Programable Logic controller questions 1-3

MODULE TITLE: PROGRAMMABLE LOGIC CONTROLLERS

TOPIC TITLE: ADDITIONAL FACILITIES

LESSON 2: ANALOGUE TO DIGITAL CONVERSION

AND SPECIAL FACILITIES

PLC - 7 - 2

© Teesside University 2011

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 begin by exploring the possibility of a digital machine

accepting signals from an analogue signal source. This is achieved in practice

by producing a digital representation before it is offered to the PLC.

Only one method of converting from analogue to digital signals is examined

although alternative methods are available. The best utilisation of the

conversion circuit is obtained when the same circuit components are used to

serve more than one analogue input channel. How the mechanism for this and

other control signals is realised in practical modules is also explained.

A separate section of the lesson looks at methods of saving programs to make

copies which can be held safely for occasions when the PLC program crashes

(fails) or when the same software needs to be duplicated for use on other

machines.

The lesson continues by taking a brief look at the trend towards improved

communications between PLCs and other computerised equipment.

Finally, a small application is used to show how some of the additional

functions which are found in larger machines are used to effect more versatile

control.

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YOUR AIMS ________________________________________________________________________________________

On completing this lesson you should be able to:

• understand the relationship between an analogue voltage value and

its digital representation

• understand the requirements of an A/D circuit to make it more

sensitive to small input voltage changes

• explain the operation of one type of A/D circuit

• explain the process of multiplexing analogue signals

• state and explain different methods of saving PLC programs

• understand the need for better communications facilities for PLCs

• explain some of the additional functions of larger PLCs.

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HANDLING ANALOGUE INPUTS ________________________________________________________________________________________

The topics covered so far have dealt, in the main, with signals and functions

which have a binary digital nature. However, a vast quantity of sensors and

transducers used in modern control and monitoring applications produce

analogue outputs. There is clearly a distinct lack of compatibility between

these signals and the operating characteristics of the microprocessor-based

machine which is required to process them. We therefore need to consider

how the analogue signals can be changed into a binary digital representation so

that the PLC will find them acceptable?

The obvious answer is to have a circuit which will accept an analogue input

and produce from it a digital output. Circuits which fulfil this function are

called analogue to digital converters as opposed to the digital to analogue

converters previously examined. FIGURE 1 shows the basic arrangement.

FIG. 1

Several different methods of A/D conversion are possible. Some methods are

used because the conversion process is very rapid, whereas others may be used

binary digital output

Analogue transducer and signal

conditioning circuit

Analogue input

0 volt common

+V

A/D circuit

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because they offer the cheapest method despite taking a little longer to carry

out the process. Whatever the method used, the most useful output will be

derived from an A/D if a large number of digital output lines are provided. A

term often used to indicate the number of outputs is called the resolution.

Hence an A/D with an eight bit resolution will have eight output lines (as in

FIGURE 1) which will provide binary patterns ranging from 00000000 up to

11111111,

i.e. 28 = 256 possible patterns exist with an eight bit device.

This means that if the range of the input value is, say, from 0 V to 2.56 V then

1/256 of this would be the voltage range associated with the least significant

bit change in the binary output. FIGURE 2 illustrates this graphically.

FIG. 2

With this arrangement any voltage between the limits 0 V and 0.01 V would be

associated with an output of 00000000 binary (00 Hex). Values above 0.01 V

but not above 0.02 V would be associated with an output of 00000001 binary

(01 Hex). The values would advance in incremental steps of 0.01 V until the

upper limit of 2.56 V was reached, the binary pattern 11111111 (FF Hex)

2.56 V 2.55 V

0.03 V

0.02 V

0.01 V

0.01 V

0.01 V

0.01 V 00H 01H 02H0V

FFH

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being the biggest number possible as an eight bit output. This is a very

mathematical approach which is being used here to establish the basic idea. In

practice this level of accuracy would not be expected but we will continue

along these lines within this text. Understandably then, the analogue value

would need to change by 0.01 V before we could be certain of a change in the

binary output. However, if the A/D were exchanged for another having a

12 bit resolution with the same analogue range then 212 = 4096 values of

output would be possible and the analogue voltage range associated with a

least significant binary bit change would now be 2.56 V × 1/4096 which is 0.000625 volts*. The increase in the number of output bits makes the A/D

more sensitive to a small change at the input.

