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MODULE TITLE: PROGRAMMABLE LOGIC CONTROLLERS

TOPIC TITLE: LADDER DIAGRAM PROGRAMMING

LESSON 1: INDUSTRIAL VERSIONS OF LADDER DIAGRAMS

PLC - 4 - 1

Published by Teesside University Open Learning (Engineering)

School of Science & Engineering

Teesside University

Tees Valley, UK

TS1 3BA

+44 (0)1642 342740

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________________________________________________________________________________________

INTRODUCTION ________________________________________________________________________________________

In this lesson we shall be looking briefly, in a general manner, at computer

high and low level languages and at some aspects of how they differ.

Subsequently we will see how a circuit diagram can be represented by the use

of a ladder diagram. Ladder diagrams are explained together with the reasons

for their use. Some practice is given in the drawing of ladder diagrams by

following unwritten, but widely used practices. This will provide experience

and a basis for future work on the programming of a PLC.

________________________________________________________________________________________

YOUR AIMS ________________________________________________________________________________________

Upon completion of this lesson you should be able to:

• understand that a microprocessor ultimately requires a program in

machine code in order to operate

• understand the difference between high and low level computer

programming languages

• describe what is meant by a ladder diagram

• explain why ladder diagrams are used

• use standard circuit diagram symbols to represent ladder diagram

components

• interpret the operation of simple ladder diagrams

• draw ladder diagrams from simple circuit diagrams

• construct ladder diagrams to obtain specific circuit behaviour.

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________________________________________________________________________________________

COMPUTER LANGUAGES ________________________________________________________________________________________

GENERAL

A computer language consist of a set of symbols that can be assembled

according to defined rules to form a sequence of source code. This set of

symbols constitutes the language character set or alphabet. Typically, the

character set of a computer language will consist of the letters of the alphabet,

the digits 0 to 9, punctuation marks and selection of mathematical operators

+,–, <, >, =, etc. The symbols are combined to form a source code and, before

a program can be executed, this code must be translated into object or

machine code such that the processor can work with it. This is done by a

translator which takes the source code and processes it or converts it into a

machine code version of its input program. Generally, there are two types of

translators:

• assemblers that have assembly language as their source code

• compilers or interpreters that have a high level language as their source

code.

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LOW LEVEL LANGUAGES – ASSEMBLY LANGUAGE AND MACHINE CODE

The operation of a computer is controlled by a sequence of binary patterns,

usually of one or more bytes fetched from memory. Each pattern defines a

specific operation that the machine can perform and this, as we have seen in an

earlier lesson, is called an op-code (operation code). The complete range of

allowed codes constitutes the machine’s instruction set. These ‘machine code’

instructions directly influence the operation of the machine and are machine

specific. A machine code of one manufacturer’s machine will not drive the

machine of another. Machine code represented in hexadecimal notation for

convenience instead of large groups of binary numbers, supports only limited

arithmetical operations and there are no instructions for such essential

operations as for example ‘read the keyboard’ or ‘display the data’ only ‘input

data from port x’ and ‘output data to port y’. For these, a series of instructions

or a ‘sub routine’ would have to be compiled, containing many machine code

instructions, which would determine at what point a key would be pressed and

possible a time delay subroutine to delay reading for elimination of key

bounce, etc. An output command may require a subroutine to repeat the

display instruction whilst a character remains on a VDU screen and so on.

Now, of course, no serious programming is performed directly in machine

code, rather the program is first written in ‘assembly language’ where each

machine code instruction, such as ‘B8 17 19’ for example, is represented by a

mnemonic ‘MOV AX 1917’. The invention of assembly languages and their

associated translators, called assemblers, took much of the pain out of

programming at machine code level.

The snippet of assembly language program below divides 100 by 50, the result

being stored in a memory location ‘BP’, where ‘BP’ is a cpu register BP.

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Example of Assembly Language

We note that there is a one-to-one mapping of mnemonic instructions to

machine code instructions. This may not seem much of an advantage but we

can see that the assembly language gives us a basic understanding of what each

instruction is doing, providing we understand and know the basic mnemonic

meaning. In this snippet the machine will perform the arithmetical operation

but first we move the data into registers which are accessed by the ALU.

