Plc report

AMITY UNIVERSITY
Rajasthan
PLC AND SCADA
SUBMITTED TO
Ms. Pushpa Gotwal
SUBMITTED BY
Priya Hada
B.tech(ECE)7th
sem
October 13, 2014

CERTIFICATE
This is to certify that PRIYA HADA, student of B.Tech. in Electronics and
Communication Engineering has carried out the work presented in Training en-
titled ”PLC and SCADA” as a part of fourth Year programme of B.Tech. in
Electronics and Communication Engineering from Amity School of Engineer-
ing and Technology, Amity University Rajasthan, under my supervision.
FACULTY
Mr. Sudhir Kumar Mishra
ASET(AUR)
i

ACKNOWLEDGMENTS
It has come out to be a sort of great pleasure and experience for me to work
on the Plc and SCADA, from Prime Vision Automation.I wish to express my
indebtedness to those who helped us i.e. the faculty of our Institute Ms. Pushpa
Gotwal, ASET during the preparation of the manual script of this text. This
would not have been made successful without his help and precious sugges-
tions.
Priya Hada
ii

Contents
CERTIFICATE i
ACKNOWLEDGMENTS ii
1 INTRODUCTION 1
1.1 AUTOMATION . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 PLC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3 SCADA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.4 Features of PLCs . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.5 History of PLCs . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.6 Hard Wired Relay Comparison . . . . . . . . . . . . . . . . . . 6
2 PROGRAMMABLE LOGIC CONTROLLER 9
2.1 INPUT MODULE . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2 OUTPUT MODULE . . . . . . . . . . . . . . . . . . . . . . . 12
2.3 Wiring In a PLC . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.3.1 Generation of Input Signal . . . . . . . . . . . . . . . . 15
2.4 PLC compared with other control systems . . . . . . . . . . . . 16
3 Ladder Logic 17
3.1 Deﬁnition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.2 Comparison to Relay Logic . . . . . . . . . . . . . . . . . . . . 17
3.3 Ladder Logic Programming . . . . . . . . . . . . . . . . . . . . 19
3.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . 19
3.4 EXAMPLES OF LADDER PROGRAMMING . . . . . . . . . 24
4 SCADA 27
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
4.2 Types of SCADA . . . . . . . . . . . . . . . . . . . . . . . . . 27
4.3 Features of SCADA . . . . . . . . . . . . . . . . . . . . . . . . 27
4.4 Manufacture of SCADA . . . . . . . . . . . . . . . . . . . . . 28
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4.5 What does SCADA MEAN? . . . . . . . . . . . . . . . . . . . 28
4.6 Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4.6.1 Hardware Architecture . . . . . . . . . . . . . . . . . . 29
4.6.2 Generic Software Architecture . . . . . . . . . . . . . . 29
4.7 Communications . . . . . . . . . . . . . . . . . . . . . . . . . 29
4.7.1 Internal Communication . . . . . . . . . . . . . . . . . 29
4.7.2 Access to Devices . . . . . . . . . . . . . . . . . . . . 29
4.8 Interfacing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
4.8.1 Application Interfaces / Openness . . . . . . . . . . . . 30
4.8.2 Database . . . . . . . . . . . . . . . . . . . . . . . . . 30
4.8.3 Scalability . . . . . . . . . . . . . . . . . . . . . . . . 30
4.8.4 Redundancy . . . . . . . . . . . . . . . . . . . . . . . . 31
4.8.5 Functionality . . . . . . . . . . . . . . . . . . . . . . . 31
4.8.6 Trending . . . . . . . . . . . . . . . . . . . . . . . . . 31
4.8.7 Alarm Handling . . . . . . . . . . . . . . . . . . . . . 32
4.8.8 Logging/Archiving . . . . . . . . . . . . . . . . . . . . 32
4.8.9 Report Generation . . . . . . . . . . . . . . . . . . . . 33
4.8.10 Automation . . . . . . . . . . . . . . . . . . . . . . . 33
5 Application Development 34
5.1 Conﬁguration . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
5.2 Development Tools . . . . . . . . . . . . . . . . . . . . . . . . 34
5.3 Object Handling . . . . . . . . . . . . . . . . . . . . . . . . . 35
5.4 Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
5.5 Engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
5.6 Potential beneﬁts of SCADA . . . . . . . . . . . . . . . . . . . 36
5.7 Where SCADA is used ? . . . . . . . . . . . . . . . . . . . . . 37
6 CONCLUSION 38
REFERENCES 39
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List of Figures
1 Complete Automation System Components . . . . . . . . . . . 1
2 Block diagram of PLC . . . . . . . . . . . . . . . . . . . . . . 4
3 Typical Small Scale Control Panel . . . . . . . . . . . . . . . . 7
4 Typical PLC Control Panel . . . . . . . . . . . . . . . . . . . . 7
5 Diagram of Counter . . . . . . . . . . . . . . . . . . . . . . . . 11
6 Diagram of Timer . . . . . . . . . . . . . . . . . . . . . . . . . 12
7 Input module of PLC . . . . . . . . . . . . . . . . . . . . . . . 12
8 Output module to PLC . . . . . . . . . . . . . . . . . . . . . . 13
9 Output module from PLC . . . . . . . . . . . . . . . . . . . . . 13
10 PLC scan cycle . . . . . . . . . . . . . . . . . . . . . . . . . . 14
11 Wiring in PLC . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
12 Basic Ladder Logic Program . . . . . . . . . . . . . . . . . . . 17
13 Electromechanical Relay . . . . . . . . . . . . . . . . . . . . . 18
14 Basic Relay logic Circuit . . . . . . . . . . . . . . . . . . . . . 18
15 Relay Logic Circuit with Jog function and Status Indicators . . . 19
16 Complex Ladder Diagram . . . . . . . . . . . . . . . . . . . . 20
17 Basic Ladder Logic Program . . . . . . . . . . . . . . . . . . . 20
18 Basic Program to show input and output . . . . . . . . . . . . . 22
19 Normally Open Contact . . . . . . . . . . . . . . . . . . . . . . 22
20 Normally Open Coil . . . . . . . . . . . . . . . . . . . . . . . 22
21 Normally Closed Contact . . . . . . . . . . . . . . . . . . . . . 23
22 Normally Closed coil . . . . . . . . . . . . . . . . . . . . . . . 23
23 Basic And Gate using Ladder Logic . . . . . . . . . . . . . . . 23
24 Basic And Gate using Ladder Logic . . . . . . . . . . . . . . . 23
25 Start/Stop of Motor by PLC . . . . . . . . . . . . . . . . . . . . 25
26 Starting of Motor . . . . . . . . . . . . . . . . . . . . . . . . . 25
27 Continous Running of motor when Start Button is Released . . . 26
28 To Stop the Motor . . . . . . . . . . . . . . . . . . . . . . . . . 26
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1 INTRODUCTION
1.1 AUTOMATION
Automation or industrial automation is the use of control systems such as com-
puters, controllers to control industrial machinery and processes, to optimize
productivity in the production of goods and delivery of services. Automation
is a step beyond mechanization. Whereas mechanization provides human op-
erators with machinery to assist them with the muscular requirements of work,
automation greatly decreases the need for human sensory and mental require-
ments.
Automation Impacts:
1. It increases productivity and reduce cost.
2. It gives emphasis on ﬂexibility and convertibility of manufacturing pro-
cess. Hence gives manufacturers the ability to easily switch from manu-
facturing Product A to manufacturing product B without completely re-
built the existing system/product lines.
3. Automation is now often applied primarily to increase quality in the man-
ufacturing process, where automation can increase quality substantially.
4. Increase the consistency of output.
5. Replacing humans in tasks done in dangerous environments.
Figure 1: Complete Automation System Components
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1.2 PLC
A Programmable Logic Controller, PLC, or Programmable Controller is a dig-
ital computer used for automation of industrial processes, such as control of
machinery on factory assembly lines. Unlike general-purpose computers, the
PLC is designed for multiple inputs and output arrangements, extended tem-
perature ranges, immunity to electrical noise, and resistance to vibration and
impact. Programs to control machine operation are typically stored in battery-
backed or non-volatile memory. A PLC is an example of a real time system
since output results must be produced in response to input conditions within a
bounded time, otherwise unintended operation will result. PLC are registered
trademarks of the Allen-Bradley Company.
PLCs have become the most predominant control elements for the discrete
event control of a mechatronics system. Simplification of engineering and pre-
cise control of manufacturing process can result in significant cost savings. The
most cost-effective way which can pay big dividends in the long run is flexible
automation; a planned approach towards integrated control systems. It requires
a conscious effort on the part of plant managers and engineers to identify areas
where automation can result in better deployment and/or utilization of human
resources and savings in man-hours or down time. Controls automation need
not be high ended and extremely sophisticated; it is the phased, step-by-step
effort to automate, employing control systems tailored to ones specific require-
ments that achieves the most attractive results. This is where programmable
logic controls have been a breakthrough in the field of automation and control
techniques.
A constant demand for better and more efficient manufacturing and pro-
cess machinery has led to the requirement for higher quality and reliability
in control techniques. With the availability of intelligent, compact solid state
electronic devices, it has been possible to provide control systems that can re-
duce maintenance, down time and improve productivity to a great extend. By
installing an efficient and user friendly electronics systems for manufacturing
machinery or processors, one can obtain a precise and reliable means for pro-
ducing quality products. One of the latest techniques in solid state controls that
offers flexible and efficient operation to the user is programmable controllers.
The basic idea behind these programmable controllers was to provide means
to eliminate high cost associated with inflexible, conventional relay controlled
systems. Programmable controllers offer a system with computer flexibility that
is suited to withstand the harsh industrial environment, has simplicity of oper-
ation/readability, can reduce machine down time and provide expandability for
future and is able to be maintained by plant technicians.
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1.3 SCADA
SCADA is widely used in industry for Supervisory Control and Data Acqui-
sition of industrial processes, SCADA systems are now also penetrating the
experimental physics laboratories for the controls of ancillary systems such as
cooling, ventilation, power distribution, etc. More recently they were also ap-
plied for the controls of smaller size particle detectors such as the L3 moon
detector and the NA48 experiment, to name just two examples at CERN.
SCADA systems have made substantial progress over the recent years in
terms of functionality, scalability, performance and openness such that they are
an alternative to in house development even for very demanding and complex
control systems as those of physics experiments.
1.4 Features of PLCs
1. PLC is an industrial computer control system that continuously monitors
the state of input devices and makes decisions based upon a custom pro-
gram to control the state of output devices.
2. It is designed for multiple inputs and output arrangements, extended tem-
perature ranges, immunity to electrical noise, and resistance to vibration
and impact.
3. Almost any production process can greatly enhanced using this type of
control system, the biggest beneﬁt in using a PLC is the ability to change
and replicate the operation or process while collecting and communicat-
ing vital information.
4. Another advantage of a PLC is that it is modular i.e. you can mix and
match the types of input and output devices to best suit your application.
With each module having sixteen ”points” of either input or output, this PLC has
the ability to monitor and control dozens of devices. Fit into a control cabinet, a
PLC takes up little room, especially considering the equivalent space that would
be needed by electromechanical relays to perform the same functions.
The main difference from other computers is that PLC are armored for se-
vere condition (dust, moisture, heat, cold, etc) and has the facility for extensive
input/output (I/O) arrangements. These connect the PLC to sensors and actua-
tors. PLCs read limit switches, analog process variables (such as temperature
and pressure), and the positions of complex positioning systems. Some even
use machine vision. On the actuator side, PLCs operate electric motors, pneu-
matic or hydraulic cylinders, magnetic relays or solenoids, or analog outputs.
3

