fundamentals of plc final

7/31/2019 Fundamentals of PLC Final

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Fundamentals of PLC


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Contents :

-Introduction

-Plc architecture

-Input/output (i/o) devices

-Input/output signals.

-Plc programming

-Examples-Timers & counters

-Logic symbols, truth tables, and equivalent ladder/plc

logic diagrams

-Number systems


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Session 1

Outline:

What is a PLC?Why Use PLCs ? Advantages.What are the Main Components of PLC?


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What is a PLC?PLC Programmable Logic Controller

So, what is it?

It is a Microprocessor-Based device

used to control equipment in an industrial applications

Applications of PLCsConveyor Systems,

Food Processing Machinery, [e.g. filling

bottles (water, soft drinks, canned food)]

Auto Assembly (e.g. automobile industry)

Fluid Level Control ( e.g. water tanks)Mixing Fluids (paint industries)

Motor speed control


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Industrial Control systems

In a traditional industrial control system,

all control devices are wired directly

Let us say that a push button is supposed to control the operation of amotor. In a traditional control system, the push button would be wireddirectly to the motor. In a PLC system, however, both the push buttonand the motor would be wired to the PLC instead. Then, the PLCs controlprogram would complete the electrical circuit between the two, allowingthe button to control the motor.


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In a PLC system:

The PLC replaces the wiring between thedevices. Thus, instead of being wired directly toeach other, all equipment is wired to the PLC

In a PLC System:1.The control program inside the PLC providesthe wiring connection between the devices.

2.The control program is the computer programstored in the PLCs memory that tells the PLCwhats supposed to be going on in the system.3.The use of a PLC to provide the wiring connectionsbetween system devices may be called soft wiring.

This soft wiring feature is useful


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Why Use PLC?

PLC system

Soft wiring

control function is

modified by just changing the control

program inside the PLC

These changes are easy and cheap

Traditional system

control function is

modified by physically changingthe wiring between the devices

This is costly and time consuming

endeavor

If you want a device in a PLC system to behave differently or to control a differentprocess element, all you have to do is change the control program. In a traditional

system, making this type of change would involve physically changing the wiringbetween the devices, a costly and time-consuming endeavor.


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Why Use PLC


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Other Advantages of PLC

In addition programming flexibility, PLC System

offers:

High reliability

Reduced costs Expandability Computing capabilities Small space requirementsAbility to withstand harsh operating conditions


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A PLC basically consists of two elements: the central processing unit

the input/output system

The central processing unit (CPU)

executes the controlprogram stored in the PLCs memory. Inessence, the CPU is the brains of a programmable controller. Itfunctions much the same way the CPU of a regular computer does,except that it uses special instructions and coding to perform its functions.The CPU has three parts: the processor the memory system

the power supply

Main Components of PLC?


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The central processing unit

Theprocessor is the section of the CPU that codes, decodes, and computes

(processes) data.The memory system is the section of the CPU that stores both the control

program and data from the equipment connected to the PLC.

Thepower supply is the section that provides the

PLC with the voltage and current it needs to operate.

CPU

is a microprocessor systemis the PLC decision making unit. The CPU

monitors the inputs and makes decisionsbased on instructions held in the programmemory. The

CPU performs relay, counting, timing, datacomparison, and

sequential operations.


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The Central Processing Unit (CPU)

3 main parts:

CPU


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The I/O system is the section of a PLC to which all of the field devices are

connected.


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Session 2

Input/Output (I/O) Devices


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The Input/Output System

The input/output (I/O) system is the section of a PLC to which allof the field devices are connected. If the CPU can be thought of as the

brains of a PLC, then the I/O system can be thought of as the armsand legs. The I/O system is what actually physically carries out thecontrol commands from the program stored in the PLCs memory.

The I/O system consists of two main parts:

the rack I/O modules

The rackis an enclosure with slots in it that is connected to the CPU.

I/O modules are devices with connection terminals to which the

field devices are wired. Together, the rack and the I/O modules formthe interface between the field devices and the PLC. When set upproperly, each I/O module is both securely wired to its correspondingfield devices and securely installed in a slot in the rack. Thiscreates the physical connection between the field equipment and thePLC. In some small PLCs, the rack and the I/O modules come prepackaged as one unit.