Hence it is necessary (1st assembler instruction) to load or ‘move’ the number

100 into register DI. Then the next instruction tells the machine to move the

number 50 into SI register as preliminary to the arithmetical operation, etc.

The third assembly language instruction tells the machine to move DI into AX,

the accumulator. The machine is then told to divide the accumulator by

HIGH LEVEL LANGUAGES

High Level Languages (HLLs) are generally the means by which we communicate

with computers. HLLs have a style that is akin to our native language and

mathematics, making it comparatively easy for us to formulate our ideas in them.

There are many different high level languages, for example BASIC, PASCAL,

FORTRAN, C++ etc. Describing a language as ‘high level’ is relative, some

people for example would describe ‘C’ as a ‘medium level language’ because of

some of the ‘machine like’ instructions that can be found therein.

In order to process, computers or microprocessors require as we have stated,

machine code instructions, each instruction being a binary pattern of one or

more ‘bytes’. Programming directly into machine code or even in assembly

language is difficult.

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As of the present there is no machine that can directly execute a HLL program;

the source code must first be translated into machine code by another program

to produce the object code with which the computer runs.

CHARACTERISTICS OF HIGH AN LOW LEVEL LANGUAGES

HLLs:

• closer to everyday language and mathematics

• more closely related to the programming task

• more concise

• easier to maintain

• more intelligible

• independent of supporting hardware.

LLL:

• machine orientated

• more economical in code

• faster to execute.

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‘BASIC’ PROGRAMMING LANGUAGE

BASIC is an acronym for Beginners ALL-purpose Symbolic Instruction Code.

The original language was designed in 1963 by John Kemeny and Thomas

Kurtz at Dartmouth College in the US and developed by students under their

direction. BASIC, based partly on versions of FORTRAN and ALGOL, was

designed to:

• be easy for beginners

• be general purpose

• be interactive

• not require computer hardware knowledge

• provide easily understandable error messages.

The designers of the language made the compiler available free, which

contributed largely to the language’s widespread use. During the 1970s

manufacturers adopted it as the language of their computers. Acorn

Computers Ltd developed a version of BASIC for the BBC for use in the BBC

computer and subsequently it become the language of more than 30 other

platforms (a platform describes some sort of framework, either in hardware or

software, which allows software to run). Many versions of BASIC were

evolved but by the latter half of the 1980s its use started to wane, though today

there are still many applications of ‘dialects’ of it.

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Example of Classic BASIC

10 INPUT "What is your name: "; U$

20 PRINT "Hello "; U$

30 REM

40 INPUT "How many stars do you want: "; N

50 S$ = ""

60 FOR I = 1 TO N

70 S$ = S$ + "*"

80 NEXT I

90 PRINT S$

100 REM

110 INPUT "Do you want more stars? "; A$

120 IF LEN(A$) = 0 THEN GOTO 110

130 A$ = LEFT$(A$, 1)

140 IF (A$ = "Y") OR (A$ = "y") THEN GOTO 40

150 PRINT "Goodbye ";

160 FOR I = 1 TO 200

170 PRINT U$; " ";

180 NEXT I

190 PRINT

It can be seen that many steps in the program are self explanatory and much

easier to understand than assembly language. Without knowing anything much

about the language it is possible to work out that the program is asking the

operator’s name then, after asking how many stars the operator wants, prints

the request.

Often a large count or an infinite loop (i.e. a routine of instructions going from

beginning to end continuously), is used to fill the display with a message.

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‘C’ PROGRAMMING LANGUAGE

There are high level languages that have some built in low level programming

facilities. ‘C’ is one such language. It was developed in 1972 by Dennis

Richie and Brian Kernighan at Bell Laboratories in the USA for use with the

‘Unix’ operating system. C began to replace BASIC in the 1970s as the

leading programming language. C is now widely used in commercial

programming applications. It is still being developed with a new revised

standard with new features to be issued in the next few years.

