Programmable logic controller questions

MODULE TITLE: PROGRAMMABLE LOGIC CONTROLLERS

TOPIC TITLE: LADDER DIAGRAM PROGRAMMING

LESSON 2: PLC VERSIONS OF LADDER DIAGRAMS

PLC - 4 - 2

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Published by Teesside University Open Learning (Engineering)

School of Science & Engineering

Teesside University

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________________________________________________________________________________________

INTRODUCTION ________________________________________________________________________________________

This is the second lesson in the topic of ladder diagrams and it deals with

various aspects related to their use with PLCs.

PLC ladder diagrams will essentially become computer programs and, as such,

are used and behave in different ways from their hard-wired equivalents.

Some of these differences are highlighted in this lesson together with the

symbols used and the importance of a suitable identification system.

The lesson also introduces internal memory relays which are common to all

PLCs. These and other circuit elements are used in practice exercises to

provide examples of ladder diagrams.

The logic theme is continued from Lesson 1 and is developed so that rungs of a

ladder may be presented in statement form instead of diagram form. Such

'Boolean' statements are used as a programming method in some PLCs.

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________________________________________________________________________________________

YOUR AIMS ________________________________________________________________________________________

Upon completion of this lesson you should be able to:

• identify and state the functions of symbols used in PLC ladder

diagrams

• specify the differences between simple hard-wired ladder diagrams

and PLC ladder diagrams

• convert from a hard-wired ladder diagram to a PLC ladder diagram

using a suitable I/O identification system

• design PLC ladder diagrams from statements of desired operation

• appreciate the differences between an internal memory relay and a

physical output relay

• interpret the operation of a PLC ladder diagram

• provide, use and interpret Boolean statements

• simplify from a 'first attempt' a PLC ladder diagram.

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________________________________________________________________________________________

PLC LADDER DIAGRAMS ________________________________________________________________________________________

In the previous lesson we examined the basics of ladder diagrams as they are

used in the electrical industry. The next stage, covered by this lesson, is to

convert the ladder diagrams so that they become suitable for the purpose of

programming a PLC. Simple circuit diagram examples are again used.

The formats of PLC and hard-wired ladder diagrams are exactly the same.

Two vertical lines represent the power supply rails across which are placed the

rungs. The load must be sited at the end of each rung immediately before the

right hand (or negative) rail. The circuit switching components, from now on

called rung elements, connect between the left hand (or positive) rail and the

rung load.

When ladder diagrams were drawn in the previous lesson they were

representative of those used in industry. Such diagrams use symbols which are

similar to circuit diagram symbols found in the British Standards

Specifications (BS 3939). The first problem, for programming purposes, is

that the BS symbols are too numerous and varied to be used for PLCs and so,

to reduce this variety, the same symbol may be used for more than one

purpose. This essentially reduces the readability by removing some of the

information which would have been present in a hard-wired ladder diagram.

This is unfortunate but cannot be avoided. However, some of the lost

information may be replaced by the identification system being used. To

illustrate this point consider the section of an electrical ladder diagram given as

FIGURE 1.

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EXAMPLE 1

FIG. 1

An electrician would recognise the symbols and would also be able to visualise

the actual components being used.

If this diagram is converted for use with a PLC then it is likely to become as

shown in FIGURE 2.

FIG. 2

The push-to-make switch and the single pole switch both share the same

symbol, that of normally open contacts. The bell now becomes a generalised

symbol for a rung load or rung output.

Clearly information is lost because the generalised symbols of rung elements

are not the same as the specific symbols used in FIGURE 1. Each rung

element must therefore be clearly and accurately identified.

If push 1 and switch 1 are inputs to a PLC and the bell is driven by an output

from the PLC then the I/O identification system must show this. In the lesson

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concerned with I/O identification it was stated that, for our purposes, each

input would be prefixed with X and each output prefixed with Y. The rung of

FIGURE 2 would then become:

FIG. 3

The numbers following the X and Y indicate the terminal number to which the

circuit component is connected.

With the identification shown:

Push 1 would be connected between input terminal 0 and the input

common.

Switch 1 would be connected between input terminal 1 and the input

common.

The bell (with its own supply) would be connected between output

terminal 0 and the output common.

So the ladder diagram indicates which terminals the devices are connected to

but it gives no indication of the type of device being used – hence the need for

input and output designation charts which were covered in a previous lesson.