The input/output arrangements may be built into a simple PLC, or the PLC may
have external I/O modules attached to a computer network that plugs into the
PLC.
Many of the earliest PLCs expressed all decision making logic in simple
ladder logic which appeared similar to electrical schematic diagrams. This pro-
gram notation was chosen to reduce training demands for the existing techni-
cians. Early PLCs used a form of instruction list programming, based on a
stack-based logic solver. The functionality of the PLC has evolved over the
years to include sequential relay control, motion control, process control, dis-
tributed control systems and networking. The data handling, storage, process-
ing power and communication capabilities of some modern PLCs are approxi-
mately equivalent to desktop computer.
Figure 2: Block diagram of PLC
1.5 History of PLCs
PLCs were ﬁrst introduced in the 1960s. The primary reason for designing such
a device was eliminating the large cost involved in replacing the complicated
relay based machine control systems. Bedford Associates (Bedford, MA) pro-
posed something called a Modular Digital Controller (MODICON) to a major
US car manufacturer. The MODICON 084 brought the world’s ﬁrst PLC into
commercial production.
When production requirements changed so did the control system. This
4

becomes very expensive when the change is frequent. Since relays are mechan-
ical devices they also have a limited lifetime because of the multitude of mov-
ing parts. This also required strict adhesion to maintenance schedules. Trou-
bleshooting was also quite tedious when so many relays are involved. Now
picture a machine control panel that included many, possibly hundreds or thou-
sands, of individual relays. The size could be mind boggling not to mention the
complicated initial wiring of so many individual devices. These relays would be
individually wired together in a manner that would yield the desired outcome.
The problems for maintenance and installation were horrendous.
These new controllers also had to be easily programmed by maintenance
and plant engineers. The lifetime had to be long and programming changes
easily performed. They also had to survive the harsh industrial environment.
The answers were to use a programming technique most people were already
familiar with and replace mechanical parts with solid-state ones which have no
moving parts.
Communications abilities began to appear in approximately 1973. The ﬁrst
such system was Modicon’s Modbus. The PLC could now talk to other PLCs
and they could be far away from the actual machine they were controlling. They
could also now be used to send and receive varying voltages to allow them to
use analog signals, meaning that they were now applicable to many more control
systems in the world. Unfortunately, the lack of standardization coupled with
continually changing technology has made PLC communications a nightmare
of incompatible protocols and physical networks.
The 1980s saw an attempt to standardize communications with General Mo-
tor’s manufacturing automation protocol (MAP). It was also a time for reducing
the size of the PLC and making them software programmable through sym-
bolic programming on personal computers instead of dedicated programming
terminals or handheld programmers.
The 1990s saw a gradual reduction in the introduction of new protocols, and
the modernization of the physical layers of some of the more popular protocols
that survived the 1980’s. PLCs can now be programmable in function block
diagrams, instruction lists, C and structured text all at the same time. PC’s are
also being used to replace PLCs in some applications. The original company
who commissioned the MODICON 084 has now switched to a PC based control
system.
1. The ﬁrst PLCs were designed and developed by Modicon as a relay re-
placer for GM and Landis.
2. The primary reason for designing such a device was eliminating the large
cost involved in replacing the complicated relay based machine control
5