You should be aware that if a PLC manufacturer offers two different A/D

modules, one having a specification of 0 to 5 V with a 12 bit resolution and the

other having a specification of 0 to 10 V with a 12 bit resolution then both will

produce a 12 bit binary output but the first will produce a binary bit change

due to a 5/4096 volt change at the input and the second, despite having a larger

input range, will need a 10/4096 volt change at the input to produce the same

one bit change at the output.

Hence, a larger analogue voltage range does not mean better sensitivity to

change, it means that a greater analogue voltage change will be necessary to

cause a one bit change at the output. FIGURE 3 highlights this.

________________________________________________________________________________________

*Strictly, it will be slightly greater than this as has been previously stated, the number of level changes is one

less than the total binary levels. In practical terms the error is insignificant.

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FIG. 3

This also illustrates that a binary digital value can never be said to equal an

analogue value. The relationship between the analogue input and the digital

output will differ with changes in the analogue range, the bit resolution and the

electronic circuitry employed to effect the conversion.

The best statement we can make is the following.

For a given system, a binary digital value at the output of an A/D

converter is representative of an analogue input value lying somewhere

between two specified values within a range.

This statement probably sounds complicated but in practice the majority of

PLC systems would probably not be so finely tuned as to make the statement a

problem. Having 4096 possible values at your disposal does not necessarily

mean that you will use all of them!

10 4096

5 4096

V

1 bit change

1 bit change

1 bit change

1 bit change

1 bit change

1 bit change

V

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A/D CONVERSION PROCESS ________________________________________________________________________________________

As already mentioned, several methods of A/D conversion are available to the

hardware engineer. This lesson is not intended describe all of the different

methods which may be employed but it is considered important that at least

one process is examined. Hopefully, by understanding one method you will be

able to work your way through others should this become necessary.

The method which we shall examine is popular, cheap to implement and

relatively straightforward to understand.

The diagram of FIGURE 4 shows a collection of components which are

interconnected to form an A/D circuit. Study the diagrams for a few moments

so that you can pick out any components which you recognise (at least two

should be familiar).

FIG. 4

D/A Circuit

Binary Up-

Counter Circuit

&

• •

• •

• •

• •

• •

• •

Analogue Output

Clock

Astable

NAND Gate

Comparator Analogue

Input + –

5

1

4Reset

Begin Conversion

Signal

3

2

Binary Digital Output to PLC

Conversion Complete

Signal

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Components 1 (two input NAND gate) and 2 (digital to analogue converter)

should be immediately recognisable. Depending upon your background you

may also have identified other devices or even the whole circuit.

Component 3 is a free running astable multivibrator circuit which is being used

as a clock signal generator. When the supply is switched on, this circuit

produces a continual output of a square, or rectangular, waveform. Its

operation is not affected by any of the other components shown in the diagram.

Component 4 is a binary up-counter. We have mentioned hardware counters in

Lesson 2 of Topic 6. The symbol is shown with two inputs and twelve outputs.

The reset input, when enabled, causes the counter to reset all of the output bits

to a logic 0 state, i.e. 000000000000 (or 000 Hex). When the reset signal is

disabled the counter becomes free to start to count up. Each pulse applied to

the count-up input will increment the counter in a true binary fashion. The

count is immediately available on the twelve output lines.

Component 5 is called an analogue comparator. It has two inputs, marked as +

and – on the symbol, and one output. This circuit accepts at its input terminals

values of analogue voltage but supplies at its output a digital voltage level.

