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a. What is Automation

Delegation of human control function to technical equipments

to increase production

to reduce costto reduce manpower

to improve safety working conditions

to reduce power consumption

to improve quality

b. Types of Automation

Automated machines can be subdivided into two large categoriesopen-loop and

closed-loop machines, which can then be subdivided into even smaller categories.Open-loop machines are devices that, once started, go through a cycle and then

stop. A common example is the automatic dishwashing machine. Once dishes are

loaded into the machine and a button pushed, the machine goes through a

predetermined cycle of operations: pre-rinse, wash, rinse, and dry, for example. A

human operator may have choices as to which sequence the machine should

followheavy wash, light wash, warm and cold, and so onbut each of these

operations is alike in that the machine simply does the task and then stops. Many

of the most

familiar appliances in homes today operate on this basis. A microwave oven, acoffee maker, and a CD player are examples.

Larger, more complex industrial operations also use open-cycle operations. For

example, in the production of a car, a single machine may be programmed to

place a side panel in place on the car and then weld it in a dozen or more

locations. Each of the steps involved in this processfrom placing the door

properly to each of the different weldstakes place according to instructions

programmed into the machine.

Other category in which automation is divided is:

a. Scientific Automation(used by scientists)

b. Industrial Automation

(building management system)

c. Office Automation

(used by non technical staff)

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c. Role ofcomputers in automation

Since the 1960s, the nature of automation has undergone dramatic changes as a

result of the availability of computers. For many years, automated machines were

limited by the amount of feedback data they could collect and interpret. Thus,their operation was limited to a relatively small number of alternatives. When an

automated machine is placed under the control of a computer, however, that

disadvantage disappears. The computer can analyze a vast number of sensory

inputs from a system and decide which of many responses it should make.

LAYOUT OF INDUSTRIAL AUTOMATION

AUTOMATION- ADVANTAGES

1. Replacing human operator in tedious task.

2. Replacing humans in tasks that should be done in dangerous environment.

3. Making tasks that are beyond human capabilities such as handle too heavy

loads, too large objects, too hot or cold substances or the requirement to make

things too fast or too slow.

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A SCADA System usually consists of the following subsystems:

A Human-Machine Interfaceor HMI is the apparatus which presents process

data to a human operator, and through this, the human operator monitors and

controls the process. A supervisory (computer) system, gathering (acquiring) data

on the process and sending commands (control) to the process.

Remote Terminal Units (RTUs) connecting to sensors in the process,

converting sensor signals to digital data and sending digital data to the supervisory

system.

Programmable Logic Controller (PLCs) used as field devices because they are

more economical, versatile, flexible, and configurable than special-purpose RTUs.

Communication infrastructure connecting the supervisory system to the RemoteTerminal Units

Features of scada There are various features of scada thats why it is mostly used

in industries present time.

The main features are described below

1. Dynamic Representation 7. Recipe Management

2. Alarms 8. Script

3. Trends 9. Networking

4. Security 10. Logging/Archiving

5. Data connectivity

6. Device connectivity

1. Dynamic Representation -SCADA should have good library of symbols so that

you can develop the mimic as per requirement. Once the operator sees the

screen he should know what is going on in. the plant

It is the most important feature of scada because due to this the complete plant can

be represented graphically on pc screen and it looks like that live plant is running.

So in scada we can create the model of a plant on which operator can continuouslymonitor the whole plant. And company mangers, supervisors are able to see that

what is happening in the plant on their pc screen.

2. ALARMS have a very important role in automation. Generally you have alarm

states for each inputs/outputs like your temperature should not cross 80 deg or

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lever should be less than 60. So if the parameters go in alarm state the operator

should be intimated with alarm. Most of the SCADA software support four types

of alarms like LOLO,LO,HI and HIHI. Deadband the value of deadband defines the

range after which a high low alarm condition returns to normal.

Alarms are the most important part of the plant control applications because theoperator must know instantly when something goes wrong. It is often equally

important to have a record of alarms and whether an alarm was acknowledged.