‘C++’ PROGRAMMING LANGUAGE

‘C++’ was developed at Bell Laboratories by B. Stroustrup in 1983 and was

originally called ‘C with Classes’. The ++ indicates a development of the

original ‘C’ programming language. Although ‘C++’ is often regarded as a

refined form of C there are significant differences even though there are large

areas of similarity where each has borrowed or incorporated from the other.

C++ was intended among other things to support many programming styles, to

give the programmer choice and to be compatible with ‘C’ as far as possible.

It is now more widely used than C.

Example of C++

This short program is designed to determine the CPU’s speed.

/ *****************************************

Find the speed of machine’s cpu

******************************************/

float determine_cpu_ speed(void)

float time_taken;

clock_t time, time2;

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print(“\Finding the speed of your processors\n\a”);

time1=clock();

asm{

push ax

push bx

move bx,COUNT

xor ax,ax

}

loopl:

asm{

dec ax

jnz loopl

dec bx

jnz loopl

pop bx

pop ax

}

time2= clock();

time_taken=(time2-time1)/CLK_TCK;

return(time_taken/COUNT*FFFF00;

}

The program demonstrates the use of embedded assembly language to directly

control the cpu’s registers and memory stack.

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VISUAL PROGAMMING LANGUAGE (VPL)

A visual programming languge is any programming language that allows the

user to program using graphical program elements rather than having to

specify elements by text. VPLs can be classified, according to the extent and

type of visual elements used, as icon based, form based or diagram based

types. Popular types of simulation software, where electrical circuit icons are

selected to compose electrical circuits, which can then be analysed and tested

for different types of input stimuli, are available. One widespread VPL

language is Ladder Logic, a programming language that simulates relay logic

commonly used in PLCs.

PROGRAMMING PLCs

Generally speaking, four methods of programming PLCs are widely employed:

• ladder diagram

• logic statement

• flow chart

• high level language.

A system may have its electrical control circuit represented by any one of these

four methods but a PLC will normally only accept its programming by the use

of one. Very few machines can be programmed by using more than one

method. This is because the programming unit is itself a complicated piece of

circuitry and making it capable of accepting more than one method would

greatly add to its cost. The user who programs and operates different PLCs

needs to know the ins and outs of the method being employed for each

machine. Details of the programming language must be obtained from the

manufacturer ’s manual because, although a particular method may be

specified, each model may be programmed differently.

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In saying this, models within a range from one manufacturer are normally

taken to have their programming upwardly compatible. This means that a

program which runs on a smaller model would be expected to run on a larger

model from the same manufacturer. This is not always the case and certainly

downward compatibility is never automatically assumed.

In this and succeeding lessons we will be concentrating on only one of the four

methods considered to be in widespread use, i.e. the ladder diagram

programming method, to construct ‘ladder diagrams’ and eventually to

program a PLC.

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________________________________________________________________________________________

LADDER DIAGRAMS ________________________________________________________________________________________

BACKGROUND

Electrical circuit diagrams can be fairly complicated not least because symbols

are used to represent actual devices which are interconnected with cables.

Interpretation of the diagram necessitates recognition of the symbols together

with an understanding of how the circuitry will behave when interconnected in

that particular way. The problem of interpretation of the circuit and its

behaviour is compounded by a circuit's complexity or sheer size. It is a

straightforward job for an electrician to handle circuits with relatively few

components but when tens, twenties or even hundreds may be involved it

becomes that much more difficult to follow the operation.

In the earlier days of control circuits the actual circuit diagrams were a mass of

interconnections with conductors crisscrossing over others. When such

circuits were installed the wiring was housed in panels and often needed to be

built by specialists in a factory before being transported to site, accompanied

by a commissioning engineer. Switchgear protection panels, machine control

panels and lift control panels are all examples of this practice.

The circuit diagrams for such panels had to be drawn so that anyone involved

with the system, from the design stage through the installation and

commissioning to subsequent alterations or fault finding, could clearly

understand and follow the operation.

The convention adopted was the ladder diagram format. This format was

accepted as the norm and was learned by everyone involved.

When manufacturers first designed PLCs they had to choose a programming

method which would be acceptable to the people who previously used control

panels.