The output level will be a logic 1 (+5 volts) if the voltage applied to the

– input is less than the voltage applied to the + input, and the output level will

be a logic 0 if the two analogue voltages are reversed. (See FIGURES 5(a) and

5(b)).

FIG. 5(a) FIG. 5(b)

Having briefly described the components of the converter we can now analyse

how they behave collectively.

–

+

Say +2.5V

Say +2V

Logic 0

Output

–

+

Say +2V

Say +2.5V

Logic 1

Output

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Assume that an analogue value of, say, 1 volt is applied to the analogue input

of the circuit. (This is an arbitrary value but it gives us a number to work

with.)

This input causes the converter to begin the conversion process. This is

achieved by the reset input being enabled and then disabled. On the

application of the reset pulse the counter output goes to 000 Hex. The counter

output is fed, as an input, to the D/A circuit. A zero input on all bits of the

R/2R D/A circuit produces a 0 volt analogue output. In the diagram the output

of the D/A is fed into the – input of the comparator.

The comparator now has 0 V on its – input and 1 V on its + input. What logic level

will the comparator have at its output?

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

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

________________________________________________________________________________________

A logic 1 should be present at the output. In turn, this output is applied, as one

of the inputs, to the two input NAND gate. When the first logic 1 arrives from

the clock, the two 1's produce a 1 to 0 change from the gate which is

recognised by the counter causing it to increment. Hence the counter output

changes to 001 Hex.

The same cycle of operations continues with each clock pulse. As the counter

output grows so does the value of analogue voltage at the D/A output until

such time as its value is just above the 1 volt applied to the comparator + input.

At this point the comparator output rapidly changes from logic 1 to logic 0.

The logic 0, now being applied to the NAND gate input, prevents any further

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clock pulses from getting through the gate to the counter. The counter output,

therefore, freezes at a binary digital value which may be taken to represent the

1 volt being applied at the circuit input. The logic level change from the

comparator is also used as a signal to the microprocessor within the PLC that

the conversion process is finished and that the digital value is available for use.

For the circuit being considered the maximum analogue voltage which can be

fed to the – input of the comparator will occur when all twelve inputs to the

D/A are logic 1. If the analogue voltage at the input is greater than this the

process will never end because the comparator output will always be a logic 1.

The analogue input value is therefore derived, or scaled down from, the upper

limit of the range stated for the A/D module.

We can summarise the use of the A/D as follows.

When the PLC is instructed to use the A/D circuit by commands within the

ladder diagram it must:

• initiate the A/D conversion (by sending a reset signal to the counter)

and

• wait until it receives the signal back from the comparator to indicate

that conversion is complete and that the binary value presently at the

counter output represents the analogue value at the input. (If the PLC

reads this value before conversion is complete then an incorrect

representation will be obtained.)

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MULTIPLEXING ________________________________________________________________________________________

As is the case with D/As the provision of the A/D facility should not be

expected within "small" machines. Users may, on the other hand, realistically

expect expandable or larger machines to support A/Ds, the most common form

being separate expansion or rack mounted modules. Each module may boast

two or four analogue channels but this is not likely to mean that each module

will have this number of physical A/D circuits. This would be wasteful of A/D

circuitry and would make each module bulky and expensive.

Remember, the circuit is only being used when directed by the ladder diagram.

In theory only one A/D circuit is in use at any one time. There is no reason to

have more than is needed.

How then can several analogue channels be provided by the use of only one

A/D circuit?

The answer to this interesting question is a technique known as multiplexing.

To make the multiplexing process easier to understand consider how the

voltage on three different lines could be measured by the use of only one

voltmeter. The diagram of FIGURE 6 shows a voltmeter connected to one of

three lines by the use of a manual selector switch. An operator would take,

and note down, a reading from the meter before switching from position 1 to

position 2.