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I/O or field devices can mainly categorized as:

They wait for a signal/data from thePLC to perform their control functions.

Examples include Lights, horns,

motors, and valves These devices stay

put until the PLC

says, You need to turn on now orYoud better open up your

valve a little more,

They supply a signal or datato the PLC. Examples are:

Switches, Push Buttons,

sensors, etc

Input device tells the PLC,

Hey, something is happeningOut hereyou need to check

this out to

see how it affects the controlprogram.


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Example on Input/Output Devices

An overhead light fixture and its corresponding wall switch are good examplesof everyday inputs and outputs. The wall switch is an input-it provides a signal for the light to turn on. The overhead light is an output it waitsuntil the switch sends a signal before it turns on.Let us pretend that you have a souped-up overhead light/switch circuitthat contains a PLC. In this situation, both the switch and the light willbe wired to the PLC instead of to each other. Thus, when you turn on

the switch, the switch will send its turn on signal to the PLC instead of to thelight. The PLC will then relay this signal to the light, which willthen turn on.


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Types of Input/Output Devices

Digital (discrete)

These are inputs and outputs

that have only two states: ON

and OFF, i.e. 1 or 0A 1 means that the device is

on and a 0 means that the

device is off.

As a result, they send/receive

simple signals to/from a PLC.

Analog

Are inputs and

outputs that

can have an

infinite number of states.

These devices can not only be on and off, butthey can also be barely on, almost totally on,

not quite off, etc. These devices send/receivecomplex signals to/from a PLC. Theircommunications consist of a variety ofsignals, not just 1s and 0s.


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Types of Input/Output Devices


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EXAMPLEThe overhead light and switch we just discussed are both examples

of discrete devices. The switch can only be either totally on or

totally off at any given time. The same is true for the light.

A thermometer and a control valve are examples of the other typeof I/O devices-analog. A thermometer is an analog input devicebecause it provides data that can have an infinite number of states.

Temperature is not just hot or cold. It can have a variety of states,including warm, cool, moderate, etc. A control valve is an analog

output for the same reason. It can be totally on or totally off,but it can also have an infinite number of settings between

these two states.


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TerminologyThe language of PLCs consists of a commonly used setof terms; many of which are unique to PLCs. In order to

understand the ideas and concepts of PLCs, an understandingof these terms is necessary.

Sensor

A sensor is a device that converts a physical condition into anelectrical signal for use by the PLC. Sensors are connected tothe input of a PLC. A pushbutton is one example of a sensorthat is connected to the PLC input. An electrical signal is sentfrom the pushbutton to the PLC indicating the condition (open/

closed) of the pushbutton contacts.


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Actuators

Actuators convert an electrical signal from the PLC into a physicalcondition. Actuators are connected to the PLC output.

A motor starter is one example of an actuator that is connected to

the PLC output. Depending on the output PLC signal the motorstarter will either start or stop the motor


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Session 3

Input/Output (I/O) signals


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More detailed for (I/P & O/P)

I/PDiscrete InputA discrete input, also referred to as a digital input, is an inputthat is either in an ON or OFF condition. Pushbuttons, toggleswitches, limit switches, proximity switches, and contactclosures are examples of discrete sensors which are connectedto the PLCs discrete or digital inputs. In the ON condition a

discrete input may be referred to as a logic 1 or a logic high. Inthe OFF condition a discrete input may be referred to as a logic0 or a logic low.


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A Normally Open (NO) pushbutton is used in the followingexample. One side of the pushbutton is connected to the firstPLC input. The other side of the pushbutton is connected to an

internal 24 VDC power supply. Many PLCs require a separatepower supply to power the inputs. In the open state, no voltageis present at the PLC input. This is the OFF condition. When thepushbutton is depressed, 24 VDC is applied to the PLC input.This is the ON condition.


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Analog InputsAn analog input is an input signal that has a continuous signal.

Typical analog inputs may vary from 0 to 20 milliamps, 4 to 20

milliamps, or 0 to 10 volts. In the following example, a leveltransmitter monitors the level of liquid in a tank. Depending onthe level transmitter, the signal to the PLC can either increase ordecrease as the level increases or decreases.