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Imagine the scene not so many years ago when a firm's representative arrived

at a factory trying to sell a computer controller. This was at a time when

scientists, people at university or the military were the only recognised users of

computers. How could the representative convince prospective users of their

ability to handle and program a device which for most was from science

fiction?

The answer was to make the transition from hard wiring to PLC as painless as

possible by adopting, as a programming method, the very standard which

industry already used. i.e. ladder diagrams.

WHAT IS A LADDER DIAGRAM?

The term 'ladder diagram' comes from the format of the diagram itself, i.e. it

resembles a step ladder, having two uprights with rungs spaced at regular

intervals running across from one upright to the other. The uprights represent

the power supply rails and the rungs contain the circuit components.

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________________________________________________________________________________________

CONVERTING FROM CIRCUIT DIAGRAMS TO LADDER DIAGRAMS ________________________________________________________________________________________

We will begin by considering simple circuit arrangements before progressing

to slightly more involved circuitry.

Example 1

The first circuit is a bell circuit. FIGURE 1 shows a circuit diagram. The

circuit has three components: a bell, a push-to-make switch and a battery (in

symbol form) interconnected with conductors.

For the bell to ring, the circuit must be completed by having the bell push

pressed. When complete, a current flows in the circuit from the battery

positive terminal. The flow path is from the battery positive, through the push

switch, through the bell and returning to the negative terminal. The current,

flowing through the bell, should make it sound.

FIG. 1

–+

Bell Push To Make

Switch

Battery Supply

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The circuit diagram shown in FIGURE 1 must now be redrawn in ladder

diagram format. The supply is placed either at the very top or the very bottom

of the diagram. It makes no difference which.

FIG. 2

Notice that although the diagram has taken-on a new shape the circuit is

exactly the same. When the push is pressed the circuit current flow is in the

same direction and flows through the components in the same order as the

circuit diagram of FIGURE 1. Each rung of the diagram can be identified by

number if required.

Example 2

Now let us consider another bell circuit. FIGURE 3 shows a bell with

indicator circuit. The circuit has two bell pushes, each capable of ringing the

bell. An indicator is placed in series with each bell push in order to indicate

which push has been operated.

–+

+ –

Rung

Battery

BellPush

Positive Upright

Negative Upright

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FIG. 3

The diagram is shown having push 1 closed to indicate the flow path of the

current in this part of the circuit. Trace the current around the circuit by

following the arrows. Notice that because indicator 1 is in series with push 1

the current flows through the indicator on its way to the bell.

The diagram of FIGURE 3 is reproduced overleaf as FIGURE 4. In this

diagram push 2 is shown closed. Complete this diagram by marking on the

flow path of the current with arrows. Start from the battery positive terminal.

–+

1 2

Indicators

Pushes

Battery Current

Flow

Bell

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FIG. 4

Convert the circuit diagram of FIGURE 4 into a ladder diagram by completing

FIGURE 5.

FIG. 5

The completed diagram of FIGURE 5 is given overleaf as FIGURE 6.

–+

+ –

Rung

–+

1 2

Battery

Bell

Closed

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FIG. 6

Check that the diagram of FIGURE 6 is the same as your completed diagram

of FIGURE 5.

Verify that the circuit of FIGURE 3 is the same as the ladder diagram of

FIGURE 6 by tracing out the current flow paths of each.

Every circuit diagram and, therefore, every ladder diagram requires a supply.

Since we know that a supply must be present we will omit it from the diagrams

in future. The value of supply voltage may be specified at the side of an

upright.

In the two examples used so far the current has been shown to flow from the

left upright to the right upright through the circuit components. This is the

proving direction of each rung, i.e. the direction of current flow from supply

positive to supply negative.

Special points of interest concerning FIGURE 6 are:

• the two bell pushes are independent of each other. The operation of

one in no way affects the other

2 2

–+

1 1

Bell

+ –

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• neither bell push has precedence

• each rung can be identified

• each circuit component is identified.

FIGURE 7 is given as an example of a ladder diagram proforma sheet. If

sheets such as these are provided as standard then there is a good chance of

maintaining good documentation. Keeping the paperwork up to date allows

for easier fault finding should it become necessary.