systems for major U.S. car manufacturers.
3. These controllers eliminated the need of rewiring and adding additional
hardware for every new configuration of logic.
4. The first PLC, model 084, was invented by Dick Morley in 1969.
5. The first commercial successful PLC, the 184, was introduced in 1973
and was designed by Michel Greenberg.
Communications abilities began to appear in approximately 1973. The first
such system was Modicon’s Modbus. The PLC could now talk to other PLCs
and they could be far away from the actual machine they were controlling.
1.6 Hard Wired Relay Comparison
At the outset of industrial revolution, especially during sixties and seventies,
relays were used to operate automated machines, and these were interconnected
using wires inside the control panel. In some cases a control panel covered an
entire wall. To discover an error in the system much time was needed, especially
with more complex process control systems. On top of everything, a lifetime of
relay contacts was limited, so some relays had to be replaced. If replacement
was required, machine had to be stopped and production as well. Also, it could
happen that there was not enough room for necessary changes. A control panel
was used only for one particular process, and it wasnt easy to adapt to the re-
quirements of a new system. As far as maintenance, electricians had to be very
skillful in finding errors. In short, conventional control panels proved to be very
inflexible. Typical example of conventional control panel is given in the follow-
ing picture. In Figure 3 you can see a large number of electrical wires, relays,
timers and other elements of automation typical for that period. The pictured
control panel is not one of the more complicated ones, so you can imagine what
complex ones looked like.
The most frequently mentioned disadvantages of a classic control panel are:
1. Large amount of work required connecting wires.
2. Difficulty with changes or replacements.
3. Difficulty in finding errors; requiring skillful/experienced work force.
4. When a problem occurs, hold-up time is indefinite, usually long.
6

Figure 3: Typical Small Scale Control Panel
Figure 4: Typical PLC Control Panel
With invention of programmable controllers, much has changed in how a pro-
cess control system is designed. Many advantages appeared. Typical example
of control panel with a PLC controller is given in the following picture.
Advantages of control panel that is based on a PLC controller can be pre-
sented in few basic points:
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1. Compared to a conventional process control system, number of wires
needed for connections is reduced by approximately 80
2. Diagnostic functions of a PLC controller allow for fast and easy error
detection.
3. Change in operating sequence or application of a PLC controller to a dif-
ferent operating process can easily be accomplished by replacing a pro-
gram through a console or using PC software (not requiring changes in
wiring, unless addition of some input or output device is required).
4. Needs fewer spare parts.
5. It is much cheaper compared to a conventional system, especially in cases
where a large number of Input/Output instruments are needed and when
operational functions are complex.
6. Reliability of a PLC is greater than that of an electro-mechanical relay or
a timer, because of less moving parts.
7. They are compact and occupy less space.
8. Use of PLC results in appreciable savings in hardware and wiring cost.
8

2 PROGRAMMABLE LOGIC CONTROLLER
Definition:
A Programmable controller is a solid state user programmable control sys-
tem with functions to control logic, sequencing, timing, arithmetic data manip-
ulation and counting capabilities. It can be viewed as an industrial computer
that has a central processor unit, memory, input output interface and a pro-
gramming device. The central processing unit provides the intelligence of the
controller. It accepts data, status information from various sensing devices like
limit switches, proximity switches, executes the user control program stored in
the memory and gives appropriate output commands to devices such as solenoid
valves, switches etc.
Input output interface is the communication link between field devices and
the controllers. Through these interfaces the processor can sense and measure
physical quantities regarding a machine or process, such as, proximity, position,
motion, level, temperature, pressure, etc. Based on status sensed, the CPU
issues command to output devices such as valves, motors, alarms, etc. The
programmer unit provides the man machine interface. It is used to enter the
application program, which often uses a simple user-friendly logic.
What is inside a PLC?
The PLC, being a microprocessor based device, has a similar internal struc-
ture to many embedded controllers and compute rs. They consist of the CPU,
Memory and I/O devices. These components are integral to the PLC controller.
Additionally the PLC h as a connection for the Programming and Monitoring
Unit or to connect to other PLCs in the field.
Components:
The PLC mainly consists of a CPU, memory areas, and appropriate circuits
to receive input/output data. We can actually consider the PLC to be a box full
of hundreds or thousands of separate relays, counters, timers and data storage
locations. They don’t physically exist but rather they are simulated and can
be considered software counters, timers, etc. Each component of a PLC has a
specific function:
1. The CPU is the brain of a PLC system. It consists of the microprocessor,
memory integrated circuits and circuits necessary to store and retrieve
information from memory. It also includes also includes communication
ports to the peripherals, other PLCs or programming terminals. The job
of the processor is to monitor status or state of input devices, scan and
solve the logic of a user program, and control on or off state of output
devices.
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2. RAM or Random Access Memory is a volatile memory that would lose
its information if power were removed , hence some processor units are
provided with battery backup. Normally CMOS (Complementary Metal
Oxide Semiconductor) type RAM is used.
3. ROM is a non volatile type of memory. This means it stores it is data
even if no power is available. This type of memory information can only
be read, it is placed there for the internal use and operation of processor
units.
4. EEPROME or Electrically Erasable Programmable Read Only Memory
is usually an add on memory module that is used to backup the main
program in CMOS RAM of the processor. In many cases, the processor
can be programmed to load the EEPROMS program to RAM, if RAM is
lost or corrupted.
5. Input Relays (contacts) - These are connected to the outside world. They
physically exist and receive signals from switches, sensors, etc. Typically
they are not relays but rather they are transistors.
6. Internal Utility Relays - These do not receive signals from the outside
world nor do they physically exist. They are simulated relays and are
what enables a PLC to eliminate external relays. There are also some
special relays that are dedicated to performing only one task. Some are
always on while some are always off. Some are on only once during
power-on and are typically used for initializing data that was stored.
7. Counters - These are simulated counters and they can be programmed to
count pulses. Typically these counters can count up, down or both up and
down. Since they are simulated they are limited in their counting speed.
Some manufacturers also include high-speed counters that are hardware
based. We can think of these as physically existing.
8. Timers - These come in many varieties and increments. The most com-
mon type is an on-delay type. Others include off-delay and both retentive
and non-retentive types. Increments vary from 1 millisecond through 1
second.
9. Output Relays (coils) - These are connected to the outside world. They
physically exist and send on/off signals to solenoids, lights, etc. They can
be transistors, relays, or triacs depending upon the model chosen.
10. Data Storage - Typically there are registers assigned to simply store data.
They are usually used as temporary storage for math or data manipulation.
They can also typically be used to store data when power is removed from
10

