Programable Logic Controller questions. PLCS

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MODULE TITLE: PROGRAMMABLE LOGIC CONTROLLERS

TOPIC TITLE: INTERFACING

LESSON 4: INPUT/OUTPUT USER PRECAUTIONS

PLC - 3 - 4

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 the previous lessons we have looked at input and output interface circuits.

There are other less obvious characteristics of PLCs but none the less

important that should be considered when selecting a PLC. The ostensibly

simple decision of selecting the correct choice of PLC from among the many

available for a particular application, for example, is crucial. In this lesson we

shall look at some of the factors which, if not considered, can be costly in

terms of safety and ‘down-time’ or make the difference between smooth

commissioning procedure of a process control system or the unpleasant task of

attempting to commission a system with equipment not correctly designed for

the job.

Some common practices of PLC manufacturers are highlighted in order to

introduce you to some of the things of which the PLC purchaser should be aware.

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YOUR AIMS ________________________________________________________________________________________

On completing this lesson you should be able to:

• choose a correctly ‘sized’ PLC

• understand common I/O identification practices

• identify the need for I/O designation charts

• follow and interpret manufacturers’ diagrams with respect to the

installation of the ‘common’ conductor

• understand the requirements of three-wire input sensors

• anticipate I/O problems before installation work begins.

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________________________________________________________________________________________

HUMAN INPUT ________________________________________________________________________________________

Programmable controllers can be seen in factories, process plants, etc. busily

working away with the indicators on the front panel flashing on and off giving

the impression that they are in total command of the situation and that humans

are unnecessary. Up to a point this is quite true. It is when something goes

wrong (a valve sticks, something jams on a conveyor system or there is a loss

of power etc.) that the human input becomes so important. Speedy corrections

and repair work are then the priority because down-time means lost production

which costs money.

The other occasions when the human input is vital are at the design,

installation and commissioning stages of a new system. A sense of urgency

can exist at the commissioning stage of a system as well as when something

goes wrong. Designers, maintenance technicians and engineers need to know

with confidence that the decisions they make will be the correct ones. Such

decisions are more likely to be correct if personnel are aware of the possible

pitfalls.

This lesson takes a step along that road.

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I/O QUANTITY ________________________________________________________________________________________

If you talk to an engineer about a programmable controller then sooner or later

the following questions will probably arise.

How many I/O has it got?

The I/O stands for, as you might imagine, inputs and outputs. The work

covered in Lessons 2 and 3 was about individual interface circuits where we

considered one input circuit or one output circuit at a time. A PLC will have a

number of such circuits consisting of inputs and outputs. The total number of

circuits is normally specified. For example, a small PLC may be said to have

20 I/O. This does not mean 20 inputs and 20 outputs, but simply 20 in total.

How many of these 20 I/O will be inputs and how many will be outputs?

Unfortunately there is no laid down standard for the proportion of inputs to

outputs. However, the majority of applications of PLCs require that more

inputs than outputs.

Consider a pneumatic cylinder which operates a ram. The cylinder is likely to

have a pneumatic solenoid valve which can be turned on or off – one output

can do this. The travel of the ram would need to be detected by sensors, one at

the extreme end of travel and another to detect that the ram has fully returned.

Here, then, two input sensors would be needed. In this example the ratio is

2 inputs to 1 output.

Consider now an application where a PLC turns on or off an alarm (or lamp).

For this application one output is needed but no feedback is required to inform

the PLC that the alarm has sounded (or that the lamp is on).

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Clearly the number of inputs and outputs required depends upon the

application being considered. The manufacturers have no idea what the

applications will be but we may say that some ratio must be decided upon

otherwise the PLC could not be made

Within the range of smaller PLCs most makers seem to go for a ratio of 1.5 to

1, i.e. for every two outputs they would provide three inputs. On this basis the

20 I/O mentioned earlier would have 12 input circuits and 8 output circuits.

THE I/O RATIO MUST NOT BE ASSUMED. IT MUST BE CHECKED BY

CONSULTING THE MANUFACTURER'S SPECIFICATIONS.

NOTE

In Lesson 3 it was mentioned that at least one manufacturer produces a PLC

without any fixed output interface circuits. That same PLC is also purchased

void of input interface circuits. The user then has the freedom of choosing the

I/O ratio. The interface modules are perfectly interchangeable, which makes

the PLC extremely flexible.