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________________________________________________________________________________________

PCL LADDER DIAGRAM SYMBOLS AND THEIR MEANING ________________________________________________________________________________________

Due to generalised symbols being used there are very few to learn with the

result that people new to programming find it easy to handle them.

We will now examine some of the symbols used for simple diagrams while

leaving other symbols (for devices not yet considered) to a later lesson.

Symbols can be classified as being:

(a) at the beginning of a rung

(b) in the middle of a rung, or

(c) at the end of a rung.

The table of FIGURE 4 indicates these generalised symbols together with

variations in manufacturers' abbreviations for the function of each.

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FIG. 4

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________________________________________________________________________________________

ADVANTAGES OF PLC LADDER DIAGRAMS ________________________________________________________________________________________

There are specific advantages in using a PLC to simulate a hard-wired system.

This section of the lesson deals with some of them. If you have an electrical

background you will, no doubt, be amused at what can be achieved by

programming.

A PLC ladder diagram may possess elements which are not available as

physical devices. This may seem strange but in actual fact some devices

shown on a diagram may actually be replaced by parts of the microcomputer

memory.

For example, internal relays are imaginary relays which do not exist in the

form of the physical relays described in Topic 2. They are, in fact, memory

cells of the computer.

Additionally, relays may be assumed to possess multiple numbers of contacts

which are electrically isolated from each other even though they do not

actually exist in that form.

Also the same contacts may appear in a diagram more than once, though this is

not the case in the physical circuit.

To illustrate what is meant by the section above we will examine one or two

problems and show how these advantages are really useful.

Lesson 1 in this topic presented a problem under the section headed

'Circuit/Ladder Design'. We shall now repeat this problem with a view to

obtaining a PLC ladder diagram solution.

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EXAMPLE 2

Design a switching circuit, to be laid out in PLC ladder diagram format, which

will operate a relay from suitable combinations of four switches.

The relay must be energised if:

Switch 1 and switch 2 are both closed

OR Switch 3 and switch 4 are both closed

OR Switch 1 and switch 3 are both closed.

We can write this down as:

Relay = (1 AND 2) OR (3 AND 4) OR (1 AND 3)

This problem must be solved by a software solution and not by a hardware

solution.

The four switches would be connected between their respective input terminals

and the input common. The diode which was required for the hardware

solution will not be needed for a software solution. Instead, the ladder diagram

will be drawn as if the input switches had multiple contacts. In fact the

program is said to interrogate the state of the inputs a multiple number of

times.

The ladder diagram for the PLC solution will be built-up in stages.

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First Stage

The relay must operate if switch 1 and switch 2 are closed. The rung for this is

shown as FIGURE 5.

R1 = (1 AND 2)

FIG. 5

Second Stage

The relay must operate if switch 3 and switch 4 are closed. The rung for this is

shown as FIGURE 6.

R1 = (3 AND 4)

FIG. 6

Notice that the same relay is being used for both rungs but it only exists once.

In most PLCs programming these two rungs would cause, what is called, a coil

duplication error. To avoid this error the two rungs should be combined to

drive an output to the common relay. FIGURE 7 shows this solution.

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R1 = (1 AND 2) OR (3 AND 4)

FIG. 7

So far in the solution the diagram looks very much like the hardware solution

of the previous lesson (without the diode).

Third Stage

The relay must operate if switch 1 and switch 3 are closed. The rung for this is

shown in FIGURE 8.

R1 = (1 AND 3)

FIG. 8

What do we need to do with this rung?

________________________________________________________________________________________

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Here again a common relay is being used and so this rung should be combined

with the diagram of FIGURE 7. The overall solution would then become as

shown in FIGURE 9:

R1 = (1 AND 2) OR (3 AND 4) OR (1AND 3)

FIG. 9

Notice that there appear to be two inputs called X001 and two inputs called

X003, when in fact only one switch is used for each. This would obviously be

an impossible hardware solution if single pole switches were used but it is an

acceptable software solution – the reason for this and the operation of the

solution to the problem will become clear in subsequent lessons.

We are now at the stage where you can attempt to provide ladder diagram

solutions to stated problems.

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EXERCISE 1

Consider a situation where the operation of three single pole switches is to be

monitored by the use of six indicator lamps.

The following list provides the monitoring requirements:

Lamp 1 will light up if switch 1 is closed

Lamp 2 will light up if switch 2 is closed

Lamp 3 will light up if switch 3 is closed

Lamp 4 will light up if switch 1 and switch 2 are closed

Lamp 5 will light up if switch 2 and switch 3 are closed

Lamp 6 will light up if switch 3 and switch 1 are closed

If this operation were to be hard-wired then either three triple pole switches or

three relays with three sets of N/O contacts would be needed. If a PLC is to be

used then three single pole switches may be connected between inputs 1, 2 and

3 and the input common. The lamps are to be connected in order from output

0 through to output 5.