the PLC. Upon power-up they will still have the same contents as before
power was removed
A counter is a simple device intended to do one simple thing - count. Using
them can sometimes be a challenge however because every manufacturer seems
to use them a different way. There are several different types of counters. There
are up-counters called CTU CNT, or CTR that only count up, such as 1, 2, and
3. There are also down counters called CTD that only count down, such as 9,
8, 7, etc. In addition to these two, there are up-down counters, typically called
UDC (up-down counter). These count up and/or down (1,2,3,4,3,2,3,4,5,...)
Figure 5: Diagram of Counter
A timer is an instruction that waits a set amount of time before doing some-
thing. As usual in industry, different types of timers are available with different
manufacturers. The most common type of timer is an On-Delay Timer. This
type of timer simply delays turning on its respective output. In other words,
after our sensor (input) turns on we wait x number of seconds before activating
a solenoid valve (output). This is the most common timer. It is often called
TON (timer on-delay), TIM (timer) or TMR (timer). Another type of timer is
an Off-Delay Timer. This type of timer is the opposite of the on-delay timer
listed above. This timer delays turning off its respective output. After a sensor
(input) sees a target we turn on a solenoid (output). When the sensor no longer
sees the target we hold the solenoid on for x number of seconds before turning
it off. It is called a TOF (timer off-delay) and is less common than the on-delay
type listed above. Very few manufacturers include this type of timer, although
it can be quite useful. The last type of timer is a Retentive or Accumulating
timer. This type of timer needs 2 inputs. One input starts the timing event (i.e.
the clock starts ticking) and the other resets it. The on/off delay timers above
would be reset if the input sensor wasn’t on/off for the complete timer duration.
This timer however holds or retains the current elapsed time when the sensor
turns off in mid-stream. For example, we want to know how long a sensor is
on for during a 1 hour period. If we use one of the above timers they will keep
resetting when the sensor turns off/on. This timer however, will give us a to-
11

tal or accumulated time. It is often called an RTO (retentive timer) or TMRA
(accumulating timer).
Figure 6: Diagram of Timer
2.1 INPUT MODULE
Input Module: Input modules interface directly to devices such as switches and
temperature sensors. Input modules convert many different types of electrical
signals such as 120VAC, 24VDC, or 4-20mA, to signals which the controller
can understand since all electrical systems are inherently noisy, electrical isola-
tion is provided between input and processor. The component most often used
for this purpose is optocoupler .Input signal from the field devices are usually 4
to 20 ma or 0-10 V.
Figure 7: Input module of PLC
2.2 OUTPUT MODULE
Output module : It interface directly to devices such as motor starters and lights
Output modules take digital signals from the PLC and convert them to electrical
signals such as 24VDC and 4 mA that field devices can understand. D to A
conversion is carried out in there modules. Usually Silicon Controlled Rectifier
12

Figure 8: Output module to PLC
(SCR), triac, or dry contact relays are used for this purpose. Normally the output
signal is 0-10 V or 4-20 ma.
Figure 9: Output module from PLC
PLC Operation: PLC operates by continually scanning the program and
acting upon the instructions , one at a time, to switch on or off the various
outputs. In order to do this PLC first scans all the inputs and stores their states
in memory. Then it carries out program scan and decides which outputs should
be high according to the program logic.
Then finally it updates these values to the output table, making the required
outputs go high. At his point PLC checks its own operating system and if ev-
erything is ok, it goes back to scanning inputs all over again.
PLC SCAN CYCLE: A PLC works by continually scanning a program.The
first step is to check the input status. This step is therefore generally referred
to as the Check Input Status stage. First the PLC takes a look at each input to
determine if it is on or off. In other words, is the sensor connected to the first
input on? How about the second input? How about the third? This goes on and
13

on through the entire program. It records this data into its memory to be used
during the next step.
Next the PLC executes your program one instruction at a time, called the
Execute Program stage. For example, if your program said that if the first input
was on then it should turn on the first output. Since it already knows which
inputs are on/off from the previous step it will be able to decide whether the
first output should be turned on based on the state of the first input. It will store
the execution results for use later during the next step.
Finally the PLC updates the status of the outputs. It updates the outputs
based on which inputs were on during the first step and the results of executing
your program during the second step. Based on the example in step 2 it would
now turn on the first output because the first input was on and your program
said to turn on the first output when this condition is true.
A new style of scanning has been implemented in the more recent years,
called rung scanning. This type basically scans each ladder rung individually in
the entire ladder logic program, updating the outputs on that rung after scanning
through the inputs. This changes the type of programming that will be used as
well. If an output is in a rung above the inputs it depends on, you will not get
the output updated until the next scan, as the program will keep scanning down
until the last rung, then start over. This style is very advantageous in certain
situations. If you want your outputs updated at the soonest possible moment,
this is the style of scanning that you want to use.
Figure 10: PLC scan cycle
SCAN TIME
Time taken by plc to execute these three steps (Checking Input status, Exe-
cuting Program, Updating Output Status) is denoted by its scan time.
COMMUNICATION
14

There are several methods to communicate between a PLC and a Program-
mer or even between two PLCs. PLCs have built in communications ports, usu-
ally 9 pin RS 232, RS-485, TTY but optionally EIA-485 or Ethernet Modbus,
BACnector DF1 os usually included as one of the communications protocols.
Other options include various ﬁeldbuses such as Device Net or Proﬁbus. Most
modern PLCs can communicate over a network to some other system , such
as a computer running a SCADA ( Supervisory Control And Data Acquistion )
system or web browser.
2.3 Wiring In a PLC
2.3.1 Generation of Input Signal
Inside the PLC housing, connected between each input terminal and the Com-
mon terminal, is an opto-isolator device (Light-Emitting Diode) that provides
an electrically isolated ”high” Logic signal to the computer’s circuitry (a photo-
transistor interprets the LED’s light) when there is 120 VAC power applied be-
tween the respective input terminal and the Common terminal. An indicating
LED on the front panel of the PLC gives visual indication of an ”energized”
input.
Figure 11: Wiring in PLC
Output signals are generated by the PLC’s computer circuitry activating a
switching device (transistor, or even an electromechanical relay), connecting
the ”Source” terminal to any of the ”Y-” labeled output terminals. The ”Source”
terminal, correspondingly, is usually connected to the L1 side of the 120 VAC
power source. As with each input, an indicating LED on the front panel of the
PLC gives visual indication of an ”energized” output
15

In this way, the PLC is able to interface with real-world devices such as
switches and solenoids. The actual logic of the control system is established
inside the PLC by means of a computer program. Although the program itself
appears to be a ladder logic diagram, with switch and relay symbols, there are
no actual switch contacts or relay coils operating inside the PLC to create the
logical relationships between input and output. These are imaginary contacts
and coils, if you will. The program is entered and viewed via a personal com-
puter connected to the PLC’s programming port.
2.4 PLC compared with other control systems
PLCs are well-adapted to a certain range of automation tasks. These are typ-
ically industrial processes in manufacturing where the cost of developing and
maintaining the automation system is high relative to the total cost of the au-
tomation, and where changes to the system would be expected during its oper-
ational life. PLCs contain input and output devices compatible with industrial
pilot devices and controls; little electrical design is required, and the design
problem centers on expressing the desired sequence of operations in ladder
logic (or function chart) notation. PLC applications are typically highly cus-
tomized systems so the cost of a packaged PLC is low compared to the cost of
a specific custom-built controller design. For high volume or very simple fixed
automation tasks, different techniques are used.
A microcontroller-based design would be appropriate where hundreds or
thousands of units will be produced and so the development cost (design of
power supplies and input/output hardware) can be spread over many sales, and
where the end-user would not need to alter the control. Automotive applications
are an example; millions of units are built each year, and very few end-users al-
ter the programming of these controllers. However, some specialty vehicles
such as transit buses economically use PLC’s instead of custom-designed con-
trols, because the volumes are low and the development cost would be uneco-
nomic.
PLCs may include logic for single-variable feedback analog control loop,
a ”proportional, integral, derivative” or ”PID controller.” A PID loop could be
used to control the temperature of a manufacturing process, for example. His-
torically PLCs were usually configured with only a few analog control loops;
where processes required hundreds or thousands of loops, a distributed control
system (DCS) would instead be used. However, as PLCs have become more
powerful, the boundary between DCS and PLC applications has become less
clear.
16

