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MODULE TITLE: PROGRAMMABLE LOGIC CONTROLLERS

TOPIC TITLE: PROGRAMMABLE FACILITIES

LESSON 1: LATCHING RELAYS AND BASIC TIMERS

PLC - 6 - 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 ________________________________________________________________________________________

This lesson covers, as revision, some of the previously learned basic material

which must be known because of its fundamental importance in PLC theory.

By continually using the basics it is possible to make advances whilst

consolidating work already covered. The explanations given continue to show

ladder diagrams in different formats so that you can learn different types of

representation.

The work covered takes us through the final steps in switching logic, and

clarifies the presentation of ladder diagrams before progressing to introduce

new items such as retentive devices and basic timer operation.

________________________________________________________________________________________

YOUR AIMS ________________________________________________________________________________________

On completing this lesson you should be able to:

• organise the order of presentation of ladder diagram elements such

that they will be suitable for the majority of PLCs

• understand the idea of programming ladder diagram blocks

• explain the difference between retentive and non-retentive device

identification and operation

• explain the basic operation of PLC timer facilities.

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________________________________________________________________________________________

STUDY ADVICE ________________________________________________________________________________________

This lesson makes reference to previous lessons so ensure that you have them

at hand.

________________________________________________________________________________________

CORRECT PROGRAMMING METHODS ________________________________________________________________________________________

All pieces of computer equipment are programmed in ways which follow

strictly laid down rules. If the programmer programs in one way and the PLC

interprets in another, then, quite often, the computer program either won't run

at all or it will run in a way different from that expected. People new to

computer equipment always seem amazed when a program doesn't run

properly due to, what appears to be, the most trivial reason. Comments such

as: "Well, it should have worked. I only missed out a comma", are all too

common. In programming a PLC the same level of exactness is required.

Admittedly the program listing may not contain commas, full stops, question

marks etc. but the characters which are allowed must be in precise order before

being keyed into the machine.

At this point we will revise some material from previous lessons and use this to

illustrate what the PLC will accept and how it interprets it.

Consider a practical circuit problem.

Four switches SW1, SW2, SW3 and SW4 are used to control a load. The load

is switched on if SW1, SW3 and SW4 are all closed at the same time or if

SW2, SW3 and SW4 are all closed at the same time. Let us examine some of

the circuit wiring possibilities.

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FIG. 1

FIG. 2

FIG.3

Electricians are trained to observe what is called correct polarity, which

essentially means that if this were a mains powered circuit the switches would

always be placed in the live conductor and the load would always be connected

to the neutral conductor. This same order of presentation was mentioned in

Lesson 1 of Topic 4 (on ladder diagrams) where the convention is that the load

is always placed last on the rung.

Load

Supply Supply

SW4SW3

SW2

SW1

Load

Supply Supply

SW4 SW3

SW2

SW1

Load

Supply Supply

SW4 SW3

SW2

SW1

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An acceptable diagram would therefore be:

FIG. 4

We should consider the question:

"does the order of presenting the switches make any difference?"

In an attempt to answer this question we will refer back to the work of

Lesson 2 in Topic 4. At this time we looked at Boolean expressions which

were written to represent the functions fulfilled by switching circuits.

The diagram and expression shown below serve as a reminder.

FIG. 5

Expression OUT 23 = 001 AND 002

OUT 23 = 001.002

Write down an expression for the circuit of FIGURE 6.

Out 23

002001

Load

Supply Supply

SW4SW3

SW2

SW1

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FIG. 6

OUT 24 =

Now write an expression for the circuit of FIGURE 7.

FIG. 7

Notice that the circuit operation is the same as the circuit of FIGURE 6.

OUT 24 =

Before going on, check your answers with those given on page 31.

You should notice that a degree of clarity has been added to the expressions by

the use of brackets.

004 003

005

Out 24

003 004

005

Out 24

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If such expressions are to be keyed-in to a PLC then it is unlikely that the

keypad would show any brackets. The clarification would therefore not be

possible.

A program listing for the expression of the diagram of FIGURE 7 is given

below in FIGURE 8.

Check through this simple listing – do you think it is correct?

STEP OP CODE OPERAND

001 LD 004

002 AND 003

003 OR 005

004 OUT 24

FIG. 8

________________________________________________________________________________________

Well, a particular machine may accept the program listing in the form given,

but the machine's interpretation of the circuit operation is not likely to be the

same as that of the actual circuit!

Consider the circuit of FIGURE 9.

FIG. 9

004 003

005

Out 24

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Write down an expression for this circuit.

OUT 24 =

Answer given on page 31.

Check the listing given below against the expression provided for the circuit of

FIGURE 9.

STEP OP CODE OPERAND

001 LD 004

002 AND 003

003 OR 005

004 OUT 24

FIG. 10

Now, compare the diagrams of FIGURE 7 and FIGURE 9 and the listings of

FIGURE 8 and FIGURE 10.