Complete the partially drawn diagram of FIGURE 10.

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FIG. 10

The solution to the problem is shown on page 31.

The diagram would have us believe that nine switches are being used when in

fact only three exist. Each of the three switches appears three times in the

diagram. The six rungs are all separate i.e. in parallel across the supply rails.

Now change the diagram of FIGURE 10 so that the monitoring operation

becomes:

Lamp 1 will light up if switch 1 is closed

Lamp 2 will light up if switch 2 is closed

Lamp 3 will light up if switch 3 is closed

Lamp 4 will light up if any two switches are closed

Lamp 5 will light up if all three switches are closed

Lamp 6 is not now used.

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No changes are necessary to the hard-wiring.

FIG. 11

Solution on page 32.

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________________________________________________________________________________________

INTERNAL MEMORY RELAYS ________________________________________________________________________________________

Internal (or memory) relays are not, as was previously stressed, physical relays

or output relays. They are internal memory cells within the microcomputer of

the PLC which are treated as if they were actual relays. They can appear in

ladder diagrams and can be used instead of an output relay when a drive to an

output circuit is not required. The use of internal relays can reduce the size of

programs which may be very important in situations where memory capacity is

limited.

The generalised output symbol is normally used together with a prefix or code

e.g. MR000 (for memory relay 0) or, as in our case, R000 for this same relay.

Consider the case below where a memory relay is used to save memory space

and at the same time improve the readability of a section of a program.

EXAMPLE 3

FIGURE 12 shows a ladder diagram having three rungs. The outputs are all

actual relays which can be used to drive circuit loads.

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FIG. 12

Examination of the diagram shows that there is a degree of commonality in the

rungs, i.e. each rung contains contacts from inputs X000 through to X003. If

these contacts are extracted from the diagram and given a rung of their own to

drive a memory relay then the original three rungs become easier to read. This

is shown by the next diagram.

FIG. 13

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The diagrams of FIGURE 12 and FIGURE 13 will behave in similar ways

when operated by a PLC. The diagram of FIGURE 13 should be easier to

follow. It also requires less programming because fewer elements exist in the

diagram. (There are 18 elements in FIGURE 12 and 14 elements in

FIGURE 13.)

Notice that the diagram of FIGURE 13 shows a set of contacts for memory

relay R000 on each of rungs 2, 3 and 4. If R000 were a physical relay then it

would need to have three sets of isolated contacts but R000 is not an actual

relay (it is, remember, a memory cell behaving as a relay i.e. being turned on

and off).

So far we have established that:

• a ladder diagram may show an input (or a set of relay contacts) more

than once

and

• an imaginary relay (called an internal relay) may be used within a

ladder diagram.

Some PLCs have only a small number of internal relays available to the user

whereas others have hundreds. However many there are, they must all be

identified individually because they are separate and unique memory cells.

The identifying number is often called the relay address.

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EXERCISE 2

In this exercise a hard-wired circuit is to be implemented by the use of a PLC.

It must, therefore, be converted into a PLC ladder diagram for programming

purposes.

Push 1 is connected to input 3

Push 2 is connected to input 5

Push 3 is connected to input 7

Limit switch is connected to input 2

R1 and R2 are to be internal relays (R000 and R001)

Indicator 1 is to be driven by output 6

Indicator 2 is to be driven by output 5

Using the above input and output designations convert the diagram of

FIGURE 14 into a correctly identified PLC ladder diagram.

Do this by completing FIGURE 15 on the next page. A solution is given on

page 33.

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FIG. 14

FIG. 15

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EXAMPLE 4

Now consider the diagram of FIGURE 16. This is a section of a ladder

diagram which uses inputs, outputs and internal memory relays.

We will examine this diagram to ascertain its circuit operation. (Remember

that X means input, R means internal memory relay and Y means a physical

output.)

We will also use this diagram to explain a slightly different way of writing

down an expression to represent each rung of the ladder.

FIG. 16

A convention previously outlined will be used during the circuit examination

i.e. working from the top left of the diagram, going across the rung and

continuing down the page.