3 Ladder Logic
3.1 Deﬁnition
Ladder logic is one form of drawing electrical logic schematics, and is a graphi-
cal language very popular for programming PLC’s. Ladder logic was originally
invented to describe logic made from relays. The name is based on the observa-
tion that programs in this language resemble ladders, with two vertical ”rails”
and a series of horizontal ”rungs” between them. Figure 12 below is a very
basic example of ladder logic used in a programmable logic controls program.
Figure 12: Basic Ladder Logic Program
3.2 Comparison to Relay Logic
The program used in a controls schematic, called a ladder diagram, is similar to
a schematic for a set of relay circuits. An argument that aided the initial adop-
tion of ladder logic was that a wide variety of engineers and technicians would
be able to understand and use it without much additional training, because of
the resemblance to familiar hardware systems. This argument has become less
relevant lately given that most ladder logic programmers have a software back-
ground in more conventional programming languages, and in practice imple-
mentations of ladder logic have characteristics such as sequential execution that
make the analogy to hardware somewhat imperfect. Electricians and data ca-
bling or control technicians still argue that this is the best graphical interface as
17

they generally do not have any computer science or digital systems background,
and are therefore taught with this interface in sequence with relay logic. Relay
Figure 13: Electromechanical Relay
logic is the precursor to ladder logic, and is a method of controlling industrial
electronic circuits by using relays and contacts. Figure 13 above shows an aver-
age mechanical relay used in older relay logic systems. The schematic diagrams
for relay logic circuits are often called line diagrams, because the inputs and out-
puts are essentially drawn in a series of lines, with the lines representing actual
wires run in the circuit. A relay logic circuit is an electrical network consisting
of lines, in which each input/output group must have electrical continuity with
all components in that group of devices to enable the output device. The Relay
logic diagrams represent the physical interconnection of devices, while the re-
lay logic circuit forms an electrical schematic diagram for the control of input
and output devices. This is why electricians and control technicians can easily
understand and interpret relay logic and ladder logic diagrams. Figure 14 below
shows a basic relay logic circuit.
Figure 14: Basic Relay logic Circuit
Figure 14 is a small, basic relay logic circuit. You can see how in relay logic
circuits the push buttons are represented with graphical drawings of a normally
closed push button for the stop button, and a normally open push button for the
start button. The coil that is marked M is a motor coil, and is a physical piece
18

of equipment in the same location as the motor, which is represented by a circle
with the letter M in the middle. The over current or overload device is repre-
sented by a normally closed coil symbol with O.L. over it. There would only be
seven wires to connect in this circuit, so this would not be very difficult to wire,
but when more inputs and outputs are added, the difficulty grows exponentially.
Figure 15 below adds four more components to the system. Two of them
are just coils from the motor apparatus that are used as inputs and the other two
are a red and green light to be utilized as output/motor status indicators for the
user.
Figure 15: Relay Logic Circuit with Jog function and Status Indicators
Ladder logic is the most widely used program for PLC where sequential
control of a process or manufacturing operation is required. Ladder logic is
useful for simple but critical control systems, or for reworking old hardwired
relay circuits. As programmable logic controllers became more sophisticated
it has also been used in very complex automation systems. Figure 15 above
shows a much more complicated ladder logic diagram than the one shown in
Figure 5.In addition there are holding/latching contacts included, but they are
not a piece of hardware. In fact, they are just the address of the respective output
being referenced, which will be discussed in greater detail later. This is still not
a very large program. Ladder logic programs can easily grow to more than 500
rungs to finish some functions.
3.3 Ladder Logic Programming
3.3.1 Introduction
Ladder logic or ladder diagrams are the most common programming language
used to program a PLC. Ladder logic was one of the first programming ap-
proaches used in PLCs because it borrowed heavily from the relay diagrams
19

Figure 16: Complex Ladder Diagram
that plant electricians already knew. The symbols used in relay ladder logic
consist of a power rail to the left, a second power rail to the right, and individual
circuits that connect the left power rail to the right. The logic of each circuit (or
rung) is solved from left to right. A common mistake made by most people is
trying to think of the diagram as having to have current across the rung for the
output to function. This has given many people trouble because of the fact that
some inputs are not inputs, which will be true when there isnt current through
this sensor. These concepts will be discussed more later. The symbols of these
diagrams look like a ladder - with two side rails and circuits that resemble rungs
on a ladder.
Figure 17: Basic Ladder Logic Program
While Ladder Logic is the most commonly used PLC programming lan-
guage, but it is not the only one. Following table lists some of the Languages
20

that are used to program a PLC.
1. Ladder Diagram()LD.
2. Functional block Diagram (FBD)
3. Structured Text (ST)
4. Instruction List (IL)
5. Sequential Functional Chart (SFC)
Ladder logic is a method of drawing electrical logic schematics. It is now a
graphical language very popular for programming Programmable Logic Con-
trollers (PLCs). It was originally invented to describe logic made from relays.
The name is based on the observation that programs in this language resemble
ladders, with two vertical ”rails” and a series of horizontal ”rungs” between
them. It is a graphical programming language, initially programmed with sim-
ple contacts that simulates the opening and closing of relays. Ladder Logic pro-
gramming has been expanded to include functions such as Counters, Timers,
shift Registers and math operations.
A program in ladder logic, also called a ladder diagram, is similar to a
schematic for a set of relay circuits. An argument that aided the initial adoption
of ladder logic was that a wide variety of engineers and technicians would be
able to understand and use it without much additional training, because of the
resemblance to familiar hardware systems.
Ladder logic is widely used to program PLCs, where sequential control of a
process or manufacturing operation is required. Ladder logic is useful for sim-
ple but critical control systems, or for reworking old hardwired relay circuits.
As programmable logic controllers became more sophisticated it has also been
used in very complex automation systems.
Ladder logic can be thought of as a rule-based language, rather than a pro-
cedural language. A ”rung” in the ladder represents a rule. When implemented
with relays and other electromechanical devices, the various rules ”execute” si-
multaneously and immediately. When implemented in a programmable logic
controller, the rules are typically executed sequentially by software, in a loop.
By executing the loop fast enough, typically many times per second, the effect
of simultaneous and immediate execution is obtained.Figure 18 shows a simpli-
ﬁed ladder logic circuit with one input and one output. The logic of the rung
above is such:
1. If Input1 is ON (or true) - power (logic) completes the circuit from the
left rail to the right rail - and Output1 turns ON (or true).
21