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O/P

Discrete Outputs

A discrete output is an output that is either in an ON or OFFcondition. Solenoids, contactor coils, and lamps are examplesof actuator devices connected to discrete outputs. Discreteoutputs may also be referred to as digital outputs. In thefollowing example, a lamp can be turned on or off by the PLC

output it is connected to.


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Analog OutputsAn analog output is an output signal that has a continuoussignal. The output may be as simple as a 0-10 VDC level that

drives an analog meter. Examples of analog meter outputs arespeed, weight, and temperature. The output signal may alsobe used on more complex applications such as a current-to pneumatic transducer thatcontrols an air-operated flow-controlvalve.


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SESSION 4

Programming

A plc


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The control program is made up of things calledinstructions. Instructions are, in essence, little computer codes that make the inputs andoutputs do what you want in order to get the result you need. There are all different kindsof instructions and they can make a PLC do just about anything (add and subtract data, timeand count events, compare information, etc.). All you have to do is program the

instructions in the proper order and make sure that they are telling the right devices what todo and voila!you have a PLC-controlled system. And remember, changing the system is asnap. If you want the system to act differently, just change the instructions in the controlprogram.

Different PLCs offer different kinds of instructions. Thats part of whatmakes each type of PLC unique. However, all PLCs use two basic

types of instructions: contacts coilsContacts are instructions that refer to the input conditions to thecontrol programthat is, to the information supplied by the inputfield devices. Each contact in the control program monitors a certain

field device. The contact waits for the input to do something in particular(e.g., turn on, turn off, etc.this all depends on what type ofcontact it is). Then, the contact tells the PLCs control program, Theinput device just did what its supposed to do. Youd better check tosee if this is supposed to affect any of the output devices.


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Coils are instructions that refer to the outputs of the control programthat is,to what each particular output device is supposed to do in the system. Like acontact, each coil also monitors a certain field device. However, unlike acontact, which monitors the field device and then tells the PLC what to do, acoil monitors the PLC control program and then tells the field device what todo. It tells the output device, Hey, the PLC just told me that the switch turnedon. That means that youre supposed to turn on now. So lets go!ExampleLet us talk again about that souped-up switching circuit, in which a wall switch

and an overhead light are connected to a PLC. Let us say that turning on theswitch is supposed to turn on the light. In this situation, the PLCs controlprogram would contain a contact that examines the input device -the wallswitch-for an on condition and a coil that references the light.When the switch turns on, the contact will

energize meaning that it will tell the PLC that the condition its been lookingfor has happened. The PLC will relay this information to the coil instruction byenergizing it. This will let the coil know that it needs to tell its referencedoutput-the light-to turn on.


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The Control Program

All PLCs use two basic instructions:

1. Contacts 2. Coils


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A PLC Scan Cycle

In PLC talk, this three-step process of monitoring the inputs, executingthe PLC control program, and changing the status of theoutputs accordingly is called the scan.


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Programming

A program consists of one or more instructions that accomplisha task. Programming a PLC is simply constructing a set ofinstructions. There are several ways to look at a program suchas ladder logic, statement lists, or function block diagrams.Ladder Logic Ladder logic (LAD) is one programming language used

with PLCs. Ladder logic uses components that resembleelements used in a line diagram format to describe hard-wiredcontrol.


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Ladder Logic Diagram

The left vertical line of a ladder logic diagram represents the

power or energized conductor. The output element or instructionrepresents the neutral or return path of the circuit. The rightvertical line, which represents the return path on a hard-wiredcontrol line diagram, is omitted. Ladder logic diagrams are readfrom left-to-right, top-to-bottom. Rungs are sometimes referred

to as networks. A network may have several control elements,but only one output coil.


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In the example program shown example I0.0, I0.1 and Q0.0represent the first instruction combination. If inputs I0.0 andI0.1 are energized, output relay Q0.0 energizes. The inputs couldbe switches, pushbuttons, or contact closures. I0.4, I0.5, and

Q1.1 represent the second instruction combination. If eitherinput I0.4 or I0.5 are energized, output relay Q0.1 energizes.