FIG. 7

Machine Name: Machine Number Panel Number

Drawn By Wired By Checked By

Date Date Date

Diagram Page Number Is There A Follow-on Page? Yes No

Rung Reference 0

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We will now proceed to deal with circuit variations. The main objective is to

provide you with practice in constructing ladder diagrams whilst highlighting

important points.

Example 3

FIGURE 8 shows a circuit diagram similar to that used in Topic 1. Inspect the

diagram to familiarise yourself with its operation.

FIG. 8

Use the space opposite to convert the circuit diagram of FIGURE 8 into a

ladder diagram. (Part of the diagram is started for you.)

SW1 SW2 SW3

R1Relay

Battery

R1/1

N/O Valve

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FIG. 9

The most acceptable diagram is given on the next page as FIGURE 10.

+V _

Rung 1

Rung 2

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Compare the diagram which you have completed as FIGURE 9 with that given

below as FIGURE 10.

FIG. 10

Your diagram may, in fact, be different and yet still be acceptable. If your

diagram is different then compare it now with FIGURE 11 which is also

correct!

FIG. 11

If both FIGURE 10 and FIGURE 11 are correct then why is one more

acceptable than the other?

R1/1 Valve +V –

Rung 1

Rung 2 R1

Relay SW1 SW2 SW3

R1/1 Valve

Relay +V –

Rung 1

Rung 2

SW1 SW2 SW3

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The answer would appear to be convention.

When we read a page of text we start at the top and work down the page

reading from left to right. We expect the text which we read at the top of the

page to 'come before' that at the bottom of the page. In this way it is supposed

to make more sense to us.

In using ladder diagrams we try to 'make more sense' by following the same

conventions and we would claim that this makes the diagram more readable.

In the case of the diagram of FIGURE 8 what 'comes before' the valve being

activated is the relay coil being energised and so it becomes more readable to

place the relay rung before the valve rung.

Of course this circuit is very simple and we can make these statements about

convention seem very convincing. More complex circuits, however, will not

be as easy to place in rung order. The hope is that if we follow the convention

complex circuits will be easier to understand.

Let us pause to check what else we can use in future diagrams.

The circuit diagrams from which the ladder diagrams have been taken have all

been shown with the circuit components arranged in a particular order.

Faithful conversion from circuit to ladder diagram has resulted in the proving

order; from left to right across the rung, checking all switch contacts first and

lastly allowing the rung current (if the switches are closed) to flow through the

load for that rung.

You are advised to follow this as another convention because ladder diagrams

used for programming PLCs are also arranged in this way.

To check that you understand what is meant by this last point look at the four

versions of the ladder diagram rung shown in FIGURE 12.

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If any of these arrangements were hard-wired then they would work, i.e. the

relay would become energised only when all three switches were closed.

Only one of these arrangements, however, would be acceptable for

programming a PLC.

Which one; (a), (b), (c) or (d)?

FIG. 12

Answer given on page 40.

NOTE: SW1 'AND' SW2 'AND' SW3 must be closed before the relay will

energise.

+V –

(a)R1 SW1 SW2 SW3

(b)R1 SW1 SW2 SW3

(c)R1 SW1 SW2 SW3

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Example 4

The next circuit diagram, shown as FIGURE 13, is provided to give you more

practice in converting from circuit to ladder diagram. The circuit looks a 'bit

of a jumble' so remember, an electrical connection is formed only where a dot

is shown at the joining of three lines. Lines which cross without a dot being

shown are not connected together.

FIG. 13

Complete the conversion to ladder diagram by using FIGURE 14. Take note

of the conventions covered so far.

R1/1 N/O

Valve 1R1

R2 R2/1 N/O

SW3

SW1 SW2

Valve 2

+ –

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FIG. 14

You can check that you have completed FIGURE 14 correctly by turning to

page 46 where a full version of the ladder diagram appears as FIGURE 29.

If you now study the operation of the circuit, which diagram do you find easier to

read?

You should find that the ladder diagram is easier to read than the circuit

diagram.

+V –

Rung 1

Rung 2

Rung 3

Rung 4

Contacts Rung Loads

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Example 5

Topic 1 covered the basics of relay operation. The next circuit diagram

(FIGURE 15) shows a relay with change-over contacts. One set is normally

open (N/O) and identified by number 1 whilst the second set is normally

closed (N/C) and identified by number 2.