Figure 18: Basic Program to show input and output
2. If Input1 is OFF (or false) - then the circuit is not completed and logic
does not flow to the right - and Output 1 is OFF (or false).
There are many logic symbols available in Ladder Logic - including timers,
counters, math, and data moves such that any logical condition or control loop
can be represented in ladder logic. With just a handful of basic symbols such as
a normally open contact, normally closed contact, normally open coil, normally
closed coil, timer and counter most logical conditions can be represented.
Normally Open Contact
Figure 19: Normally Open Contact
This can be used to represent any input to the control logic such as a switch
or sensor, a contact from an output, or an internal output. When solved the
referenced input is examined for a true (logical 1) condition. If it is true, the
contact will close and allow logic to flow from left to right. If the status is
FALSE (logical 0), the contact is open and logic will NOT flow from left to
right.
Normally Open Coil
This can be used to represent any discrete output from the control logic.
When ”solved” if the logic to the left of the coil is TRUE, the referenced output
is TRUE (logical 1).
Figure 20: Normally Open Coil
Normally Closed Contact
22

When solved the referenced input is examined for an OFF condition. If the
status is OFF (logical 0) power (logic) will flow from left to right. If the status
is ON, power will not flow.
Figure 21: Normally Closed Contact
Normally Closed Coil
When ”solved” if the coil is a logical 0, power will be turned on to the
device. If the device is logical 1, power will be OFF. Normally Open Contact
Figure 22: Normally Closed coil
Basic AND and OR Gates The AND is a basic fundamental logic condition
that is easy to directly represent in Ladder Logic. Figure 8 shows a simplified
AND gate on a ladder rung.
Figure 23: Basic And Gate using Ladder Logic
Figure 24: Basic And Gate using Ladder Logic
In order for Light1 to turn TRUE, Switch1 must be TRUE, AND Switch2
must be TRUE. If Switch1 is FALSE, logic (not power) flows from the left rail,
23

but stops at Switch1. Light1 will be TRUE regardless of the state of Switch2. If
Switch1 is TRUE, logic makes it to Switch2. If Switch2 is TRUE, power cannot
flow any further to the right, and Light1 is FALSE. If Switch1 is TRUE, AND
Switch2 is TRUE - logic flows to Light1 solving its state to TRUE. The OR is
a logical condition that is easy to represent in Ladder Logic. Figure 13 shows
a simple OR gate. Notice the differences in logic between the OR and AND
gates.
If Switch1 is TRUE, logic flows to Light1 turning it to TRUE. If Switch2
is TRUE, logic flows through the Switch2 contact, and up the rail to Light1
turning it to TRUE. If Switch1 AND Switch 2 are TRUE Light1 is TRUE. The
only way Light1 is FALSE is if Switch1 AND Switch2 are FALSE. In other
words, Light1 is TRUE if Switch1 OR Switch2 is TRUE.
Basic Timers and Counters
Many times programs will call for action to be taken in a control program
based on more than the states of discrete inputs and outputs. Sometimes, pro-
cesses will need to turn on after a delay, or count the number of times a switch
is hit. To do these simple tasks, Timers and Counters are utilized.
A timer is simply a control block that takes an input and changes an output
based on time. There are two basic types of timers. There are other advanced
timers, but they wont be discussed in this report. An On-Delay Timer takes
an input, waits a specific amount of time, allows logic to flow after the delay.
An Off-Delay Timer allows logic to flow to an output and keeps that output
true until the set amount of time has passed, then turns it false, hence off-delay.
Figure 14 above shows an On-Delay Timer with a 10 second delay before it
passes the logic through it.
A counter simply counts the number of events that occur on an input. There
are two basic types of counters called up counters and down counters. As its
name implies, whenever a triggering event occurs, an up counter increments the
counter, while a down counter decrements the counter whenever a triggering
event occurs.
3.4 EXAMPLES OF LADDER PROGRAMMING
Programming For Start/Stop of Motor by PLC Often we have a little green
”start” button to turn on a motor, and we want to turn it off with a big red
”Stop” button. The pushbutton switch connected to input X1 serves as the
”Start” switch, while the switch connected to input X2 serves as the ”Stop.”
Another contact in the program, named Y1, uses the output coil status as a seal-
in contact, directly, so that the motor contactor will continue to be energized
after the ”Start” pushbutton switch is released. You can see the normally-closed
24

contact X2 appear in a colored block, showing that it is in a closed (”electrically
conducting”) state.
Figure 25: Start/Stop of Motor by PLC
Starting of Motor If we were to press the ”Start” button, input X1 would
energize, thus ”closing” the X1 contact in the program, sending ”power” to the
Y1 ”coil,” energizing the Y1 output and applying 120 volt AC power to the real
motor contactor coil. The parallel Y1 contact will also ”close,” thus latching the
”circuit” in an energized state.
Figure 26: Starting of Motor
Logic for Continous Running of motor When Start Button is Released
Now, if we release the ”Start” pushbutton, the normally-open X1 ”contact” will
25

return to its ”open” state, but the motor will continue to run because the Y1 seal-
in ”contact” continues to provide ”continuity” to ”power” coil Y1, thus keeping
the Y1 output energized.
Figure 27: Continous Running of motor when Start Button is Released
To Stop the Motor To stop the motor, we must momentarily press the
”Stop” pushbutton, which will energize the X2 input and ”open” the normally-
closed ”contact,” breaking continuity to the Y1 ”coil:” When the ”Stop” push-
button is released, input X2 will de-energize, returning ”contact” X2 to its nor-
mal, ”closed” state. The motor, however, will not start again until the ”Start”
pushbutton is actuated, because the ”seal-in” of Y1 has been lost.
Figure 28: To Stop the Motor
26

4 SCADA
An industrial SCADA system will be used for the development of the controls of
the four LHC experiments. This paper describes the SCADA systems in terms
of their architecture, their interface to the process hardware, the functionality
and the application development facilities they provide.
4.1 Introduction
Widely used in industry for Supervisory Control and Data Acquisition of in-
dustrial processes, SCADA systems are now also penetrating the experimental
physics laboratories for the controls of ancillary systems such as cooling, ven-
tilation, power distribution, etc.
SCADA systems have made substantial progress over the recent years in
terms of functionality, scalability, performance and openness such that they are
an alternative to in house development even for very demanding and complex
control systems as those of physics experiments.
4.2 Types of SCADA
1. D+R+N ( Development +Run + Networking)
2. R+N ( Run +Networking )
3. Factory focus
4.3 Features of SCADA
1. Dynamic process Graphic
2. Alarm summery
3. Alarm history
4. Real time trend
5. Historical time trend
6. Security (Application Security)
7. Data base connectivity
27