An alarm occurs when something goes wrong. It can signal that a device or

process has ceased operating within acceptable, predefined limits or it can

indicate breakdown, wear or process malfunction

3. Trends - Trends means graphical representation of data or a parameter .There

are two types of trends available in scada.

(A). Real Time Trend

(B). Historical Trend

(A). Real Time Trend - This trend is used for real time monitoring of varying

parameters in the field (like Temperature) in graph form on the screen of pc. So the

user can notice the value changes and status of the parameter in real time. So it is

useful for monitoring varying quantities by graph representation .These are

dynamic and updated continuously during run time. It allows user to chart up to 4

data values.

(B). Historical Trend - By historical trend feature it is

possible to record the data values up to 10 years so operator can check the data of

last ten years when needed .These are not dynamic and provide a snapshot only of

particular time. It allows user to chart up to 8 data values at a time.

4. SECURITY is on facility people generally look for. You can allocate certain

facilities or features to the operator, process people, engineering dept and

maintenance dept. for example operators should only operate the system, he

should not be able change the application. The engineers should have access tochanging the application. The engineers should have access to changing the

application developed.

When a new app is created the default user name is Administrator with an access

level of 9999(which allows access to all security commands).

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5. Data connectivity- It means that the data values in scada can be shared or

monitored on an other application like excel etc. and the data can be stored as a log

file for future use and can access later when needed.

There are 2 options by which data can communicate with other software.

(A). Dynamic Data Exchange (DDE)- If it is required to communicate data

between two softwares in same pc then this option is selected. Or we can say DDE

is the facility developed by Microsoft for communication of data between two

applications or programs.

Example - Data exchange between Scada and Excel.

(B). Suite link This option is selected when the communication of data is

between two devices via Modem or a cable. Also by this option the scada on

different computers can be exchanged. So we can say it is used when hardwarecommunication is required.

Example Data exchange between two different computers.

6. DEVICE CONNECTIVITY you will find there are hundreds of automation

hardware manufacturer like Modicon, Siemens, Allen Bradly, ABB. Everybody has

there own way of communication or we can say they have there own

communication protocol. SCADA software should have connectivity to the

different hardware used in automation. It should not happen that for Modicon Iam buying one software and for Siemens another one. The software like Aspic or

Wonderware has connectivity to almost all hardware used in automation.

7. RECIPE MANAGEMENT is an additional feature. Some SCADA software support

it, some do not. Most of the plants are manufacturing multi products. When you

have different products to manufacture, you just have to load the recipe of the

particular product.

Example - In wonder ware scada maximum 12 recipes can be prepared at a time

and maximum numbers of items that can be used are 63. So we can say that 12recipes can be prepared by different combinations of these items.

Also these recipe files can be stored and can be transferred to other scada system or

other computer. So if a company has different plants at different locations then

same recipe can be used in each plant there is no need to create new recipe file for

each plant.

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8. Script script is a way of writing logic in scada software. Each scada has its

own instruction set and according to then program is written and system follows

this program and works accordingly it.

So whenever a script is written the system follows the instructions written in script

and works.

There are various types of scripts.

1. Application It is linked to entire application means it is common for complete

application.

2.Window It is linked to a specificWindow.

3. Key It is linked to a specific key or key combination on the keyboard

4. Condition It is linked to a discrete Tag name or expressions.

5. Data change It is linked to a tag name.

9. Networking Networking feature enables operator to link scada with different

devices like plc or with other computers and also various scada systems on

different computers can be linked together via network by which they can

communicate with each other.

These are the various features of scada that makes it very useful in industries at

Present time.

And today almost in all large industries scada systems are used.

10. 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 permanent 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.

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GENERAL TERMINOLOGY

What is a Tag- a tag is a logical name for a variable in a device or local memory

(RAM). Tags that receive data from some external devices such as programmable

logic controllers or servers are refereed to as I/O tags. Tags that receive datainternally from software are called memory tags.

Analog Tags- store a range of values. EX temp, flow, density etc

Discrete tags- to store values such as 0 or 1. EX on/off status of a pump, valves,

switches etc.

System tags- store information generated while the software is running including

alarm info and system time and date.

String tags- are used to store ASCII strings a series of characters or whole word.