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Statement listA statement list (STL) provides another view of a set ofinstructions. The operation, what is to be done, is shown on theleft. The operand, the item to be operated on by the operation,is shown on the right. A comparison between the statementlist shown below, and the ladder logic shown on the previouspage, reveals a similar structure. The set of instructions in this

statement list perform the same task as the ladder diagram.


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Function Block Diagrams

Function Block Diagrams (FBD) provide another view of a set ofinstructions. Each function has a name to designate its specifictask. Functions are indicated by a rectangle. Inputs are shownon the left-hand side of the rectangle and outputs are shown onthe right-hand side. The function block diagram shown below

performs the same function as shown by the ladder diagramand statement list.


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Introduction to PLC Programming

1. System Block Diagram

2. Basic Components and Their Symbols3. Ladder Diagram Fundamentals

4. Applications

PLC Block Diagram


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Basic Components and Their Symbols

Head Push Button Switches

Basic Components


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Limit Switches (LS)

Limit switches can be mechanical or light activated switches

Examples: limit switches on the refrigerator door that turns

ON the inside or to open doors in supermarkets

Basic Components

Basic Components


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Basic Components

Relays or Contactors Electromagnetic devices)

Basic Components


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Push Button (PB) Switches

Normally Open

(NO or N/O)Normally Closed(NC or N/C)

One PB withboth

NO and NCcontacts

Basic Components

Basic Components


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Basic Components

Relays Symbols

Basic Components


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Basic Components

CR

control relay

(internal relay or software relay)

When coil CR1 is energized,

all the N/O CR1 contacts will

be closed and all the N/C CR1

contacts will be open.

Likewise, if coil CR1 is de-energized, all the N/O CR1 contacts

will be open and all the N/C CR1 contacts will be closed.

A contact labeled CR indicates that it is associated with a relay coil.

Each relay will have a specific number associated with it. The

range of numbers used will depend upon the number of relays in

the system.


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EXAMPLES

SESSION 5


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Example: AND Circuit


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Example: AND/OR Circuit

The left vertical line of a ladderlogic diagram represents the

power or energized conductor.The output element orinstruction represents the neutralor return path of the circuit.

Example: Two Rungs LD


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Example: Two Rungs LD

AC M t St t (C t t )


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AC Motor Starter (Contactor)

AC motor starters

are used to allowcontrol circuitry (low

voltage/low current)

to control high

current high voltage

power circuit

AC Motor Starter


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AC Motor Starter

Three heavy-duty N/O main contactors, one light-dutyN/C auxiliary contactors, and one light-duty N/C

overload contactor

AC Motor Starter (Contd)


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AC Motor Starter (Contd)

Terminals are labeled

Designers refer to theselabels on the schematic

diagram

as shown in the figure.

The coil of the contactor all

of the main contacts and

the auxiliary contacts

The overload contactor is independent on

the contactor

coil and only operates under overload

conditions


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Example: AC Motor Start/Stop Circuit


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AC Motor Start/Stop Circuit (Contd)


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p ( )

When the Start pushbutton is depressed the CPU

receives a logic 1 from input I1.

This causes the I1 contact to close. All

three inputs are now a logic 1. The CPU sends a logic 1 to

output [Q1. The motor starter is energized and the motor

starts.


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AC Motor Start/Stop Circuit (Contd)


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When the Start pushbutton is pressed, output [Q1 is now

true and on the next scan, when normally open contactQ1 is solved, the contact will close and output [Q1 will

stay on even if the Start pushbutton has been released.

AC M t St t/St Ci it ( td)


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The motor will continue to run until the Stop pushbutton is

depressed. Input i2 will now be a logic 0 (false).

The CPU will send a binary 0 to output [Q1.

The motor will turn off.

AC Motor Start/Stop Circuit (contd):

AC Motor Start/Stop Circuit (contd):


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AC Motor Start/Stop Circuit (cont d):

When the Stop pushbutton is released i2 logic function

will again be true and the program ready for the next

time the Start pushbutton is pressed.

AC Motor Start/Stop Circuit: Adding


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Indicator lightsfor RUN and

STOP conditions

are included.