How many inputs and outputs are LIKELY in:

(a) a 120 I/O machine? ....................................................................................................

(b) a 40 I/O machine? ....................................................................................................

________________________________________________________________________________________

Using the 1.5 : 1 ratio; a 120 I/O machine would have 72 inputs and 48

outputs; a 40 I/O machine would have 24 inputs and 16 outputs.

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Now put yourself in the situation of having wired a 20 I/O controller to handle a

particular application. All, or most, of the I/O have been used up. Some time later the

application is altered and more than 20 I/O are needed.

Jot down any ideas you may have to solve this problem.

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

________________________________________________________________________________________

The solution to this type of problem is covered in the next section.

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EXPANSION ________________________________________________________________________________________

When the problem arises that an application requires some changes to be made

with the consequence that the PLC presently employed will not cope with the

increase in I/O needed then this could mean that the PLC has become

redundant and that a larger machine must be bought. To handle such situations

some machines are made to be expandable. This obviously does not mean that

the case is stretched and more electronics are forced inside. What it does mean

is that expansion units housing additional inputs and/or outputs, which may be

plugged into the controller, can be purchased.

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IF YOU HAVE AN APPLICATION WHICH IS LIKELY TO BE EXPANDED

THEN ENSURE, BEFORE PURCHASE, THAT THE PLC TO BE USED IS

A TYPE WHICH SUPPORTS EXPANSION.

________________________________________________________________________________________

I/O SIZES ________________________________________________________________________________________

Programmable Logic Controllers are available in various sizes. The size is

related, in particular, to the number and type of I/O it can control.

As a rough guide the following classification may be used.

SMALL PLCs – generally control from 12 up to 128 I/O.

MEDIUM PLCs – control from 128 up to 1024 I/O.

LARGE PLCs – control systems having over 1024 I/O.

Some manufacturers will state the maximum I/O which a PLC can support.

This is not necessarily the number of I/O that the PLC has, it is often the total

I/O capacity of the PLC itself plus the full expansion possibilities.

DON'T BE CAUGHT OUT. CHECK THE NUMBER OF I/O THE PLC

ACTUALLY HAS SEPARATELY FROM THE ADDITIONAL I/O WHICH

CAN BE OBTAINED FROM ANY EXPANSION ARRANGEMENTS.

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________________________________________________________________________________________

I/O IDENTIFICATION ________________________________________________________________________________________

Lesson 1 of this topic touched on the idea of connecting input conductors and

switches to the input terminals of a PLC. The text stated that once an input

device was connected it would thereafter be associated with the number of the

terminal. This seems reasonable for small PLCs but when large PLCs or even

medium PLCs which have been expanded are considered, some problems

arise. Coupled with the numbering of devices inside the PLCs, which we have

not yet taken into consideration, it becomes apparent that very clear and

unambiguous identification of parts is required. The identification system

chosen by the manufacturer (and these are varied) must be clearly understood

by the user (and the programmer). The same system of identification used

outside the PLC by the installing and commissioning engineer must be used in

the programming of the controller.

Although manufacturing standards and networking protocols are being

developed (as we shall see in a later lesson), such as ‘MAP’ the

‘Manufacturers Automation Protocol’, various manufacturers still use their

own identification systems. One can imagine the problems faced by an

engineer when fault finding on different PLCs. In order to avoid potential

design and maintenance problems, many companies use only one

manufacturer’s range of controllers. This provides a manufacturer with a

‘captive’ customer base. Larger firms, however, may employ different

personnel individually trained to various manufacturers’ equipment standards

whilst others, not aware of different manufacturing standards, unwisely expect

the field engineer to be familiar with every make of PLC produced.

If a manufacturer identifies inputs or outputs by the use of an octal system, for

example, it allows easier identification of the inputs and outputs by the PLC.

The user, however, needs to exercise a little caution. For example, suppose a

prospective user sees a PLC with the last terminal marked as 40. If the octal

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system has been used then this means that the terminal is not the fortieth, as

might be expected, but is actually the the thirty-second because 40 octal is

equivalent to (4 × 8) + (0 × 1) = 32 decimal.

If forty were needed then obviously this PLC would not suffice.