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FIG. 6

A second reading (for line 2) would be taken and noted. Finally the switch

position would be changed to position 3 to allow the third reading to be taken.

This switching (or multiplexing) process allows all three readings to be taken

but not all at the same time.

The difficulty of not having access at all times to all readings would not be a

problem for a microprocessor system because it itself is a sequential device i.e.

it does one task at a time. So, if we now remove the voltmeter and put an A/D

circuit in its place, and change the 3 way selector switch for a 4 way type, then

we have the means of handling four analogue voltage channels.

Obviously the manual selection part of the process is not going to be

acceptable because the CPU within the PLC must be in charge of the

proceedings. Multiplexed analogue switch integrated circuits are

manufactured for just this purpose. The channel selection is obtained by the

application of a binary digital selection code which must be provided by the

microprocessor from information derived from the channel number specified

within the ladder diagram or program listing provided. The diagram of

FIGURE 7 gives a slightly more complete picture of the circuit together with

the control lines necessary to effect total control of the A/D process.

Line 1

Line 2

Line 3

Manual Selector Switch

Neutral/Common

V

1

2

3

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FIG. 7

Consider now the format of a typical conversion instruction and the anticipated

action which will take place. Examine FIGURE 8 which shows a rung of a

ladder diagram.

FIG. 8

When the processor is executing the program and arrives at this point in the

diagram contacts 0703 and 0705 are ANDed. If both contacts are closed then

FUN **

03

DM015

0703 0705

1 of 4 Selector

A/D Circuit

Tristate Buffer

Enable

Begin Conversion

Conversion Complete

Analogue Channel Selection Code

Control Signals

1 2 3 4

Analogue Input

Channels

Common S1 S2

Data Bus

Common

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the processor executes the function box, otherwise it passes on to the next

rung. The function of analogue to digital conversion would be specified by a

FUNction number placed in the ** position of the diagram. The data in the

next box down specifies the number of the channel which is presently being

used as an analogue input. The next box holds the data specifying where the

binary digital representation of the analogue channel is to be stored within the

memory of the system, after conversion has taken place.

If the function is executed the action will follow the steps listed below.

(a) Set the 03 channel code to select the correct analogue input through the

multiplexor.

(b) Issue the 'begin conversion' command by resetting the counter.

(c) Wait for the 'conversion complete' signal from the comparator.

(d) When the conversion complete signal arrives enable the tristate buffer to

pass the counter value through to the data bus of the system.

(e) Accept the digital data and store it in the specified locations i.e. a group of

memory locations (probably 16) which are specified by the DM015 value

(data memory channel 15).

(f) Go to the next rung in the program.

This step-by-step approach may be a simplification of the action but it should

serve our purposes for this text.

At some later point in the program the data previously brought in and stored

will be addressed to check if the analogue value is related to an output

switching or other corrective action. For example, if input channel three is

from a temperature sensor then perhaps an output will switch off or on a

heating element, on the basis of the converted value.

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________________________________________________________________________________________

SAVING AND PROTECTING PROGRAMS ________________________________________________________________________________________

In a previous lesson the means of protecting the user program from accidental

corruption or complete loss, in the event of a power failure, was discussed.

The use of battery-backed RAM was put forward as one method which can be

employed as a possible solution. However, there are instances when this

protection is not wholly adequate. Consider, for example, when there is a need

to replace the battery-backing battery due to its age or it becoming damaged in

some way. Would this mean that the program, which may be very long, will be

lost when the battery is removed? (Which must, incidentally, be done with the

power off.) Well, in some well designed machines the battery may not be the

only backing provided. A super capacitor may also be incorporated with the

specific purpose of maintaining an adequate supply of voltage across the

memory circuits for a short duration, so that the program is not lost. After the

battery is removed the capacitor takes over its job, but only for a matter of

minutes instead of the years which the battery itself would provide. Changing

of the battery must, therefore, be carried out quickly and the supply returned

with the minimum of delay. On paper it may seem that every possibility has

been catered for, but no one within the computer control field should ever rely

upon only one copy of a valuable piece of software which is installed within a

user machine. From the beginning of training the rule has always been:

• make a master copy and keep it safe!