RUN indicator

light is connectedto output Q1

STOP indicatorlight is connected

to output Q2.


Light Indicators



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Light Indicators (contd):



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When the PLCstarts the motor

output [Q1 is ON.

The N/O Q1 isclosed and output

[Q1 turns theRUN indicator ON.

The N/C q1contacts is open

and the STOP

indicator lightconnected to

output [Q2 is

OFF.


Light Indicators (contd):



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AC Motor Start/Stop Circuit: AddingLight Indicators (contd):



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Limit Switch

A limit switch could

be used to stop themotor or prevent the

motor from being

started.

If the access door is

open, the normallyopen contacts of LS1

connected to input I3

are open and the

motor will not start.

When the access door is closed, the N/O

contacts on the limit switch (LS1) are closed.

Input I4 is now ON, and the motor will start

when the Start pushbutton is pressed.


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session 6

Timers & counters

Ti


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Timers

Timers are

devicesthat count

increments

of time.

In this example

timers are used to

control the length

of time between

signal changes.


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S7-200 Timers

S7-200 timers are provided with resolutions of 1 millisecond,10 milliseconds, and 100 milliseconds. The maximum value ofthese timers is 32.767 seconds, 327.67 seconds, and 3276.7seconds, respectively. By adding program elements, logic canbe programmed for much greater time intervals.

Hard-Wired Timing Circuit

Timers used with PLCs can be compared to timing circuits usedin hard-wired control line diagrams. In the following example, anormally open (NO) switch (S1) is used with a timer (TR1). Forthis example the timer has been set for 5 seconds. When S1

is closed, TR1 begins timing. When 5 seconds have elapsed,TR1 will close its associated normally open TR1 contacts,


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illuminating pilot light PL1. When S1 is open, de energizing TR1,the TR1 contacts open, immediately extinguishing PL1. This typeof timer is referred to as ON delay. ON delay indicates that once

a timer receives an enable signal, a predetermined amount oftime (set by the timer) must pass before the timers contactschange state.

On-Delay (TON)


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When the On-Delay timer (TON) receives an enable (logic 1) atits input (IN), a predetermined amount of time (preset time - PT)

passes before the timer bit (T-bit) turns on. The T-bit is a logicfunction internal to the timer and is not shown on the symbol.The timer resets to the starting time when the enabling inputgoes to a logic 0.

In the following simple timer example a


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In the following simple timer example, aswitch is connected toinput I0.3, and a light is connected to

output Q0.1.

When the switch is closed input 4 becomes alogic 1, whichis loaded into timer T37. T37 has a time base of100 ms (.100seconds). The preset time (PT) value has beenset to 150. Thisis equivalent to 15 seconds (.100 x 150 ). Thelight will turn on

15 seconds after the input switch is closed. Ifthe switch wereopened before 15 seconds had passed, thenreclosed, the timerwould again begin timing at 0.


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A small sample of the flexibility of PLCs is shown in thefollowing program logic. By reprogramming the T37 contact as

a normally closed contact, the function of the circuit is changedto cause the indicator light to turn off only when the timer timesout. This function change was accomplished without changingor rewiring I/O devices.

Retentive On-Delay (TONR)The Retentive On-Delay timer (TONR)


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functions in a similarmanner to the On-Delay timer (TON). There is one difference.The Retentive On-Delay timer times as long as the enabling

input is on, but does not reset when the input goes off. Thetimer must be reset with a RESET (R) instruction.

The same example used with the On-Delay timer will be


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p yused with the Retentive On-Delay timer. When the switch isclosed at input I0.3, timer T5 (Retentive timer) begins timing.If, for example, after 10 seconds input I0.3 is opened the timer

stops. When input I0.3 is closed the timer will begin timing at10 seconds. The light will turn on 5 seconds after input I0.3has been closed the second time. A RESET (R) instructioncan be added. Here a pushbutton is connected to input I0.2.If after 10 seconds input I0.3 were opened, T5 can be reset by

momentarily closing input I0.2. T5 will be reset to 0 and begintiming from 0 when input I0.3 is closed again.