8. Device connectivity
9. Scripts
10. Recipe management
4.4 Manufacture of SCADA
Modicon (Telemecanique) Visual look
1. Allen Bradly : RS View
2. Siemens: win cc
3. KPIT : ASTRA
4. Intelution : Aspic
5. Wonderware : Intouch
4.5 What does SCADA MEAN?
SCADA stands for Supervisory Control And Data Acquisition. As the name
indicates, it is not a full control system, but rather focuses on the supervisory
level. As such, it is a purely software package that is positioned on top of hard-
ware to which it is interfaced, in general via Programmable Logic Controllers
(PLCs), or other commercial hardware modules.
SCADA systems are used not only in industrial processes: e.g. steel mak-
ing, power generation (conventional and nuclear) and distribution, chemistry,
but also in some experimental facilities such as nuclear fusion. The size of such
plants range from a few 1000 to several 10 thousands input/output (I/O) chan-
nels. However, SCADA systems evolve rapidly and are now penetrating the
market of plants with a number of I/O channels of several 100 K: we know of
two cases of near to 1 M I/O channels currently under development.
SCADA systems used to run on DOS, VMS and UNIX; in recent years all
SCADA vendors have moved to NT and some also to Linux.
28

4.6 Architecture
4.6.1 Hardware Architecture
One distinguishes two basic layers in a SCADA system: the ”client layer” which
caters for the man machine interaction and the ”data server layer” which handles
most of the process data control activities. The data servers communicate with
devices in the field through process controllers. Process controllers, e.g. PLCs,
are connected to the data servers either directly or via networks or fieldbuses
that are proprietary (e.g. Siemens H1), or non-proprietary (e.g. Profibus). Data
servers are connected to each other and to client stations via an Ethernet LAN.
The data servers and client stations are NT platforms but for many products
the client stations may also be W95 machines. Fig.1. shows typical hardware
architecture.
4.6.2 Generic Software Architecture
However, it is possible to have dedicated servers for particular tasks, e.g. his-
torian, datalogger, alarm handler. Fig. 2 shows a SCADA architecture that is
generic for the products that were evaluated.
4.7 Communications
4.7.1 Internal Communication
Server-client and server-server communication is in general on a publish-subscribe
and event-driven basis and uses a TCP/IP protocol, i.e., a client application sub-
scribes to a parameter which is owned by a particular server application and
only changes to that parameter are then communicated to the client application.
4.7.2 Access to Devices
The data servers poll the controllers at a user defined polling rate. The polling
rate may be different for different parameters. The controllers pass the requested
parameters to the data servers. Time stamping of the process parameters is typ-
ically performed in the controllers and this time-stamp is taken over by the data
server. If the controller and communication protocol used support unsolicited
data transfer then the products will support this too.
The products provide communication drivers for most of the common PLCs
and widely used field-buses, e.g., Modbus. Of the three fieldbuses that are rec-
29

ommended at CERN, both Profibus and Worldfip are supported but CANbus
often not.
4.8 Interfacing
4.8.1 Application Interfaces / Openness
The provision of OPC client functionality for SCADA to access devices in an
open and standard manner is developing. There still seems to be a lack of de-
vices/controllers, which provide OPC server software, but this improves rapidly
as most of the producers of controllers are actively involved in the development
of this standard The products also provide:
1. an Open Data Base Connectivity (ODBC) interface to the data in the
archive/logs, but not to the configuration database,
2. an ASCII import/export facility for configuration data,
3. a library of APIs supporting C, C++, and Visual Basic (VB) to access data
in the RTDB, logs and archive. The API often does not provide access to
the product’s internal features such as alarm handling, reporting, trending,
etc.
The PC products provide support for the Microsoft standards such as Dy-
namic Data Exchange (DDE) which allows e.g. to visualise data dynamically in
an EXCEL spreadsheet, Dynamic Link Library (DLL) and Object Linking and
Embedding (OLE).
4.8.2 Database
The configuration data are stored in a database that is logically centralised but
physically distributed and that is generally of a proprietary format. System
(RDBMS) at a slower rate either directly or via an ODBC interface.
4.8.3 Scalability
Scalability is understood as the possibility to extend the SCADA based control
system by adding more process variables, more specialised servers (e.g. for
alarm handling) or more clients. The products achieve scalability by having
multiple data servers connected to multiple controllers. Each data server has its
own configuration database and RTDB and is responsible for the handling of a
sub-set of the process variables (acquisition, alarm handling, archiving).
30

4.8.4 Redundancy
The products often have built in software redundancy at a server level, which
is normally transparent to the user. Many of the products also provide more
complete redundancy solutions if required.
4.8.5 Functionality
Access Control Users are allocated to groups, which have defined read/write
access privileges to the process parameters in the system and often also to spe-
cific product functionality.
MMI The products support multiple screens, which can contain combi-
nations of synoptic diagrams and text. They also support the concept of a
”generic” graphical object with links to process variables. These objects can
be ”dragged and dropped” from a library and included into a synoptic diagram.
Most of the SCADA products that were evaluated decompose the process
in ”atomic” parameters (e.g. a power supply current, its maximum value, its
on/off status, etc.) to which a Tag-name is associated. The Tag-names used to
link graphical objects to devices can be edited as required. The products include
a library of standard graphical symbols, many of which would however not be
applicable to the type of applications encountered in the experimental physics
community.
Standard windows editing facilities are provided: zooming, re-sizing, scrolling...
On-line configuration and customisation of the MMI is possible for users with
the appropriate privileges. Links can be created between display pages to navi-
gate from one view to another.
4.8.6 Trending
The products all provide trending facilities and one can summarise the common
capabilities as follows:
1. the parameters to be trended in a specific chart can be predefined or de-
fined on-line
2. a chart may contain more than 8 trended parameters or pens and an unlim-
ited number of charts can be displayed (restricted only by the readability)
3. real-time and historical trending are possible, although generally not in
the same chart
31

4. historical trending is possible for any archived parameter
5. zooming and scrolling functions are provided
6. parameter values at the cursor position can be displayed
The trending feature is either provided as a separate module or as a graphical
object (ActiveX), which can then be embedded into a synoptic display. XY and
other statistical analysis plots are generally not provided.
4.8.7 Alarm Handling
Alarm handling is based on limit and status checking and performed in the
data servers. More complicated expressions (using arithmetic or logical expres-
sions) can be developed by creating derived parameters on which status or limit
checking is then performed. The alarms are logically handled centrally, i.e., the
information only exists in one place and all users see the same status (e.g., the
acknowledgement), and multiple alarm priority levels (in general many more
than 3 such levels) are supported.
It is generally possible to group alarms and to handle these as an entity
(typically filtering on group or acknowledgement of all alarms in a group). Fur-
thermore, it is possible to suppress alarms either individually or as a complete
group. The filtering of alarms seen on the alarm page or when viewing the alarm
log is also possible at least on priority, time and group. However, relationships
between alarms cannot generally be defined in a straightforward manner. E-
mails can be generated or predefined actions automatically executed in response
to alarm conditions.
4.8.8 Logging/Archiving
The terms logging and archiving are often used to describe the same facility.
However, logging can be thought of as medium-term storage of data on disk,
whereas archiving is long-term storage of data either on disk or on another per-
manent storage medium.
Logging is typically performed on a cyclic basis, i.e., once a certain file size,
time period or number of points is reached the data is overwritten. Logging of
data can be performed at a set frequency, or only initiated if the value changes
or when a specific predefined event occurs. Logged data can be transferred to an
archive once the log is full. The logged data is time-stamped and can be filtered
when viewed by a user. The logging of user actions is in general performed
together with either a user ID or station ID. There is often also a VCR facility
to play back archived data.
32