The contacts each close different circuits to make the indicators operate.

Change-over operation of the contacts ensures that only one indicator is on at

any point in time.

With switch 1 open the relay is not energised. The N/O contacts are open and

so indicator 1 is off. The N/C contacts on the other hand are closed and so

indicator 2 will be on.

Convert the circuit shown in FIGURE 15 into a 3 rung ladder diagram by

completing FIGURE 16 overleaf.

FIG. 15

Ind.2.

+ –

R1/2 R1

Ind.1.

R1/1 Relay

Battery

SW1

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Ladder diagram.

FIG. 16

The completed diagram is given on page 40.

+V –

Rung 1

Rung 2

Rung 3

Symbol for N/C Contacts

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Example 6

FIG. 17

In the circuit of FIGURE 17 the switches are connected together in parallel

which means that the relay will energise if any one (or more) of the switches

(SW1, SW2 or SW3) is closed.

So the relay is energised if SW1 'OR' SW2 'OR' SW3 is closed.

When the relay is energised its N/O contacts will close and the bell will ring.

Convert this circuit diagram to a ladder diagram.

Bell

–

SW1

SW2

SW3 R1/1 N/O

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FIG. 18

Answer given on page 41.

By now you should be quite confident about taking a circuit diagram and

converting it into a ladder diagram. One more exercise is provided to finish

off this section.

Example 7

At first sight the circuit of FIGURE 19 may seem to be quite involved but if

you apply the same technique as used with the previous circuits a four rung

ladder diagram should emerge.

+V –

Rung 1

Rung 2

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FIG. 19

Ladder diagram.

FIG. 20

+V –

Rung 1

Rung 2

Rung 3

Rung 4

Ind.2. + –

P1 R1

Ind.1.

R1/2

R1 R2/2 P2

R1/1

R2/1

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The solution is reproduced as part of the Self-Assessment Questions on

page 47 (FIGURE 30). Use it to check you answer.

________________________________________________________________________________________

READING LADDER DIAGRAMS ________________________________________________________________________________________

Now that some practice has been gained in converting from circuit to ladder

diagrams (and it can't be stressed enough that as much practice as possible is

needed) the next logical step is to read and interpret ladder diagrams so that

circuit behaviour can be fully established.

We will start with circuits that have few components and progress to slightly

more involved arrangements.

Example 8

The first exercise in this section is based on the diagram of FIGURE 21 where

the ladder diagram has only two rungs. What we need to do is examine each

rung in turn to establish which parts are carrying a current and therefore which

circuit loads are actually working.

If no current is flowing in either rung then the general question must be asked:

What will happen if .............................................................?

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FIG. 21

Examine rung 1 first.

P1 is shown as a push-to-break switch which means that it is normally closed.

A current would flow from the positive supply rail, through P1 and into the

relay R1 before returning to the negative supply rail. The relay will be

energised unless the P1 switch is pressed, in which case the current flow will

be interrupted and the relay will de-energise.

When relay R1 is energised all associated contacts will operate i.e. any N/O

contacts will close and any N/C contacts will open.

Examine rung 2.

The R1/1 (N/O) contacts will be closed when the relay is energised. These

contacts supply current to the indicator IND1. IND1, therefore, will indicate

that the relay is energised.

+V –

Rung 1

Rung 2

P1 R1

R1/1 IND.1

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What will happen if ............................ P1 is pressed?

....................................................................................................................................................

________________________________________________________________________________________

Answer given on page 41.

Example 9

The circuit illustrated by the ladder diagram of FIGURE 22 has a few more

components. SW1 and SW2 are both push-to-make switches. SW3 is a push-

to-break switch. The relay has two sets of isolated, normally open contacts.

Examine rung 1

If the circuit supply has just been switched on is this rung complete?

If it is, then relay R1 will be energised.

Reading the diagram as it is drawn SW1, SW2 and R1/1 switch contacts are all

open therefore rung 1 is not complete and relay R1 is not energised.

Examine rung 2

Is this rung complete?