The max string length is 131 characters.Visibility- used to control visibility of an object based on the value of discrete tag

name or expression.

Blink- used to make an object blink based on the value of the discrete tagname or

expression.

Orientation- used to make an object rotate based on the value of a tagname

/expression.

Disable- used to disable the touch functionality of objects based on the value of a

tagname of expression. Often used as a part of a security strategy.

Value display links- provides the ability to use text object to display the value of adiscrete, analog or string tagname.

Percent fill links- used to provide ability to vary the fill level of a filled shape

according to the value of an analog tagname or an expression that computes to

an analog value.

Application security- to an application is optional. It provides the application

developer with the ability to control whether or not specific operators are

allowed to perform specific functions within an application Security is based on

the concept of operator logging on to the application and entering his user name

and password and access level. For each operator access to any protectedfunction is granted upon verification of his password and access level.

WHAT IS INTOUCH

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Wonderware InTouch provides a single integrated view of all your controls and

information resources. Intouch enables engineers, supervisors, operators and

managers to view to view and interact with the working of entire operation

through graphical representations of their production processes.

Basic Things in INTOUCH 9.5(wonderware scada)

1. Window type There are 3 types of windows displayed during work with

Intouch 9.5.

(A). Application manager In this window user has asked to input the location of

saving the project, project name. The user enters these things to proceed.

(B). Window Maker In this window the project is developed or created by the

user.It is of three types and user has to select which type of window he wants to use.

(a). Replace Type If this type is selected to create a new project then it

automatically closes the opened window at that time.

(b). Overlay window If this type of window is selected then it will appear on the

top of current window and is larger than that window.When overlay window is

closed any window that is hidden behind it will reappear. And if we click any

window behind overlay window it will bring that window to foreground as activewindow.

(c). Popup window It is similar to overlay window except that it always stays on

the top of all other open windows even if another window is clicked. Usually

popup window requires a response from the user in order to be removed.

(C). Window viewer It is the RUN time window where we can see our project in

running condition and we can mot modify the project in this window and we have

to go again in window maker to modify the project.

To switch betweenWindows maker and windows viewer a button is provided in

the upper right corner in the window.

When we are in window maker a button named RUN TIME will display and

clicking on that will take project in run mode and in run mode window clicking on

DEVELOPMENT will take it back again in window maker.

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2. Tags- Tag defines a process variable. It is the name given to a process variable.

Three types of tags are used while working with scada,

(A). Analog Tag The tag names given to analog variables such as temperature,

distance etc are known as analog tags.

They are divided in two parts

(a). Real analog Tags In this category the actual value is taken or displayed. Ex-

2.3, 5.6 etc.

(b). Integer analog Tags In this category the approx value is taken or displayed.

Ex- 2, 6,100 etc.

Examples filling of tank, temperature etc.

(B). Discrete Tag The tag names given to discrete variables or the items whose

value can only be discrete such as switches, lights etc. It has only two values either

one or zero.

Examples Light, Switches etc.

(C). String Tags These Tags are used to display some message like reset, high

voltage zone etc.

(3). Number of Tags In wonderware scada minimum no of tags is 64 and

maximum are 64000.and the user can use the according to its need.

(4). Package The scada is available in there packages and user can select any one

according to need.

(A). DR Type In this package the user gets the feature of both development and

run Time means user can develop, modify and can see the project in run time.

(B). DRN Type In this package the user gets three features Development, Run

Time, Networking. This package is most costly.

(C). R Type This type of package is known as factory focus and user cant

develop and modify projects. It is used only to see the plant status. The companies

that provide scada also create the plant project of the company for first time and

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install it. And the company members can only see the status of the plant and they

are not able to change it.

License of scada The companies that make scada also provide license for certain

period as users need. The license is provided in two ways-

(1). Hard disk Type In this type they installs scada on the system and locate the

product key or license on local

Hard disk. So this scada can not be used on other computer.

(2). Dongle Type In this type of license the user can use the scada on different

system also because the license is provided in an usb device so the user inserts the

license via usb in the system when he wants to use scada.