The same rule must apply to PLCs but how is it achieved?

If the programming has been developed on a personal computer, then it could

simply be a matter of saving the program to a CD-ROM or to a universal serial

bus (USB) flash memory card or memory stick.

If, on the other hand, the programming has been done on a stand alone

controller then other methods may be possible.

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One method is to use an EPROM emulator. This requires the PLC to be fitted

with an additional I.C. socket into which is plugged a RAM chip which carries

its own small battery fitted to its back. The program is copied from the system

into this additional RAM chip which is later removed and carefully put into

storage. A RAM chip would normally lose its program immediately it was

removed from its socket but because it carries its own battery the program

remains safely stored. This use of a RAM chip effectively produces a device

which emulates an EPROM, i.e. a memory chip which can be programmed but

when it is removed does not lose its program.

This chip can be inserted into another system where the program is again

copied, this time going into the system RAM. By this method the chip can be

carried from machine to machine to have the same program loaded into each

system.

Another possibility is to have the program copied from the system and 'burnt'

into an EPROM. The EPROM can then, like the emulator, be carried from

machine to machine. The EPROM may also be plugged into a PROM reader

to which can be connected a printer in order that a hard copy printout of the

program can be obtained.

Very often, PLCs support the use of EPROMs, even allowing each EPROM to

carry more than one program so that rapid changeovers can be made, but the

PLC itself will not always have the ability to program them. In such cases extra

hardware, in the form of a PROM programmer, must be purchased before the

benefit of the EPROM facility can be fully realised.

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________________________________________________________________________________________

SPECIAL FACILITIES ________________________________________________________________________________________

As PLCs have developed they have become more powerful. The general need

for upward compatibility dictates that any program which will run on a small

machine should also run on a more powerful version from the same range.

This is quite understandable because if a user wishes to expand an application

necessitating a more powerful PLC then, if this also means incurring expensive

downtime while a new program is being developed, some reluctance to change

may be experienced. Downtime due to hardware changes may be an

unavoidable necessity but the system, once installed, should be up and running

as quickly as possible. This is true even if it means temporarily using the old

software from the smaller system while the new program is being developed.

Downward compatibility of software cannot be assured because programs

which run on a larger machine may utilise ladder functions and large quantities

of memory which are just not available on the smaller machine.

Just what additional functions will be provided depends upon the manufacturer

as no overall standard exists as yet, though many harmonising protocols have

been agreed as we shall see in the next lesson. The trend has been, however,

towards providing mathematical and logical functions as well as increasing

communication possibilities between PLCs and other microprocessor-based

devices. The provision of additional mathematical and logical functions makes

the programming of these PLCs similar to the programming of

microprocessor-based equipment in assembly language. The functions are

approaching those expected at this level but the use of utility type routines

keeps the programmer separated from the normal problems associated with

assembly programming.

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MATHEMATICAL AND DATA MANIPULATION FUNCTIONS

The simplest of microprocessors are expected to be able to perform tasks such

as addition and subtraction, whilst others may have the ability to multiply and

divide. Processors which don't have multiplication or division within their

instruction set can still be made to perform these functions and others, by the

use of small subroutine programs. It is not surprising, therefore, that such

functions may be included within PLCs even though they are not truly logic

functions as would be used in Boolean algebra. When PLCs are executing

arithmetic functions the data is likely to be assumed to be in binary coded

decimal form (BCD) so that the process conforms to decimal and not to binary

arithmetic. Using modern processors, sixteen bit (i.e. 4 four-bit) BCD

characters would be handled together, the data being obtained from specified

data channels comprising sixteen memory cells.