You should notice that the two diagrams are different but the two listings are

the same!

If the listing were keyed in, which circuit operation would the PLC perform?

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

________________________________________________________________________________________

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The answer is that the circuit of FIGURE 9 is the most likely interpretation.

Were you correct?

If this is the case then how can we program the PLC such that it will behave in

the same way as the circuit of FIGURE 7?

Checking the diagram we can see that OUT 24 will be energised if input 004 is

closed at the same time as one or other or both of inputs 003 and 005. If the

circuit is redrawn but the operation remains the same then we can obtain

another diagram which is equivalent to that of FIGURE 7. This has been done

with FIGURE 11 below.

FIG. 11

Examine the diagram to ascertain similarity of operation.

A suitable listing may be:

STEP OP CODE OPERAND

001 LD 003

002 OR 005

003 AND 004

004 OUT 24

Check this listing against the diagram of FIGURE 11.

003 004

005

Out 24

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Notice now that the circuit behaves in the same way as that of FIGURE 7 but

the listings for the two circuits are not the same.

GENERAL RULE

It is wise and the convention to draw ladder diagram circuits with multiple ORed

contacts nearest to the positive rail (left upright). For example, the circuit of

FIGURE 12 (below) would be better drawn as the circuit of FIGURE 13.

FIG.12

FIG. 13

Similarly the circuit of FIGURE 14 would be better drawn as the circuit of

FIGURE 15.

002 001

003

Out 20

004

001 002

003

Out 20

004

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FIG. 14

FIG. 15

For you own benefit, write down a program listing for the diagram of FIGURE 13.

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

________________________________________________________________________________________

Out 25

006

001 002004

003

005

002 004

003

Out 25

005

006

001

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A suitable answer is given on page 31.

We can now return to the question posed earlier. Does the order of presenting

the switches make any difference?

In the circuit diagram sense the answer is no. However, in the ladder diagram

sense the derived listing is more likely to produce the anticipated operation if

the previously stated general rule is observed. A suitable ladder diagram for

reliable programming would be as shown in FIGURE 16.

FIG. 16

We may now attempt to compile program listings for slightly more

complicated switching circuits.

Consider the circuit of FIGURE 17.

FIG. 17

OUT 22 001

002

006 003

004

Load Y001SW1X001

SW2 X002

SW3 X003

SW4 X004

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Compile a ladder diagram listing for this circuit.

STEP OP CODE OPERAND

001

002

003

004

005

006

007

Now examine the circuit of FIGURE 18. I hope that you agree that the circuits

of FIGURE 17 and FIGURE 18 behave differently!

FIG. 18

Now compile a listing for the diagram of FIGURE 18.

OUT 22 001 006

002 003

004

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STEP OP CODE OPERAND

001

002

003

004

005

006

007

Compare this listing with the previous listing (for FIGURE 17). It is very

likely that the two listings have turned out to be the same! The two circuits,

however, are not the same. If your two listings are the same how would you

expect the PLC to know which circuit behaviour to perform? Clearly only one

interpretation can be made by the machine and that is likely to be for the

circuit of FIGURE 18. How then can we program for the FIGURE 17 circuit

operation?

To avoid misinterpretation between the programmer and the machine some

manufacturers utilise a block programming approach. This means that the

rung must be diagrammatically broken up into distinct blocks before

programming. The diagram of FIGURE 17 would become two blocks as

shown in FIGURE 19.

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FIG. 19

Each block is programmed as a separate rung and then linked together before

driving the output.

A suitable listing may be:

STEP OP CODE OPERAND

001 LD 001

002 AND 006 BLOCK a

003 OR 002 ................................................

004 LD 003 BLOCK b

005 OR 004 ................................................

006 AND LD ..............................THE LINK

007 OUT 22

The AND LD instruction logically ANDs the two partially completed blocks, a

and b.

If block programming appears too troublesome there are other ways of

obtaining the same circuit behaviour. The two diagrams of FIGURE 20 and

001

002

006

Block a Block b

OUT 22 003

004

LINK

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FIGURE 21 will produce the same effect as FIGURE 19. Inspect each circuit

in turn to check that you agree that this is so.

FIG. 20

R001001 006

002

OUT 22003 R001

004

By using a memory relay (R001)

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FIG. 21

When programming PLCs there are often many acceptable solutions to a given

problem. It is only in the simplest of circuits that just one correct solution is

likely.

The diagrams of FIGURES 20 and 21 have the disadvantage of making the

program listings longer because more circuit elements are specified. This also

means that more memory will be taken up when these are programmed into the

PLC. Program memory is limited (often by only a given number of steps

being allowed) and should not be wasted by inefficient programming methods.