Companies that make scada -

There are various companies in world that makes scada software. The main

companies are listed below-

Company Name Scada Name

Siemens(Germany) Wincc(window control center)

Wonderware Intouch

Allen breadlly(Usa) R-s View(Rockwell solutions)KPIT(Indian) Ashtra

Ge-Fanuc Cimplicity

Note KPIT is only Indian company that makes scada. KPIT stands for Krishna

Palli Institute of Technology.

ADVANTAGES OF SCADA SYSTEM

1. A SCADA system is "normally" significantly cheaper than a DCS.

2. SCADA can continue operating even when telecommunication are temporarily

lost.

3. SCADA systems allow a smaller number of operators to control a large number

of individual assets.

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4. SCADA systems were designed to be used on large scale systems with

remote assets over a very large geographical area.

5. SCADA system improves operation, maintenance and customer service and

provides rapid response to emergencies.

6. It provides a high level of system reliability and availability.

CONCLUSION

SCADA is a control system with

More interfaces and efficient storage

More record or device oriented configuration

But system wide configuration tools are needed

Are less expensive than DCS, but offer different functionality than DCS And finally various applications

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Introduction to PLC

1-1. The Role of the Programmable Logic Controllers (PLC)

Programmable logic controllers, also calledprogrammable

controllers or PLCs, are solid-state members of the computer family,

using integrated circuits instead of electromechanical devices toimplement control functions. They are capable of storing instructions,

such as sequencing, timing, counting, arithmetic, data manipulation,

and communication, to control industrial machines and processes.

In an automated system, the PLC is commonly regarded as the

heart of the control system. With a control application program (stored

within the PLC memory) in execution, the PLC constantly monitors the

state of the system through the field input devices' feedback signal. It

will then depend on the program logic to determine the course of

action to be carried out at the field output devices.

The PLC may be used to control a simple and repetitive task, or afew of them may be interconnected together with other host controllersor host computers through a sort of communication network, in orderto integrate the control of a complex process.Programmable controllers have many definitions. However, PLCscan be thought of in simple terms as industrial computers with

specially designed architecture in both their central units (the PLCitself) and their interfacing circuitry to field devices (input/outputconnections to the real world).a) Input DevicesIntelligence of an automated system is greatly depending on theability of a PLC to read in the signal from various types of automatic

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sensing and manual input field devices.Push-buttons, keypad and toggle switches, which form the basicman-machine interface, are types of manual input device. On the otherhand, for detection of work-piece, monitoring of moving mechanism,checking on pressure and or liquid level and many others, the PLC willhave to tap the signal from the specific automatic sensing devices like

proximity switch, limit switch, photoelectric sensor, level sensor and soon. Types of input signal to the PLC would be of ON/OFF logic oranalogue. These input signals are interfaced to PLC through varioustypes of PLC input module.

b) Output DevicesAn automatic system is incomplete and the PLC system is virtuallyparalyzed without means of interface to the field output devices. Someof the most commonly controlled devices are motors, solenoids, relaysindicators, buzzers and etc.However, other output devices such as the buzzers andalarms are merely meant for notifying purpose. Like input signalinterfacing, signal from output devices are interfaced to the PLCthrough the wide range of PLC output module.

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1-2. Characteristic Functions ofa PLCA programmable controller is currently defined by the NationalElectrical Manufacturers Association (NEMA) as a digital electronicdevice that uses a programmable memory to store instructions and to

implement specific functions such as logic, sequence, timing,6

counting, and arithmetic to control machines and processes. However,this definition is so broad as to encompass nearly every solid-statedevice used in manufacturing, from a simple timer to a mainframecomputer. Instead of this definition it is more useful to examine themost important and essential characteristics ofa PLC thatportray its unique aspects whichare:1) It is field programmable by the user. This characteristic allowsthe user to write and change programs in the field without rewritingor sending the unit back to the manufacturer for this purpose.2) It contains programmed functions. PLCs contain at least logic,timing, counting, and memory functions that the user can accessthrough some type of control-oriented programming language.3) It scans memoryand inputs and outputs ( I/O ) in adeterministic manner. This critical feature allows the control