So what should a prospective user do? Would it be practical, for example, to

count the actual number of terminals?

If the PLC is large or medium then this is impractical. The answer is to:

CHECK CAREFULLY THE ACTUAL NUMBER OF I/O WITH THE

SUPPLIER BEFORE PURCHASE.

Where a decimal numbering is used we may expect no problems, but variations

may still exist.

Example

Consider a typical 20 I/O small PLC. The identification may be such that the

numbers from 0 to 11 are used for twelve inputs and the numbers from 12 to

19 used for the eight outputs. This presents little problem to the PLC. If a

certain number is used within a program then that number can be associated

either with the range 0 to 11 or the range 12 to 19 to ascertain whether an input

or an output is being specified.

With another 20 I/O system the manufacturer may number the inputs from 0

to 11 and the outputs from 0 to 7. If this is the case then specifying a number

within the program (say 6) does not indicate whether the 6 means input 6 or

output 6.

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To avoid any confusion additional information must be supplied. The standard

method of providing this information would be to use a prefix which denotes

the part of the system intended.

e.g. IN 6 or OUT 6

I 6 or O 6

In the lessons concerned with programming we shall use a set of prefixes

which some other systems employ:

'X' will represent an input

'Y' will represent an output.

Other letters will be necessary for parts of the PLC not yet covered but these

will be explained at a more suitable time.

⎫ ⎬ ⎪

⎭⎪ would give a clear indication.

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________________________________________________________________________________________

I/O DESIGNATION ________________________________________________________________________________________

An engineer who has the responsibility of designing a PLC system will

generally attack and solve the problems faced on the basis of previous training

and experience.

Two engineers, given the same problem, will come up with solutions which

will work but are very likely to be different in some way.

We might say that this does not matter as long as the problems are solved and

the system works.

An entirely different problem is faced when the system develops a fault of

some kind. A fault finding technician may be reluctant to work on a system

which someone else has installed unless he has some very important help. The

help is in the form of information. It is vital that records are kept and are

available for use, when needed, by the technician. Records which show the

up-to-date designation of inputs and outputs, any preset values, a copy of the

program, etc. must always be at hand. If changes are made then the record

must be amended to show these. It is unfortunate that some personnel neglect

the paperwork resulting in longer down-time and more costly repairs.

FIGURES 1 and 2 (opposite) are examples of what are termed input and

output designation charts respectively. These are simply terms for lists of

PLC input and output labels representing various input and output pieces of

equipment. The designation X000, for example, is an input label for an input

start push button, whereas Y005 represents a motor contactor that would

activate a motor, in this case a cutter motor in some process. Such basic

information can save much time and frustration when proving a PLC program

or when a fault develops.

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FIG. 1

FIG. 2

MACHINE NAME MACHINE No.(s) LOCATION INSPECTED BY ALTERATIONS – SEE CHART No.(s)

OUTPUT TYPE TERMINAL No.

0 1 2 3 4 5 6 7

OUTPUT VOLTAGE SUPPLIED AND ISOLATED FROM:

OUTPUT Y000 Y001 Y002 Y003 Y004 Y005 Y006 Y007

DESCRIPTION FORWARD TRAVERSE CONTACTOR REVERSE TRAVERSE CONTACTOR DOOR OPEN LAMP ‘NO COMPONENT’ LAMP CUTTER MOTOR CONTACTOR

2 A, 250 V a.c. RELAY CONTACTS

PLC MODEL INSTALLED BY DATE INSTALLED DATE

PLC No.

OUTPUT DESIGNATION CHART No.

VOLTAGE 120 V a.c. 120 V a.c. 120 V a.c. 120 V a.c. 120 V a.c.

MACHINE NAME MACHINE No.(s) LOCATION INSPECTED BY ALTERATIONS – SEE CHART No.(s)

INPUT CIRCUIT TERMINAL No.