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Rung 1

X000 is an input switch which is normally open. If this switch closes then a

current can flow along rung 1, through X000 and through the normally closed

input of X001. This current would switch on the internal relay R000. When

R000 is switched on (i.e. energised) then all N/O contacts with R000 reference

will close. Rung 1 now has the possibility of supplying current to the R000

relay, via X001, by either of two paths – X000 contacts are closed and so too

are the R000 contacts. If the input switch X000 now opens, the supply to

R000 relay will be flowing through its own R000 contacts. This means that

once energised R000 will remain energised unless the flow of current is

interrupted by opening the X001 switch. Relay R000 is said to be self

retaining.

Rung 2

If the R000 relay is energised (as already described) then the R000 normally

open contacts in rung 2 will be closed and will remain closed until the R000

relay is de-energised. For internal relay R001 to become energised the input

switch X002 will need to be closed at the same time as the R000 contacts are

closed. If this happens, R001 will energise and all contacts carrying that

reference will operate. If, at any time, X002 opens or R000 de-energises then

internal relay R001 will de-energise.

Rung 3

Whenever internal relay R001 is energised the R001 contacts in rung 3 will

close with the consequence that output Y000 will be driven on.

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The overall operation of the circuit is such that the output Y000 can only be on

if X002 input is closed and X001 is closed and has been ever since X000 was

last closed.

The three rungs of this ladder diagram may be described in statements. Some

PLCs are programmed by a statement method.

Consider the first rung:

FIG. 17

X000 and R000 are in parallel. Either X000 OR R000 could close to allow a

current through, therefore these two contacts would be stated as:

(X000 OR R000)

These contacts are in series with the normally closed input of X001 and series

connections are defined by the AND expression. In this case if X001 has NOT

been operated then current can flow to the output of the rung. Therefore, the

X001 contact is stated as:

AND NOT X001

This is written as AND X001

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The total expression for rung 1 then becomes

expressed as:

Note that:

the + sign stands for logical OR

the . sign stands for logical AND

the —— (bar) means the inverse or logical NOT

Logic expressions written down in this way are called Boolean Algebra

statements.

Now you write down the Boolean statements for rung 2 and rung 3.

Rung 2 .....................................................................................................................................

Rung 3 .....................................................................................................................................

________________________________________________________________________________________

Check your answers against those given on page 33.

X000 R000 X001 R.+( ) = 000

X000 OR R000 AND X001 R( ) = 000

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EXERCISE 3

Now draw a rung of a ladder to satisfy the following Boolean expression.

FIG. 18

This diagram is shown as FIGURE 22 on page 27 but do not check yours

against that one just yet.

Examine the rungs that you have drawn as FIGURE 18.

There are many possible paths for a current to flow along from the positive

supply rail to energise the relay.

How many different paths can you find?

________________________________________________________________________________________

Your answer should be six.

Y006 X001 X002 R000 X005 X003 R001. .= + +( ) +( )

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Five of these six paths are specified by logic statement and are shown below.

Check these against the paths you can find and then complete the sixth

statement.

1.

2.

3.

4.

5.

6.

Answer given on page 33.

These six statements can be drawn out as six paths feeding the Y006 common

output. Complete the diagram of FIGURE 19 by drawing in each of the paths.

X001 AND X005 AND R001 Y006

X001 AND X003 AND R001 Y

=

= 0006

X002 AND X005 AND R001 Y006

X002 AND X003 AND R0

=

001 Y006

R000 AND X005 AND R001 Y006

Y006

=

=

=

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FIG. 19

Now re-examine your diagram of FIGURE 19. This diagram could represent a

'first attempt' ladder diagram which can be improved by being simplified. To

do this we examine each rung searching for common elements i.e. the same

numbered element appearing in every rung.

If any are found then they are extracted and are placed in the rung alongside

the common output element. In the diagram of FIGURE 19 R001 is common

to all rungs and so it is extracted.

The diagram is now simplified to become FIGURE 20.

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FIG. 20

The diagram can now be further simplified by extraction of elements which

occur in some, but not all, parts of a rung.

X005 occurs in three parts and occurs in three different parts. These

spearate occurrences can be collected and placed in a further simplified

drawing, shown as FIGURE 21.

X003

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FIG. 21

Notice that the diagram is being simplified by working away from the output

towards the supply rail.

The last remaining common elements are in the form of two sets of (X001 OR

X002 OR R000). These can be combined to form the final, fully simplified,

version of the six paths which we started with. FIGURE 22 shows the finished

diagram.

Check this diagram against your diagram of FIGURE 18 – they should be the

same. This is not surprising because we have really worked away from this

diagram and then worked back to it. In practice the unsimplified version

would provide the starting point.