Consider, then, carrying out an arithmetic operation on the result of an A/D

conversion from a twelve bit converter. 12 binary digits give bit patterns

ranging from 000000000000 to 111111111111, i.e. 212 = 4096 possible values.

Assume that the data bit pattern from the conversion was 001101111111 from

which the decimal value 512 is to be subtracted. The PLC would have some

difficulty in subtracting a decimal number from a binary number and coming

up with the correct answer. The two numbers would need to have the same

number base so that the correct result can be obtained. For this reason the

function set should possess the ability to conduct binary to BCD and BCD to

binary conversions. Hence twelve-bit binary data taken from the A/D

conversion and stored in a data memory channel would be taken out of that

channel to be converted from binary to BCD before being placed back into

data memory as a 16 bit BCD number. The next rung in the ladder diagram

would be the subtraction of the decimal 512. After the subtraction the sixteen

bit BCD result would be stored back into another specified data memory

channel.

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Another typical operation is carried out by the COMPARE function. The

diagram of FIGURE 9 shows a simple oven arrangement. The operator would

set the value of the required temperature by placing a value on the thumbwheel

switches.

FIG. 9

The ladder diagram controlling the process reads-in the set point value from

the thumbwheel switches and stores it in memory. The analogue signal is then

read-in, converted into a binary value and stored. Some scaling of this value

may be required otherwise it could be immediately converted into a sixteen bit

BCD value before being placed back into a memory channel. The two BCD

values can now be handled by the compare function included as the next rung

in the diagram (see FIGURE 10).

• • PLC LESS

EQUAL

MORE

0 2 2 4

Temperature Sensing Transducer

Heating Element

Digital Output Module

Analogue Input Module

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FIG. 10

The rung is checked to see if it is complete. If it is, the compare function will

go ahead. The first data value is obtained from its specified location and it is

compared bit by bit with the data value found in the second specified location.

Flags, single bit memory cells, are set to 1 or reset to 0 on the result of the

compare operation. Three flags are normally employed. One flag is set if the

first number is greater than the second, otherwise it is reset. A second flag is

set if the two numbers are equal to each other, otherwise it is reset. The third

flag is set if the second number is greater than the first, otherwise it also is

reset. Therefore, immediately after the compare operation one of the three

flags must be set. If one, or more, of the flags are to be checked then this must

be the next operation to be carried out by the ladder diagram. If the compare

function shows the temperature to be less than the desired value then the

relevant flag is used to activate a physical relay which supplies the heating

elements. All three flags could be used to supply indicator outputs to show the

operator the present state of the oven.

CMP FUN ** DM010 DM025

'Greater Than' Flag Contacts

'Equal To' Flag Contacts

'Less Than' Flag Contacts

'Less Than' Flag Contacts

'Too Large' Output

'Equal' Output

'Too Low' Output

'Too Low' Output } Heater Control

Panel Indicator

Lamps

Enabling contacts

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An analogue output module could also be used to supply an analogue

voltmeter which has been calibrated to indicate the oven temperature.

It should be clear by now that the more special functions that exist the easier it

becomes to execute the programming of control tasks for a wide variety of

applications. Engineers who are used to programming microprocessor-based

equipment in machine code or assembly language take very readily to the full

range of special functions available because the function operations are very

similar to those that they already use. People new to these types of functions

often find great difficulty at first, partly because machine manuals don't seem

to be written with the "beginner" in mind. Manufacturers mistakenly assume

that prospective users are already acquainted with the intricacies of the special

functions which, of course, they frequently are not. This often deters users

from experimenting, which does not yield the full potential of the machine.

Now attempt the following Self-Assessment Questions.

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________________________________________________________________________________________

SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________

1. An A/D module with 12 bit resolution has an input voltage range of 0 V

to 5 V.

Estimate the value of analogue input voltage which would produce the

following output values:

(a) 005 Hex

(b) 0AA Hex

(c) 100000100111 binary

(d) 001101011010 binary.