Off-Delay (TOF)


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The Off-Delay timer is used to delay an output off for a fixedperiod of time after the input turns off. When the enabling bitturns on the timer bit turns on immediately and the value is set

to 0. When the input turns off, the timer counts until the presettime has elapsed before the timer bit turns off.

S7-200 Timers


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The S7-200s have 256 timers. The specific T number chosen

for the timer determines its time base and whether it is TON,TONR, or TOF.

Timer Example


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Timer Example

In the following example a tank will be filled with twochemicals, mixed, and then drained. When the Start Button ispressed at input I0.0, the program starts pump 1 controlled byoutput Q0.0. Pump 1 runs for 5 seconds, filling the tank withthe first chemical, then shuts off. The program then starts pump2, controlled by output Q0.1. Pump 2 runs for 3 seconds fillingthe tank with the second chemical. After 3 seconds pump 2shuts off. The program starts the mixer motor, connected to

output Q0.2 and mixes the two chemicals for 60 seconds. Theprogram then opens the drain valve controlled by output Q0.3,and starts pump 3 controlled by output Q0.4. Pump 3 shuts offafter 8 seconds and the process stops. A manual Stop switch isalso provided at input I0.1.


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Counters are represented by boxes in ladder logic. Countersincrement/decrement one count each time the input transitions

from off (logic 0) to on (logic 1). The counters are reset whena RESET instruction is executed. S7-200 uses three types ofcounters: up counter (CTU), down counter (CTD), and up/downcounter (CTUD).

S7-200 Counters


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S7 00 Cou e sThere are 256 counters in the S7-200, numbered C0 throughC255. The same number cannot be assigned to more thanone counter. For example, if an up counter is assigned number45, a down counter cannot also be assigned number 45. The

maximum count value of a counter is 32,767.Up CounterThe up counter counts up from a current value to a preset value(PV). Input CU is the count input. Each time CU transitions froma logic 0 to a logic 1 the counter increments by a count of 1.

Input R is the reset. A preset count value is stored in PV input.If the current count is equal to or greater than the preset valuestored in PV, the output bit (Q) turns on (not shown).


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Down CounterThe down counter counts down from the presetvalue (PV) eachtime CD transitions from a logic 0 to a logic 1. When the current

value is equal to zero the counter output bit (Q) turns on (notshown). The counter resets and loads the current value with thepreset value (PV) when the load input (LD) is enabled.

Up/Down CounterThe up/down counter counts up or down fromth t l


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the preset valueeach time either CD or CU transitions from a logic 0 to a logic 1.

When the current value is equal to the preset value, the outputQU turns on. When the current value (CV) is equal to zero, theoutput QD turns on. The counter loads the current value (CV)

with the preset value (PV) when the load input (LD) is enabled.Similarly, the counter resets and loads the current value (CV)

with zero when the reset (R) is enabled. The counter stopscounting when it reaches preset or zero.

Counter ExampleA counter might be used to keep track of the number


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of vehicles in a parking lot. As vehicles enter the lot through anentrance gate, the counter counts up. As vehicles exit the lot

through an exit gate, the counter counts down. When the lot isfull a sign at the entrance gate turns on indicating the lot is full.


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As cars leave the lot the exitgate opens. Input I0.1 transitions from a logic 0 to a logic 1,

decrementing the count by 1. When the count has reached 150output Q0.1 transitions from a logic 0 to a logic 1. The ParkingLot Full sign illuminates. When a car exits, decrementing thecount to 149, the sign turns off.


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Session 7Logic Symbols, Truth Tables,

and Equivalent Ladder/PLC

Logic Diagrams

EQUIVALENT LADDER/LOGIC DIAGRAMS


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Electrical Relay Diagram

and

PID Symbols

www.industrialtext.com 1-800-752-8398

ELECTRICAL RELAY DIAGRAM SYMBOLS


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PID SYMBOLS


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INSTRUMENT IDENTIFICATION LETTERING


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session8

Number Systems

Number Systems


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Since a PLC is a computer, it stores information in the form ofOn or Off conditions (1 or 0), referred to as binary digits (bits).Sometimes binary digits are used individually and sometimesthey are used to represent numerical values.Decimal SystemVarious number systems are used by PLCs. All

number systemshave the same three characteristics: digits, base, weight. Thedecimal system, which is commonly used in everyday life, hasthe following characteristics:

Ten digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9Base 10

Weights 1, 10, 100, 1000, ...