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FIG. 22

Simplifying a ladder diagram provides a diagram with less elements and this

allows the human eye to take in the information more easily i.e. the diagram

becomes easier to read. Further to this, any reduction in the number of

elements reduces the program size when keying-in this diagram to become a

PLC program. Less time is needed at the programming stage and less

computer memory is needed to hold this simplified version.

Now attempt the Self-Assessment Questions on page 34.

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________________________________________________________________________________________

SOLUTIONS TO QUESTIONS IN LESSON ________________________________________________________________________________________

EXERCISE 1

FIG. 23

(This should match your FIGURE 10.)

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FIG. 24

(This should match your FIGURE 11.)

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EXERCISE 2

FIG. 25

(This should match your FIGURE 15)

BOOLEAN STATEMENTS FROM PAGE 21.

Rung 2. (X002 . R000) = R001

Rung 3. R001 = Y000

EXERCISE 3

6. R000 AND X003 AND R001 Y006=

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________________________________________________________________________________________

SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________

1. Convert the hard-wired diagram of FIGURE 26 into a PLC ladder

diagram using suitable I/O identification if:

P1 is connected to input terminal 6

P2 is connected to input terminal 3

P3 is connected to input terminal 2

P4 is connected to input terminal 1

the relay is connected to output terminal 5.

FIG. 26

2. Write down a Boolean statement which expresses the operation of the

PLC version of the rung of Question 1.

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3. A relay is to be energised if:

Switch 1 AND switch 2 are closed

OR Switch 3 AND switch 4 are closed

OR Switch 4 AND switch 2 are closed.

Draw a PLC ladder diagram for these switching sequences if the switch

numbers are matched to the input terminal numbers.

4. State the function of a physical output relay which cannot be achieved by

an internal memory relay.

5. The operation of an output relay is given by the following expression:

Draw out this expression as a ladder diagram in its simplest form.

From this diagram produce another expression (equivalent to that above),

which describes the simplified version of the ladder.

Y007 X001 X002 R000 R002 R002 X001 X004 R. . . . . .= ( ) + 0000

X003 R003 R002 X002 R003 X003 R002. . . . .

( ) + ( ) + ..X006( )

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________________________________________________________________________________________

ANSWERS TO SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________

1. FIGURE 27 is the solution in ladder diagram form.

FIG. 27

2. Boolean expression for the rung of FIGURE 27

Y005 X006 Y005 X001 X003 X002= + +( ). .

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3. The diagram of FIGURE 28 provides the solution. Note that the

relay/output number was not specified (any number may be used).

FIG. 28

4. Internal memory relays behave as if they were physical output relays in

every respect but one. They are, in reality, memory cells within the

microcomputer and, as such, are not made available to the user in the

form of a switched pair of terminals to which external loads may be

connected. In short, only physical output relays can drive external loads,

internal memory relays cannot.

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5. The diagram of FIGURE 29 is the simplified version of the expression for

Y007.

FIG. 29

To prove to yourself that the simplification diagram will behave in the

same way as the original, trace out through the diagram all of the original

paths from the supply line to the load. Each path should be contained

within the simplified version. In this case four paths were indicated by

the original expression.

The expression which is the same as the original can now be written as:

Y007 R002 R000 X001 X002 X004

R003 X0

. . .

.

= ( ) +( )( ) + 003 X002 X006.( ) +( )( )

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________________________________________________________________________________________

SUMMARY ________________________________________________________________________________________

The lesson started with the foundations of PLC ladder diagrams; the symbols

in common use, the likely loss of and possible retrieval of some of the

diagrammatic information when converting from hard-wired ladder to PLC

ladder.

Simple diagrams involving switching sequences were used to show some of

the methods of design and to stress the advantages gained by using PLCs

instead of hard wiring.

The differences between internal and external relays were specified, together

with their advantages. Both may be called relays but a clear distinction must

be made between them because confusion can arise.

Boolean statements were introduced as an alternative method of expressing the

contents of particular rungs and the interconnections employed between

elements. This method is in itself very valuable because it provides a way of

expressing the actual rung without the need to draw it out. Additionally, some

PLCs may be programmed by this method and so it is wise to become familiar

with it at this stage.

The lesson finished by looking at ladder rung simplification techniques. Such

techniques make the final version of the ladder diagram easier to read and

allow it to be programmed more quickly. It will also take up less memory

space when programmed into the machine.

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