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If relay R1 is not energised then the N/O contacts R1/2 will be open, therefore

this rung is not complete and the indicator (IND1) will be off.

FIG. 22

What will happen if ...........................SW1 is momentarily pressed?

Current will flow through SW1, through SW3 (N/C) and into relay R1 – the

relay will then be energised.

When the relay is energised all of its associated contacts will operate i.e. R1/1

N/O contacts will close and R1/2 N/O contacts will also close.

The current in rung 1 now has two possible paths from the supply rail. If SW1

is released then the current to the relay will flow through the R1/1 contacts

which are now closed and the relay will remain energised. (These contacts act

as self retaining contacts to keep R1 energised.)

SW1

SW2

+V – R1

Ind.1R1/2

R1/1

SW3

Rung 1

Rung 2

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The R1/2 contacts, now also being closed, will supply current to IND1 to turn

it on.

If the relay is energised then the indicator will be on.

What will happen if .......................... SW2 is momentarily pressed?

....................................................................................................................................................

________________________________________________________________________________________

What will happen if .......................... SW3 is momentarily pressed?

....................................................................................................................................................

________________________________________________________________________________________

Answers given on page 42.

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________________________________________________________________________________________

CIRCUIT/LADDER DESIGN ________________________________________________________________________________________

When first designing a circuit in ladder diagram format to obtain specific

circuit behaviour, most students check for correct operation on paper but many

do not check for incorrect operation!

Checking for incorrect operation can be more important (and more difficult)

than checking for correct operation. This is certainly the case when designing

for PLC implementation. If the circuit is large then checking for incorrect

operation can take more time than the actual design work. This is because

every possible set of conditions should be considered and not just those which

are desired. Thankfully modern PLCs are very user friendly, possessing

facilities to help prove the operation and therefore avoiding drastic

consequences.

We can attempt to illustrate this point by considering a straightforward

switching problem.

We are asked to design a switching circuit, laid out in 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)

The diagram of FIGURE 23 provides the solution to the first two terms of the

problem. The third term requires a little more thought.

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FIG. 23

To some, the solution might appear to be simple. Merely join the mid-points

between the switches together and in that way the requirement of the relay

being energised when switch 1 and switch 3 are closed will be met. FIGURE

24 shows this condition.

Before rubbing our hands together and congratulating ourselves we must now

examine for incorrect operation.

What will happen if .................? must be used to check for every possibility

before correct operation can be established.

FIG. 24

+V –

R1 1 2

1 and 2

3 and 4

+V –

R1 1 2

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With this switching arrangement what will happen if .......... switch 4 and switch 2 are

closed?

Clearly the relay will energise. This is an unwanted combination of switches

and so some method of allowing current to flow from switch 1 to switch 3 and

yet not allowing current to flow from switch 4 to switch 2 must be found. A

diode placed at the mid-point should cure this problem. FIGURE 25 shows the

complete solution.

FIG. 25

Now attempt the Self-Assessment Questions which follow the answers to in-

text questions that begin overleaf.

+V –

R1 1 2

1 and 2

1 and 3

3 and 4

Diode

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________________________________________________________________________________________

ANSWERS TO QUESTIONS IN LESSON ________________________________________________________________________________________

QUESTION FROM PAGE 24

The acceptable arrangement is (c).

Example 5

Completed FIGURE 16 from page 28.

+V –

Rung 1

Rung 2

SW1 R1

R1/1

R1/2

Rung 3

IND.1

IND.2

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Example 6

Completed FIGURE 18 from page 30.

Example 8

Answer to question on page 34

If P1 is pressed then the flow of current to the relay will be interrupted and the

relay will de-energise. The relay contacts R1/1 will open and the indicator

IND1 will go off.

If P1 is released then the relay will energise and the indicator will be on.

SW1

SW2

SW3

+V –

Bell R1/1

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Example 9

Answer to first question on page 36

If SW2 is pressed after SW1 has already been pressed then no difference in the

state of the circuit components will be noticed because the relay is already

energised.

Answer to second question on page 36

If SW3 is pressed after SW1 (or SW2) has been pressed then the flow of

current to the relay will be interrupted, the relay will de-energise and the

indicator will go off. The action of SW3 is to unlatch the self retaining circuit.

