.values..Once.entered,.the.program.and.associated.variables.are.stored.in.the.CPU.

In.addition.to.these.basic.elements,.a.PLC.system.may.also.incorporate.an.operator.interface.device.to.simplify.monitoring.of.the.machine.or.process.

ProgrammingDevice

OperatorInterface

Central Processing Unit(CPU)

InputModule

OutputModule

In.the.simple.example.shown.below,.pushbuttons.(sensors).connected.to.PLC.inputs.are.used.to.start.and.stop.a.motor.connected.to.a.PLC.output.through.a.motor.starter.(actuator)..No.programming.device.or.operator.interface.are.shown.in.this.simple.example.

Motor Starter

StartPushbutton

StopPushbutton

Inputs

Output

PLC

Motor

SF/DIAG

6

Hard-Wired Control. Prior.to.PLCs,.many.control.tasks.were.performed.by.contactors,.control.relays,.and.other.electromechanical.devices..This.is.often.referred.to.as.hard-wired control..Circuit.diagrams.had.to.be.designed,.electrical.components.specified.and.installed,.and.wiring.lists.created..Electricians.would.then.wire.the.components.necessary.to.perform.a.specific.task..If.an.error.was.made,.the.wires.had.to.be.reconnected.correctly..A.change.in.function.or.system.expansion.required.extensive.component.changes.and.rewiring..

OL

M

CR

CR

L1T1

T2

T3

L2

L3

OL

OL

OL

M

M

CR

MMotor

StartStop

460 VAC

24 VAC

1

2

Advantages of PLCs. PLCs.not.only.are.capable.of.performing.the.same.tasks.as.hard-wired.control,.but.are.also.capable.of.many.more.complex.applications..In.addition,.the.PLC.program.and.electronic.communication.lines.replace.much.of.the.interconnecting.wires.required.by.hard-wired.control..Therefore,.hard-wiring,.though.still.required.to.connect.field.devices,.is.less.intensive..This.also.makes.correcting.errors.and.modifying.the.application.easier..

Some.of.the.additional.advantages.of.PLCs.are.as.follows:

•. Smaller.physical.size.than.hard-wire.solutions.•. Easier.and.faster.to.make.changes.•. PLCs.have.integrated.diagnostics.and.override.functions.•. Diagnostics.are.centrally.available.•. Applications.can.be.immediately.documented.•. Applications.can.be.duplicated.faster.and.less.expensively.

Siemens Modular PLCs. Siemens.SIMATIC PLCs.are.the.foundation.upon.which.our.Totally Integrated Automation (TIA).concept.is.based..Because.the.needs.of.end.users.and.machine.builders.vary.widely,.SIMATIC.PLCs.are.available.as.conventional.modular.controllers,.embedded.automation.products,.or.as.PC-based.controllers.

16 Practical SCADA for Industry

Figure 2.4 Programmable logic controller (PLC) system

Another device that should be mentioned for completeness is the smart instrument which both PLCs and DCS systems can interface to.

2.2.4 Smart instrument Although this term is sometimes misused, it typically means an intelligent (microprocessor based) digital measuring sensor (such as a flow meter) with digital data communications provided to some diagnostic panel or computer based system.

Figure 2.5 Typical example of a smart instrument