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engineer to determine precisely how the machine or process willrespond to the program.4) It provides errorchecking and diagnostics. A PLC will

periodically run internal tests of its memory, processor, and I/Osystems to ensure that what it is doing to the machine or process iswhat it was programmed to do.5) It can be monitored. A PLC will provide some form of monitoringcapability, either through indicating lights that show the status ofinputs and outputs, or by external device that can display programexecution status.6) It is packaged appropriately. PLCs are designed to withstand thetemperature, humidity, vibration, and noise found in most factoryenvironments.7) It has general purpose suitability. Generally a PLC is not

designed for a specific application, but it can handle a wide varietyof control tasks effectively.7

1-3. PLC BasicsA PLC consists of a Central Processing Unit (CPU) containingprocessor, executive memory and application memory, Input andOutput Interfacing modules, which are directly connected to the fieldI/O devices. The program controls the PLC so that when an inputsignal from an input device is turned on, the appropriate response ismade. The response normally involves turning on or off an output

signal to some sort of output devices.A simplified model of a PLC is shown in Figure 1-4. The inputmodules convert the high-level signals that come from the fielddevices to logic-level signals that the PLC's processor can readdirectly. The logic solver reads these inputs and decides what theoutput states should be, based on the user's program logic. The outputmodules convert the logic-level output signals from the logic solverinto the high-level signals that are needed by the various field devices.The program loader is used to enter the user's program into the

memory or change it and to monitor the execution of the program.

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The previous PLC Block Diagram, illustrating its basic functionality.The control engineer (user) enters the control program on the programloader. The program loader writes this program into the memory. The

logic solver reads the states of the sensors through the input modules,then uses this information to solve the logic stored in the user memory(program) and also writes the resulting output states to the outputdevices through the output modules.1-4. Construction ofa PLCSome PLCs are integrated into a single unit, whereas others aremodular. Integrated PLCs are sometimes calledshoebox orbrickPLCs because of their small size. If an integrated PLC is availablewith the capabilities that a user needs, it is usually the mosteconomical option. ModularPLCs consist of optional componentsrequired for a more complex control application, as selected andassembled by the user.The ModularPLCs include the following components, asdemonstrated in Figure 1-5:1- The CPU module, containing the CPU and its memory.2- Input and output modules (I/O modules), to allow the PLC to

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read sensors and control actuators.3- A power supply module, to provide power to the CPU and oftento provide power to drive sensors and low-power actuatorsconnected to I/O modules.4- A rack or bus, so that the CPU module can exchange data withI/O modules. In some PLCs, this component isn't requiredbecause each module plugs directly into its neighboring module.A PLC system with these components is all that is needed tocontrol an automated system. Since a PLC must be programmedbefore it can be used, another component is required:

Aprogramming unit is necessary to create the user-program

and send it to a PLC CPU module's memory.9

Additional optional PLC components are often available, including:

Communications adapters forremote I/O, so that a centralcontroller can be connected to remote sensors and actuators.

Network interfaces to allow interconnecting of PLCs and/orother controllers into distributed control systems.

Operator interface devices to allow data entry and/or data

monitoring by operators.

Description of the PLC Software2-1. Operating System and Application ProgramsThe CPU module of a PLC comes with a very different operating

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system program than those used in most other computers, and comescomplete with application programs programmed into the CPU'smemory. The operating system program causes the PLC to startwhen power is turned on, to run the user-program when the PLCis switched into run mode, and to respond to the usercommandsbyrunning the appropriate application programs. The application

programs allow the user to enter programs and data into the PLC'smemory. Some Parts of the user accessible memory are retainedeven when the PLC's power is disconnected.A PLC retains its operating system, application programs, userprograms, and some data in retentive memory (sometimes callednonvolatile memory) while the PLC is tuned off and even whendisconnected from the power supply. A PLC can therefore resumerunning a user program as soon as power is restored, although PLCs

are often programmed to require some operator action beforerestarting (for safety reasons).The PLC operating system makes the PLC run user-programs verydifferently from the way other computers run user-programs. A PLCoperating system executes an initialization step once each time it is

put into run mode, and then repeatedly makes the PLC executes ascan cycle sequence as long as the PLC remains in run mode. Thisbasic scan cycle inherent in all PLCs is shown in Figure 2-1.Every time the PLC finishes one scan cycle and starts another, theoperating system also restarts a watchdog timer. The watchdog timerruns while the scan cycle executes. If the watchdog timer reaches its

pre-set value before being restarted (if a scan cycle takes unusuallylong to complete), the PLC will immediately fault, and stop running.After faulting, the PLC usually needs operator intervention before itcan resume running.