0 1 2 3 4 5 6 7 10 11 12 13

INPUT X000 X001 X002 X003 X004 X005 X006 X007 X010 X011 X012 X013

DESCRIPTION OF SENSOR/SIGNAL PANEL START BUTTON PUSH TO MAKE PANEL STOP BUTTON PUSH TO MAKE END OF FORWARD TRAVEL – LIMIT SWITCH END OF REVERSE TRAVEL – LIMIT SWITCH LEFT-HAND GATE SENSOR – MICRO SWITCH RIGHT-HAND GATE SENSOR – MICRO SWITCH COMPONENT POSITION – INDUCTIVE PROXIMITY

VOLTAGE: CURRENT:

PLC MODEL INSTALLED BY DATE INSTALLED DATE

PLC No.

INPUT DESIGNATION CHART No.

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________________________________________________________________________________________

INPUTS ________________________________________________________________________________________

Lesson 1 covered some aspects of input wiring and Lesson 2 the fundamentals

of input interfaces. This section expands upon these.

An earlier section used a 20 I/O PLC as an example to illustrate I/O

identification. The example quoted 12 inputs and 8 outputs. From Lesson 2

we learned that an input requires an input interface circuit, therefore it follows

that 12 inputs require 12 input interface circuits. Each circuit has two

terminals. If a d.c. input circuit is used then one terminal supplies a current to

the switch and the other terminal receives the current back to complete the

flowpath.

How many input terminals do we need?

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

________________________________________________________________________________________

For 12 inputs, each requiring two terminals means that 12 × 2 = 24 terminals

are needed. These terminals must be accommodated on the case of the PLC.

In an attempt to make the PLC. smaller and cheaper sometimes half of these

terminals are combined as a common terminal.

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The common terminal either supplies all the current to all the input switches or

receives all of the current back from any closed switches (depending on how

the interface circuit has been wired).

FIGURE 3 illustrates these two possibilities.

FIG. 3

Notice that both circuits are wired exactly the same way and so it does not

matter (with input switches) in which direction the current is flowing.

If this were shown on a manufacturer’s diagram it would probably look like

FIGURE 4.

Input terminals

Current flowing into the common terminal (b)

No.1 closed

No.3 closed

Common

Input terminals

Current flowing out of the common terminal (a)

No.1 closed

No.3 closed

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FIG. 4

Notice that the common has been shown with only one line whereas in the

diagrams of FIGURE 3 two conductors are connected to the common terminal.

The manufacturer’s diagram may suggest to some people that a joint should be

made in the wiring at the node point where the inputs join the common

conductor. This is not a recommended practice. It is always best to use twin-

cored cables and run both cores back to the controller.

It has been known in the past for some installation personnel to have wired

inputs in the same way as the wiring on an automobile, no doubt thinking that

because only 24 V d.c. was being used it would be acceptable to use a metal

frame as the common conductor. This would undoubtedly work but it is an

extremely dangerous and unsafe thing to do. Any mains fault anywhere in the

whole installation will give rise to an a.c. voltage, possibly at mains level,

being injected into the common terminal and any input terminal with its switch

closed at that time. Serious damage to the installation would result with the

safety of personnel at risk (burnouts and fire).

Such practice also means that a number of PLCs could be sharing the loading

with circulating currents flowing through the circuit protective conductors.

Common

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ENSURE THAT ALL INPUT WIRING HAS ITS OWN INSULATED

CONDUCTORS AND NEVER BE TEMPTED TO USE ANY OTHER

MEANS OF COMMON CONNECTION.

The last item to mention about inputs for the time being concerns the use of

transistorised sensors. Lesson 2 indicated that a detector which was being used

as an input sensor could be used instead of a switch. Various conditions were

stated as points to be checked to ensure that the sensing device works

satisfactorily.

Some sensing devices which have a transistor output are provided with a three

core cable. FIGURE 7 in Lesson 2 only shows two conductors being used.

The third core usually requires a d.c. supply to power the sensor circuit and to

provide a signal to switch the transistor at the required time.

FIG. 5

This type of sensor can provide a problem because not all PLC manufacturers

take this into account and so do not provide a suitable supply. Users of this

type of sensor may find themselves faced with the additional expense of having

to purchase extra power supplies for this purpose. The type of supply required

normally needs to have a regulated and stabilised smooth output which is not

cheap to provide.

Common

Input

+ V

Sensor circuit

3 cores

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CHECK BEFORE PURCHASE WHETHER THE PLC HAS AN

AUXILIARY SUPPLY SUITABLE FOR TRANSISTORISED INPUTS.

FIGURE 6 shows the terminal arrangement provided on a commercially

available PLC.