2. Two 10 bit A/D circuits have the following input voltage ranges:

(a) 0 V to 5 V

(b) 0 V to 10 V.

Which one would be more sensitive to an input voltage change and why?

3. In the A/D circuit of FIGURE 4 reproduced below, explain the purpose

and operation of the component identified as (5).

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FIG.4 (Reproduced)

4. Why do some larger machines need to have a binary to BCD and a BCD

to binary conversion function?

5. State two different methods of saving a back-up copy of a PLC program.

D/A Circuit

Binary Up-

Counter Circuit

& •

• •

• •

• •

• •

• •

•

Analogue Output

Clock

Astable

NAND Gate

Comparator Analogue

Input + –

5

1

4Reset

Begin Conversion

Signal

3

2

Binary Digital Output to PLC

Conversion Complete

Signal

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________________________________________________________________________________________

ANSWERS TO SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________

1. 12 bit resolution gives 4096 possible values.

The bit value range, in theory, will be 1/4096 of 5 V i.e. 0.00122 volts.

(a) 005 Hex should be representative of an analogue value between

5 × 0.00122 V and 6 × 0.0122 V i.e. 0.0061 V and 0.00732 V

(b) OAA Hex is (0 × 256) + (10 × 16) + (10 × 1) = 170 decimal. The analogue value should be between

170 × 0.00122 and 171 × 0.00122 i.e. 0.2074 V and 0.2086 V

(c) 1000 0010 0111 binary is 827 Hex.

827 Hex is (8 × 256) + (2 × 16) + (7 × 1) = 2087 decimal. The analogue value should be between

2087 × 0.00122 and 2088 × 0.00122 i.e. 2.546 V and 2.547 V

(d) 0011 0101 1010 binary is 35A Hex.

35A Hex is (3 × 256) + (5 × 16) + (10 × 1) = 858 decimal. The analogue value should be between

858 × 0.00122 and 859 × 0.00122 i.e. 1.047 V and 1.048 V

2. Both A/D circuits have 10 bit resolution which means 1024 possible

values. Hence:

(a) would have a bit sensitivity of 1/1024 × 5 V = 5/1024 V (b) would have a bit sensitivity of 1/1024 × 10 V = 10/1024 V.

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Therefore (a) would be more sensitive to a small change requiring

only 5/1024 V to produce a one bit change at the output whereas (b)

requires a 10/1024 V change.

3. Component (5) in FIGURE 4 is the analogue comparator device.

This component accepts values of analogue voltage at each of its two

inputs, compares these values with each other and produces an output

dependent upon their relationship. As a means of identification each

input terminal is identified by either a + or a – symbol. If the voltage at

the – terminal is less than that at the + terminal then the output will be a

logic 1. If the voltage at the – terminal is greater than that at the +

terminal the output will be a logic 0.

4. In some machines certain functions, typically mathematical operations,

are executed with the expectation that the data being handled will be in

BCD form. However, the data may not be in this form but may instead be

in binary. So a conversion from binary to BCD may be needed

immediately before the mathematical operation.

A second example of the need for conversion between these two can be

seen from a typical application.

If you imagine an operative setting a value on a set of thumbwheel

switches (it could be a temperature, a number of turns on the coil winding

machine etc.), then the value would be set up in a BCD form on these

switches. When the PLC reads the switches then BCD data will be taken

in. If the operation requires this data to be compared with the value of an

internal counter, for example, then a direct comparison cannot be made

unless a conversion is first carried out.

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5. Any two from:

(a) CD-ROM

(b) EPROM emulator

(c) EPROM.

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________________________________________________________________________________________

SUMMARY ________________________________________________________________________________________

This lesson has covered aspects of analogue to digital conversion from the

point of view of digital representations, methods of effecting the conversion,

the need for multiplexing and the ladder functions which set up the control

signals to produce the action expected. Methods of saving programs to

different mediums and a brief look at some of the additional functions

available were also considered in this lesson.

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