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PLC ArchitectureAs mentioned previously, some PLCs are integrated into a singleunit and some are modular. A modular PLC consists of several

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components that can be connected by being plugged into a commonbus or rack. Every PLC needs:

A CPU module

A power supply module

At least one I/O module

An Integrated PLC contains all of those components in a singlecase, so the I/O capabilities of an integrated PLC are decided by themanufacturer, not by the user. Some integrated PLCs can beexpanded by having additional I/O modules plugged into expansionsockets, making them somewhat modular.Modular PLCs must contain a CPU module, a power supply, andI/O modules in components purchased separately and plugging theminto the same rack. Some manufacturers offer CPU modules with fewbuilt-in high speed I/O capabilities as CPUs for modular systems,

making them somewhat integrated.3-1. The CPU ModuleAs explained in Figure 3-1, the CPU module contains the centralprocessing unit and its memory. The memory includes PROM(programmable read-only memory) containing the PLCs operatingsystem, driver programs, and application programs, and RAM wherethe user-written programs and working data are stored. PLCmanufacturers offer various types of retentive memory to save userprogramsand data while power is removed, so that the PLC can

resume execution of the user-written control program as soon aspower is restored. If the PLC has one of the following retentivememory options, it doesnt have to be reprogrammed each time it is15

turned on, so a keyboard and monitor dont need to be included as apart of every Plc

3-2. THE RACKOR BUSDuring every scan cycle, a CPU module reads and writes I/Omodules that are part of the modular PLC. The CPU module is

connected to each of those I/O modules via a set of parallelconductors called a bus. In some modular systems, the bus is in abackplane circuit card in a rack, and all PLC modules are plugged intoslots in the rack. In other modular systems, I/O modules are pluggedinto the side of the CPU module or into the side of an I/O module thatis already plugged into the CPU, so bus conductors are connectedthrough the I/O modules.

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Bus conductors are used for data that the CPU can send to orreceive from the I/O modules, several bits at a time. The CPU mustspecify which of the I/O modules the CPU wants to read from or writeto. I/O module addresses are assigned automatically according to howfar the module is located away from the CPU module along the bus.Some bus conductors are used for miscellaneous control signals

passed between the CPU module and I/O modules and to providepower to run the circuitry inside I/O modules. The bus does notprovide power to operate the sensors or actuators attached to I/Omodules.3-3. THE POWER SUPPLYAs shown in Figure 3-2, a power supply module converts availablepower to dc power at the level(s) required by the CPU and I/O moduleinternal circuitry. Usually, the available power is typically 60 Hz/120 V

ac or 50 Hz/220 V ac, although power supply modules are availablefor other input power characteristics. Output power must drive thecomputer circuitry at 5 V dc. Power supply modules may be connectedto the bus or may have to be wired to the CPU module in modular PLCsystems.

3-4. I/O MODULESInput and output modules (I/O modules) allow the PLC to beconnected to sensors and actuators. The I/O modules isolate the lowvoltage,low-current signals that the PLC uses internally from thehigher-power electrical circuits required by most sensors andactuators. The user purchases the types of I/O modules that areneeded for the sensors and actuators that need to be used, and theuser can connect several different types (or several of the same type)19

of I/O modules to a PLC's bus. I/O modules offered by PLCmanufacturers are designed to work with that manufacturer's CPUmodule, so the user can be confident that compatibility won't be a

problem.

Most manufacturers have a wide range of I/O modules that theuser can select from, including:1. Digital I/O modules, which are used to connect the PLC to sensorsand actuators that can only be switch on and off. Modules areavailable for a variety of dc and ac voltages and currents. Eachmodule typically can be connected to several digital sensors and/orto several digital actuators of similar electrical characteristics.