Working from left to right the points to note are:

(a) The terminal marked DCO is a d.c. output provided as a supply suitable

for three-wire d.c. input sensors. In this case the DCO terminal is the

positive of the supply and the negative is the next terminal which is

marked as C for common.

FIG. 6

(b) Next come the 12 input terminals. Notice that the first is identified as 0

and the twelfth is marked as 13.

of the input sensors are terminated at this point. (Lots of conductors into

only one terminal is not easy.)

CommonCommon

D.C. supply to power

3 wire sensors

Terminals provided for inputs

DCO C 0 1 2 3 4 5 6 7 10 11 12 13 C

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________________________________________________________________________________________

OUTPUTS ________________________________________________________________________________________

To conclude this lesson we will now turn our attention to some of the external

factors concerning the use of outputs.

RELAY TYPE OUTPUTS

Lesson three covered aspects of relay outputs from the point of view of the

actual circuitry employed. It is now time for us to review this work by

considering practical usage.

It has been stated that a typical contact rating is about 2 A. If we can apply

this to the 20 I/O PLC model discussed earlier then we can expect 8 outputs

each rated at 2 A.

How much load can we drive in total?

Users new to PLCs may be excused for thinking that if a controller has, say,

eight output relays and each relay contact has a rating of 2 A then

8 × 2 A = 16 A of load may be connected.

This is, to say the least, a dangerous assumption because quite often the

manufacturer does not expect every relay to carry its full rated current and

consequently the internal conductors,which may actually be thin printed circuit

board tracks, will not be rated to dissipate the heat of this full loading. One or

two relays could carry full load but although 8 × 2 A suggests a possible total load of 16 A the manufacturer is likely to recommend a maximum of around

10 A.

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USERS BEWARE, CHECK THE TOTAL LOADING AS WELL AS THE

INDIVIDUAL CONTACT RATINGS.

A further important point to note is that some controllers have one contact

from each of a number of relays connected together and brought out as a

common terminal. This could be to reduce the number of terminals used and

therefore reduce the overall size of the unit. However, other controllers

employing common contacts bring out two terminals for each relay. The

important point about the use of commons is that once the user has decided on

a voltage to be used for one set of commoned contacts then this same voltage

should be used for all contacts which share the same common connection.

FIGURE 7 and FIGURE 8 show some of the internal arrangements which are

used. An eight relay unit is shown in the diagrams.

Notice again that the first contact is numbered 0 (when programming starts this

can take a little getting used to).

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(a) (b)

FIG. 7

FIGURE 7(a) shows eight relay contacts. In this diagram all of the contacts

are separate i.e. none of them is internally commoned. If a common

connection is required then the user must join together the terminals external to

Relay contacts

Common

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the PLC. When all eight pairs of terminals are separate then the eight output

circuits could have different values of voltage connected to them. Not many

applications are likely to need eight different values of voltage but the freedom

of choice is there as long as the voltages used are within the rating of the

contacts.

In some manuals, users are recommended to leave unused one set of contacts

between two widely different values of voltage. For example, if one output

uses 12 V d.c. and the next uses 240 V a.c. then increasing the insulation

between these voltages by having an unused set of contacts between them

would be the recommendation. This assumes that not all outputs are going to

be needed.

FIGURE 7(b) shows an arrangement where two sets of four contacts are

commoned. Two values of output voltage may be used with this internal

method of connection. Outputs 0, 1, 2 and 3 would share one voltage and

outputs 4, 5, 6 and 7 would share another voltage. One value of voltage could

be used for both.

FIGURE 8 shows yet another possible arrangement. One contact from each

relay has been internally commoned and brought out to one terminal. Only

one value of load voltage is allowed and so all loads must have the same

voltage rating.

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FIG. 8

It is not usual for manufacturers of small PLCs to provide any form of

overcurrent protection with relay outputs. Any protective device for the load

(fuse, thermal cutout, etc.) would be supplied by the user within the external

wiring. Adequate protection must be provided if the load uses mains voltage.

Relay contacts

Common

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TRANSISTOR OUTPUTS

We shall now examine one or two points relating to transistor outputs and

which are worthy of consideration.