4.8.9 Report Generation
One can produce reports using SQL type queries to the archive, RTDB or logs.
Although it is sometimes possible to embed EXCEL charts in the report, a ”cut
and paste” capability is in general not provided. Facilities exist to be able to
automatically generate, print and archive reports.
4.8.10 Automation
The majority of the products allow actions to be automatically triggered by
events. A scripting language provided by the SCADA products allows these
actions to be defined. In general, one can load a particular display, send an
Email, run a user defined application or script and write to the RTDB.
The concept of recipes is supported, whereby a particular system configura-
tion can be saved to a file and then re-loaded at a later date.
33

5 Application Development
5.1 Configuration
The development of the applications is typically done in two stages. First the
process parameters and associated information (e.g. relating to alarm condi-
tions) are defined through some sort of parameter definition template and then
the graphics, including trending and alarm displays are developed, and linked
where appropriate to the process parameters. The products also provide an
ASCII Export/Import facility for the configuration data (parameter definitions),
which enables large numbers of parameters to be configured in a more efficient
manner using an external editor such as Excel and then importing the data into
the configuration database.
However, many of the PC tools now have a Windows Explorer type devel-
opment studio. The developer then works with a number of folders, which each
contains a different aspect of the configuration, including the graphics.
The facilities provided by the products for configuring very large numbers
of parameters are not very strong. However, this has not really been an issue
so far for most of the products to-date, as large applications are typically about
50K I/O points and database population from within an ASCII editor such as
Excel is still a workable option.
On-line modifications to the configuration database and the graphics is gen-
erally possible with the appropriate level of privileges.
5.2 Development Tools
The following development tools are provided as standard:
1. a graphics editor, with standard drawing facilities including freehand,
lines, squares circles, etc. It is possible to import pictures in many formats
as well as using predefined symbols including e.g. trending charts, etc. A
library of generic symbols is provided that can be linked dynamically to
variables and animated as they change. It is also possible to create links
between views so as to ease navigation at run-time.
2. a data base configuration tool (usually through parameter templates). It is
in general possible to export data in ASCII files so as to be edited through
an ASCII editor or Excel.
3. a scripting language
34

4. an Application Program Interface (API) supporting C, C++, VB
5. a Driver Development Toolkit to develop drivers for hardware that is not
supported by the SCADA product.
5.3 Object Handling
The products in general have the concept of graphical object classes, which sup-
port inheritance. In addition, some of the products have the concept of an object
within the configuration database. In general the products do not handle objects,
but rather handle individual parameters, e.g., alarms are defined for parameters,
logging is performed on parameters, and control actions are performed on pa-
rameters. The support of objects is therefore fairly superficial.
5.4 Evolution
SCADA vendors release one major version and one to two additional minor
versions once per year. These products evolve thus very rapidly so as to take
advantage of new market opportunities, to meet new requirements of their cus-
tomers and to take advantage of new technologies.
As was already mentioned, most of the SCADA products that were evalu-
ated decompose the process in ”atomic” parameters to which a Tag-name is as-
sociated. This is impractical in the case of very large processes when very large
sets of Tags need to be configured. As the industrial applications are increas-
ing in size, new SCADA versions are now being designed to handle devices
and even entire systems as full entities (classes) that encapsulate all their spe-
cific attributes and functionality. In addition, they will also support multi-team
development.
As far as new technologies are concerned, the SCADA products are now
adopting:
1. Web technology, ActiveX, Java, etc.
2. OPC as a means for communicating internally between the client and
server modules. It should thus be possible to connect OPC compliant
third party modules to that SCADA product.
5.5 Engineering
Whilst one should rightly anticipate significant development and maintenance
savings by adopting a SCADA product for the implementation of a control sys-
35

tem, it does not mean a ”no effort” operation. The need for proper engineering
can not be sufficiently emphasised to reduce development effort and to reach
a system that complies with the requirements, that is economical in develop-
ment and maintenance and that is reliable and robust. Examples of engineering
activities specific to the use of a SCADA system are the definition of:
1. a library of objects (PLC, device, subsystem) complete with standard ob-
ject behaviour (script, sequences, ...), graphical interface and associated
scripts for animation,
2. templates for different types of ”panels”, e.g. alarms,
3. instructions on how to control e.g. a device ...,
4. a mechanism to prevent conflicting controls (if not provided with the
SCADA),
5. alarm levels, behaviour to be adopted in case of specific alarms, ...
5.6 Potential benefits of SCADA
The benefits one can expect from adopting a SCADA system for the control of
experimental physics facilities can be summarised as follows:
1. a rich functionality and extensive development facilities. The amount of
effort invested in SCADA product amounts to 50 to 100 p-years!
2. the amount of specific development that needs to be performed by the
end-user is limited, especially with suitable engineering.
3. reliability and robustness. These systems are used for mission critical
industrial processes where reliability and performance are paramount.
In addition, specific development is performed within a well-established
framework that enhances reliability and robustness.
4. technical support and maintenance by the vendor.
For large collaborations, as for the CERN LHC experiments, using a SCADA
system for their controls ensures a common framework not only for the devel-
opment of the specific applications but also for operating the detectors. Opera-
tors experience the same ”look and feel” whatever part of the experiment they
control. However, this aspect also depends to a significant extent on proper
engineering.
36

5.7 Where SCADA is used ?
Electric power generation, transmission and distribution: Electric utilities use
SCADA systems to detect current flow and line voltage, to monitor the opera-
tion of circuit breakers, and to take sections of the power grid online or offline.
Water and sewage: State and municipal water utilities use SCADA to mon-
itor and regulate water flow, reservoir levels, pipe pressure and other factors.
37

6 CONCLUSION
With the speed of changing technology today it is easy to lose sight or knowl-
edge of the basic theory or operation of programmable logic. Most people sim-
ply use the hardware to produce the results they desire. Hopefully, this report
has given the reader a deeper insight into the inner workings of programmable
logic and its role in mechanical operations. The idea of programmable logic is
very simple to understand, but it is the complex programs that run in the lad-
der diagrams that make them difficult for the common user to fully understand.
Hopefully this has alleviated some of that confusion.
SCADA is used for the constructive working not for the destructive work
using a SCADA system for their controls ensures a common framework not
only for the development of the specific applications but also for operating the
detectors. Operators experience the same ”look and feel” whatever part of the
experiment they control. However, this aspect also depends to a significant
extent on proper engineering
38

References
[1] A Daneels, W Salter. ”Technology Survey Summary of Study Report” ,
IT-CO/98-08-09, CERN, Geneva 26th Aug 1998.
[2] A Daneels, W Salter, ”Selection and Evaluation of Commercial SCADA
Systems for the Controls of the CERN LHC Experiments” Proceedings of
the 1999 International Conference on Accelerator and Large Experimental
Physics Control Systems, Trieste, 1999, p.353.
[3] G Baribaud et al., ”Recommendations for the Use of Fieldbuses at CERN
in the LHC Era” Proceedings of the 1997 International Conference on Ac-
celerator and Large Experimental Physics Control Systems, Beijing, 1997,
p.285.
[4] www.wikipedia.com
[5] literature.rockwellautomation.com/idc/groups
39

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