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2. Analog I/O modules, which are used to connect the PLC tosensors that can provide electrical signals which are proportional toa measured value or to actuators that vary their output

proportionally with the electrical signals they receive from an outputanalog module. A single analog I/O module can typically only beconnected to a few sensors or actuators of similar electricalcharacteristics.3. Miscellaneous intelligent I/O modules, each with its own built-inmicroprocessor and memory. Intelligent I/O modules are designedfor special purposes such as counting high-frequency signals or

providing servo control of motors.4. Communication interface modules, which are intelligent I/Omodules that handle the exchange of data via a communicationlink. The user-program in the CPU writes data to the

communication interface module, and the module ensures that it isplaced on the communication network. Similarly, thecommunication interface module can accept data from othercomputers via the communications network and hold it until theCPU reads it from the module. Modern CPU modules can beconnected directly to communication networks, so communicationinterface modules are needed only if communications requirementsexceed the CPU's built-in capabilities.20

When most PLCs power up, they perform a self-check that includes

searching the bus to determine how many modules are present, inorder to optimize the data exchange that will be performed each scancycle. The PLC often exchanges different amounts of information withdifferent types of I/O modules, so optimization of communication alsorequires the PLC to know what type of module is in each slot of therack. In a truly optimized data exchange, a PLC will not waste timereading from an output module and will not try to write data to an inputmodule. (The CPU module's memory will still contain an input imageand an output image data word for each slot, but the data won't reflect

sensor or actuator states).3-4-1. Digital I/O ModulesDigital input modules allow a CPU module to read input image datawords from the module. Each individual bit of a data word reflects theopen or closed state of a single switch or switched sensor.Digital output modules accept output image data words from theCPU module. Each bit of the data word will turn a single actuator on or

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off.Digital I/O modules primarily provide electrical isolation between thelow-power internal circuits of the PLC and the (typically) higher-powercircuits containing sensors or actuators. A digital output module also

provides a buffering feature so that after the CPU writes a data wordto the output module, the module will retain that data word (holdingsome actuators on and others off) until the next scan cycle, when anew output image data word will be written to the output module.a) Digital Input Modules

PLC inputs must convert a variety of logic levels to the 5VDClogic levels used on the data bus. This can be done with circuitssimilar to those shown Figure 3-3. Basically the circuits conditionthe input to drive an optocoupler. This electrically isolates theexternal electrical circuitry from the internal circuitry. Other circuit

components are used to guard against excess or reversed voltagepolarity.

b) DigitalOutput ModulesNote that the PLC outputs must convert the 5VDC logic levels onthe PLC data bus to external voltage levels. This can be done withcircuits similar to those shown in Figure 3-5. Basically the circuitsuse an optocoupler to switch external circuitry. This electricallyisolates the external electrical circuitry from the internal circuitry.Other circuit components are used to guard against excess orreversed voltage polarity.

3-4-2. Analog I/O ModulesSometimes, the control system requires the PLC to either monitoran analog voltage or produce an analog voltage. Because analog I/Omodules can interpret continuous signals, analog I/O interfaces areused in applications, such as batching and temperature control, wherethe simple two-state capabilities of discrete I/O systems areinsufficient.

a) Analog Input ModulesControl systems sometimes receive analog signals from analogtransducers like flow transducers, humidity transducers, load celltransducers, potentiometers, pressure transducers, vibrationtransducers, temperature transducers etc.Analog input modules digitize analog input signals, thereby

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bringing analog information into the PLC (see Figure 3-6). Themodules store this multi-bit information in register locations insidethe PLC.

b) Analog Output ModulesAnalog output interfaces are used in applications requiring thecontrol of field devices that respond to continuous voltage orcurrent levels like analog valves, actuators, meters, electric motordrivesetc. An example of this type of field device is a volumeadjust valve (see Figure 3-8). This type of valve, which is used inhydraulic-based punch presses, requires a 010 VDC signal to varythe volume of oil being pumped to the press cylinders, therebychanging the speed of the ram or platen.