Although power transistors will be used for the output to the load, the current

rating of each transistor is likely to be very limited. This will mainly be due to

the fact that in an effort to reduce the overall size of the unit no heat sink, or

only a very small one, would be used on each transistor. When a power

transistor is used without a heat sink the power rating must be reduced to a

value of current which will not overheat and burn out the device. A heatsink

would normally carry away and dissipate into the surrounding air the heat

produced by the additional current. If you can imagine, say, eight power

transistors all trying to give off heat at the same time then you will appreciate

that the air temperature inside the enclosure will rise. This will make it even

more difficult for individual transistors (certainly those in the middle of a row)

to give off their share of the heat. A further and most important factor is the

ambient temperature outside of the enclosure – if this is high then the transistor

current will need to be further reduced.

With all this in mind it is not easy to make decisions which will ensure

reliability of the unit when in service unless the total loading is kept well

within the manufacturer's recommendations. For example, some

manufacturers not only give a maximum value of current for any one transistor,

but also supply a graph of total loading for the unit against values of ambient

temperature. FIGURE 9 (opposite) shows an example of this type of graph.

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FIG. 9

This graph must not be used for any practical purpose other than illustrating

points in this lesson. If you are using transistorised output controllers then you

should use the graph (if there is one) supplied by the maker.

On the assumption that the graph of FIGURE 9 has been supplied by a

manufacturer to indicate the maximum total value of current which can be

safely drawn by an eight transistor unit, attempt to answer the following

questions.

Total transistor unit loading (amperes)

0 2 4 6 8 10

A m

b ie

n t

te m

p er

a tu

re (

°C )

Line of maximum load

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1. What would be the maximum total transistor current at an ambient temperature

of 40°C.

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

________________________________________________________________________________________

2. A total transistor current load of 7.5 A is required to be drawn from the unit.

What temperature range would be suitable?

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

________________________________________________________________________________________

3. Explain why the line of the graph slopes in the direction shown by FIGURE 9.

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

________________________________________________________________________________________

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4. Explain what you think might happen if a load of 5 A is drawn when the ambient

temperature is 47°C.

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

________________________________________________________________________________________

5. Suggest what action would be advisable if the transistorised unit were to be

installed inside a metal enclosure for mechanical protection reasons.

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

________________________________________________________________________________________

Answers given on page 27.

Just like relay outputs, transistorised outputs may be expected to be commoned

and so care is needed when choosing the value of output voltage as all

commoned circuits should share the same voltage supply. The better PLC

makers will provide a fuse to protect each transistor, others may provide

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protection within a common line, whilst some expect the user to provide

whatever protection they think necessary.

TRIAC OUTPUTS

Triacs, sometimes called static relays, are often provided to switch mains a.c.

voltages. Some units have enhanced circuitry so that the triac switches on at

the zero crossover of the a.c. waveform. This should extend the life of the

device and result in less switching interference experienced by other devices

on the same line.

For mains wiring it is likely to be recommended that only one phase of the

supply be used to feed all the triac loads and that a mains fuse be incorporated

within this phase conductor. Discrimination in protection may be needed for

each individual triac load but this is often left to the discretion and good sense

of the user.

If the triac is being switched by a firing pulse but its load current is very small

then it may not maintain conduction over each of the half cycles of the a.c.

This is due to the load current being below the minimum holding current for

the device. The only remedy for this, apart from changing the load, is to

artificially increase the load current by connecting an additional dummy load

across the terminals of the proper load.

CHECK THE VALUE OF THE TRIAC MINIMUM HOLDING CURRENT

AGAINST THE VALUE OF CURRENT DRAWN BY THE LOAD – THE

LOAD CURRENT MUST BE LARGER.

You should now attempt the Self-Assessment Questions on page 28.

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________________________________________________________________________________________

ANSWERS TO QUESTIONS ON PAGES 24 AND 25 ________________________________________________________________________________________

1. Reading from the graph 40°C shows a maximum current of 6.2 A.

2. Reading from the graph any ambient temperature up to a maximum value

of 36.5°C would allow 7.5 A to be safely drawn.

3. The slope of the graph is dictated by the fact that at lower ambient

temperatures more current can be drawn because the heat can be more

easily dissipated into the surrounding air. At higher ambient temperatures

the unit will have difficulty in transferring its heat to the air which is

already warm.