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________________________________________________________________________________________

NOTES ________________________________________________________________________________________

...................................................................................................................................................

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________________________________________________________________________________________

SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________

1. (a) What is the difference between computer assemblers and compilers?

(b) State one advantage of assembly language over machine code.

2. Identify, by naming, each of the ladder diagram symbols shown in

FIGURE 26.

FIG. 26

(a) (b)

(e)

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3. The diagram of FIGURE 27 shows three circuit components connected in

series. Write down an expression for this circuit arrangement.

FIG. 27

R1 =

4. The diagram of FIGURE 28 shows three circuit components connected in

parallel. Write down an expression for this circuit arrangement.

FIG. 28

R2 =

Relay R2

A B C Relay R1

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5. Answer the following questions about the diagram of FIGURE 29.

(a) How will R1 become energised?

(b) When will valve 1 become activated?

(d) Is the operation of valve 2 in any way dependent upon the operation

of valve 1?

FIG. 29

FIGURE 29 is a completed version of FIGURE 14. Use it to check your

answer for FIGURE 14.

Rung 1

Rung 2

Rung 3

Rung 4

SW1

R1/1

SW3

+V –

R2/1

Valve 2

Valve 1

SW2

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6. Answer the following questions about the diagram of FIGURE 30.

(a) If R1 is energised will IND2 be on?

(b) If R2 is energised and P2 is pressed what happens to R1?

operation of the other relay?

FIG. 30

FIGURE 30 is a completed version of FIGURE 20. Use it to check your

answer for FIGURE 20.

Rung 2

Rung 3

Rung 4

R1/2

+V –

R2/2 IND.2

IND.1

R2/1

Rung 1

R1/1

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________________________________________________________________________________________

ANSWERS TO SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________

1. (a) Assemblers have assembly language as their source code whereas

compilers or interpreters have high level languages as their source

codes.

(b) Assembly language gives us a basic understanding of what each

instruction in machine code is doing providing we understand and

know the basic mnemonic meaning.

making it comparatively easy for the programmer to formulate ideas

in them.

2. (a) normally open circuit contact

(b) normally closed circuit contact

(d) push-to-break switch

(e) indicator element

3. R1 = A AND B AND C

4. R2 = A OR B OR C

5. (a) R1 will be energised if SW1 and SW2 are closed.

(b) Valve 1 will become activated whenever relay R1 is energised.

will be energised when SW3 is closed.

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(d) The operation of each valve is independent of the other. Each is

dependent upon its own relay.

6. (a) If R1 is energised then contacts R1/1 will be open and R2 cannot

become energised. If R2 is not energised then IND2 will be off.

(b) When P2 is pressed R2 will de-energise. The R2/1 contacts will

close causing relay R1 to energise.

contacts. The operation of either is dependent upon the other being

de-energised. At any point in time only one relay may be energised –

which one depends upon the last push switch to have been pressed.

If both push switches are held down then neither relay (or indicator)

can operate.

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________________________________________________________________________________________

SUMMARY ________________________________________________________________________________________

We have been introduced briefly to low and high level computer programming

languages and their relative advantages. Many high level languages are

especially designed for particular tasks. Ladder diagram programming was

developed and is widely used for the programming of programmable logic

controllers. We have been introduced to the symbols used in ladder diagram

programming and have seen how we convert an electrical circuit into a ladder

diagram. The ability to convert circuits to ladder diagrams is important and as

a consequence has been emphasised in the lesson. Such ladder designs would

normally be followed by testing, fault finding and correction. These activities

will be encountered in the practical assignments which you will undertake after

the theory has been assimilated. The fundamental theme of logic switching

has been introduced at this stage but will take on more importance in the

second lesson of this topic.

One point which should be noted is that most of the circuits used within this

lesson contain relays and were supplied from d.c. supply lines. The relays

were used for simplicity and the d.c. supplies for convenience. Ladder

diagrams are used with other circuit components and a.c. supplies are

obviously also used. Do not make the mistake of thinking that only d.c.

circuits are presented in ladder diagram format.

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