Binary SystemThe binary system is used by programmable controllers. The


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binary system has the following characteristics:Two digits 0, 1Base 2Weights Powers of base 2 (1, 2, 4, 8, 16, ...)In the binary system 1s and 0s are arranged into columns. Eachcolumn is weighted. The first column has a binary weight of20. This is equivalent to a decimal 1. This is referred to as theleast significant bit. The binary weight is doubled with each

succeeding column. The next column, for example, has a weightof 21, which is equivalent to a decimal 2. The decimal value isdoubled in each successive column. The number in the far lefthand column is referred to as the most significant bit. In thisexample, the most significant bit has a binary weight of 27. This

is equivalent to a decimal 128.

Converting BinaryThe following steps can be used to interpret adecimal


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d cto Decimal number from a binary value.1) Search from least to most significant bit for 1s.

2) Write down the decimal representation of each columncontaining a 1.3) Add the column values.In the following example, the fourth and fifth columns from the

right contain a 1. The decimal value of the fourth column fromthe right is 8, and the decimal value of the fifth column fromthe right is 16. The decimal equivalent of this binary number is24. The sum of all the weighted columns that contain a 1 is thedecimal number that the PLC has stored.

In the following example the fourth and sixth columns from the


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g pright contain a 1. The decimal value of the fourth column fromthe right is 8, and the decimal value of the sixth column from

the right is 32. The decimal equivalent of this binary number is40.

Bits, Bytes, and Words Each binary piece of data is a bit. Eightbits make up one byte.

Two bytes, or 16 bits, make up one word.

Logic 0, Logic 1 Programmable controllers can only understand ai l th t


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signal thatis On or Off (present or not present). The binary system is a

system in which there are only two numbers, 1 and 0. Binary 1indicates that a signal is present, or the switch is On. Binary 0indicates that the signal is not present, or the switch is Off.

BCD

Binary-Coded Decimal (BCD) are decimal numbers where


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each digit is represented by a four-bit binary number. BCD iscommonly used with input and output devices. A thumbwheel

switch is one example of an input device that uses BCD. Thebinary numbers are broken into groups of four bits, each grouprepresenting a decimal equivalent. A four-digit thumbwheelswitch, like the one shown here, would control 16 (4 x 4) PLC

inputs.

Hexadecimal

Hexadecimal is another system used in PLCs The hexadecimal


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Hexadecimal is another system used in PLCs. The hexadecimalsystem has the following characteristics:

16 digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, FBase 16Weights Powers of base 16 (1, 16, 256, 4096 ...)The ten digits of the decimal system are used for the first tendigits of the hexadecimal system. The first six letters of the

alphabet are used for the remaining six digits.A = 10 D = 13B = 11 E = 14C = 12 F = 15

The hexadecimal system is used in PLCs because it allows thestatus of a large number of binary bits to be represented in asmall space such as on a computer screen or programming


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small space such as on a computer screen or programmingdevice display. Each hexadecimal digit represents the exactstatus of four binary bits. To convert a decimal number to a

hexadecimal number the decimal number is divided by the baseof 16. To convert decimal 28, for example, to hexadecimal:

Decimal 28 divided by 16 is 1 with a remainder of 12. Twelve isequivalent to C in hexadecimal. The hexadecimal equivalent ofdecimal 28 is 1C.The decimal value of a hexadecimal number is obtained bymultiplying the individual hexadecimal digits bythe base 16 weight and then adding the results.

In the following examplethe hexadecimal number 2B is convertedto its decimal equivalent of 43.


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Conversion of Numbers

The following chart shows a few numeric values indecimal,binary, BCD, and hexadecimal representation.


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Adviser :

D YEHIA EL MASHAD


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Dr. YEHIA EL MASHAD

prepared by:

ABDALLA MOKTAR ABD EL MONSER

HASAN ADEL HASAN

HODA HOSNY ABBAS

SHAIMAA MOHAMMED BARAKAT

SHAIMAA TALAAT

fundamentals of plc final

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