Therefore LOW AMBIENT TEMPERATURE – LARGER CURRENT

and HIGH AMBIENT TEMPERATURE – SMALLER CURRENT

4. At an ambient temperature of 47°C the graph indicates that the current

should not be more than 3.5 A. A current of 5 A is therefore too large. It

is likely that the unit will supply this load for a period of time and then

one or more transistors may overheat and burn out. During the period of

time that the unit is overloaded the heat produced may affect other

components which are being used for other parts of the controller.

Components other than the power transistors may also fail.

5. If the controller were to be placed inside a metal enclosure then the free

passage of air and consequently the rate of cooling would be affected.

The ratings provided for the controller would need to be down-rated or

else some form of forced cooling would need to be employed.

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________________________________________________________________________________________

SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________

1. A 24 I/O controller has an I/O ratio of 2 : 1.

How many inputs and how many outputs does it have?

2. Explain how it is possible for a manufacturer to leave the I/O ratio to be

decided by the user.

3. Explain briefly how a PLC may be able to handle a greater I/O capacity

than it houses.

4. State the 'range' of I/Os expected from a medium size PLC.

5. When does it become necessary to use prefix letters in an I/O numbering

system?

6. Explain the value of using I/O designation charts.

7. What installation differences are there when a three-wire proximity

detector is used instead of a simple switch?

8. A controller which incorporates transistor outputs is to be used for a

particular application. Explain the important points to be considered

when deciding the output loading.

9. Why is it sometimes necessary for a dummy load to be connected across

the normal load terminals when the output circuit incorporates a power

triac?

Teesside University Open Learning (Engineering)

________________________________________________________________________________________

NOTES ________________________________________________________________________________________

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

Teesside University Open Learning (Engineering)

________________________________________________________________________________________

ANSWERS TO SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________

1. A 2 : 1 ratio means 2 inputs for every output. If the controller has 24 I/O

then 16 inputs and 8 outputs have been provided.

2. The way a manufacturer leaves the I/O ratio to be decided by the user is

by designing and selling the PLC void of any input or output interface

circuits. The user decides how many of each type of interface would be

necessary and purchases them separately. Not all manufacturers think

that this is a good practice.

3. Expandable controllers are produced with the control electronics to

handle the I/O circuits which they house as well as additional I/O which

may or may not be required. Not all controllers are expandable and there

is a practical limit to the additional I/O which can be used.

4. The medium range is not rigidly specified but can be taken as between

128 I/O and about 1024 I/O.

5. Prefix letters (or some other suitable method) become necessary in an I/O

numbering system if some of the inputs have the same numbers as some

of the outputs. The prefix provides clarification as to which of the

numbers stated are meant as inputs and which are meant as outputs.

6. I/O designation charts are provided as information for personnel in an

attempt to make fault finding on, or alterations to, a system easier.

Knowing the number and position of each input or output makes it much

easier to follow what is (or is not, in the case of a fault) happening.

Teesside University Open Learning (Engineering)

7. A simple switch input will require two conductors (or a twin) to be run

from the switch to the controller. A three-wire sensor requires three

conductors (or a three core) to be run. The three-wire sensor will also

need a supply, normally low voltage d.c., to provide a power source for

the electronics inside the sensor itself.

8. At least three factors should be considered.

(a) The load voltage must be well within the maximum voltage rating of

the transistor.

(b) The load current must be well within the maximum current rating of

the transistor.

checked against the load current/ambient temperature graph. This

factor is often overlooked.

9. When a triac conducts it not only has a maximum safe value of current, it

also has a minimum conduction value. If a load is so small that it does

not draw sufficient current through the triac then the triac will cease

conduction i.e. it will switch off. When this is the case additional current

must be drawn by increasing the size of the load. A dummy load

connected across the normal load draws the extra current so that

conduction is maintained.

Teesside University Open Learning (Engineering)

________________________________________________________________________________________

SUMMARY ________________________________________________________________________________________

This lesson has attempted to highlight some of the factors which a PLC user

should be aware of regarding I/O requirements.

Warnings have been provided in box format and in each case an explanation

has been offered to show the relevance of the warning. Hopefully the

explanations will add credibility to the warnings and ensure that these points

are remembered.

Teesside University Open Learning (Engineering)

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