OUT 22001 006

001 006

003

004

003

004

002

By splitting the circuit up and identifying every possible flow path

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________________________________________________________________________________________

LATCHING RELAY OPERATION ________________________________________________________________________________________

In certain applications of programmable controllers it may be required for the

system to "remember" the occurrence of an event for future use. In the human

sense we remember events by storing them in our memory and this same

method is used by PLC programmers. The advantage with a PLC is that the

event can easily be stored as data within the electronic memory circuits and

can, just as easily, be forgotten by having the data cleared from the same

circuits. The PLC can, therefore, be programmed to "remember" or "forget"

quite readily.

We don't need to look very far to find a practical example of an event which a

PLC may be required to remember. Consider, for instance, the operation of a

pelican crossing.

The green road traffic light is normally biased to be on for the majority of the

time so that the road traffic is kept on the move. When a pedestrian wishes to

cross the road a push switch must be pressed to make the traffic lights change

from green through to red to cause the traffic to stop. The change in the road

traffic lights will not happen immediately the push has been operated, but the

controller will remember the switch press event. The pedestrian may stop

pressing the push switch once the event has been recognised and remembered.

This is signalled back to the pedestrian by the lighting up of the wait sign.

We can design a circuit to fulfil this part of the operation. Consider the circuit

diagram shown in FIGURE 22.

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FIG. 22

The circuit is quite readily converted into ladder diagram format for PLC

programming (returning to ladder diagrams using the X, Y, etc. format).

FIGURE 23 shows an equivalent ladder diagram.

FIG. 23

X001

R000

Y004

not complete

PUSH

R1/1

RELAY

R1/2

R1/3

WAIT SIGN

Relay used to remember the pressing of the push switch

Relay retaining contacts

Contacts used to signal to the next step in the light change sequence (rung not complete)

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When the push switch (X001) closes the R000 relay turns on. Once on, all

contacts carrying the R000 identification will operate. In the diagram the N/O

R000 contacts connected across the X001 input will close causing the R000

relay to be retained or held in the on state. The X001 contacts can now be

opened by the pedestrian releasing the push switch but the R000 relay will

remain in the on state, hence remembering that the switch has been pressed.

The R000 contacts in the second rung will also close and so the Y004 output,

supplying the wait sign, will also be on.

The R000 contacts in the third rung must be used to supply a signal for the rest

of the light change procedure to be initiated.

This arrangement, therefore, remembers the event of pushing the switch.

However, once R000 is energised by the closing of the input contacts (X001) it

will remain on permanently – even after the pedestrian has crossed the road.

Only one method would be available to turn off the R000 relay and that would

be to switch off the supply! Otherwise R000 is said to be latched in the on

state.

Clearly the ladder diagram requires alteration to provide a means of resetting

the R000 relay to the off state. A simulated loss of supply can be performed

by programming a suitable set of N/C contacts in series with the relay. This

inclusion into the first rung in shown in FIGURE 24. The contacts (arbitrarily

called R013) may, for instance, open when the red and amber road lights

operate. This would extinguish the wait sign and reset the R000 relay at a

suitable point in the light change sequence.

In this simplified explanation the setting and resetting of the relay is controlled

from the same rung of the diagram. Further, the relay may be reset by a loss of

power supply which may be a decided disadvantage in applications where the

resetting must be controlled.

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FIG. 24

________________________________________________________________________________________

RETENTIVE LATCHING RELAYS ________________________________________________________________________________________

If certain relays are on before a power failure but are subsequently not on when

the power is restored after the power failure then the operation of the PLC will

be affected.

For this reason most manufacturers allocate an area of fixed size within the

battery-backed RAM which may be used for retentive latching relay purposes.

If relays within this area are latched on and a power failure occurs the latching

is remembered during the power failure because of the battery backing. When

power is restored and the PLC restarts its program it can continue from the

point at which power was lost. Physical output relays are not normally

retentive.

The programmer needs to know how many retentive relays have been

provided. The PLC needs to be able to determine whether the programmer

X001

R000

Y004

not complete

R013

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intends the use of a retentive relay or a non-retentive relay within a given

program. To achieve this, retentive relays may require an identifying letter or

may use identifying numbers within a range allocated for this purpose.

Alternatively, they may require a different programming method to be used to

signify the retentive device.

When latching devices are used the SET conditions and the RESET conditions

are sometimes specified by different lines of programming which may appear

some distance apart (this method needs to be watched out for).

In almost all cases if both the set and the reset conditions arise at the same

time then the reset conditions will have priority (the latching device will not

latch on).

The two diagrams below are provided as examples of retentive and non-

retentive programming methods used by leading PLC manufacturers.

FIG. 25

X004

R000

X004

L000

X005

L000

X005

Non-retentive through power failure

Retentive through power failure

The difference is in the use of R or L relay identification.

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FIG. 26

At this point we will move away from straightforward switching logic to

examine some of the other programmable facilities commonly available within

the PLCs. Such facilities, which are not strictly switching, may not follow

Boolean rules. This gives wider possibilities of variations between

manufacturers. It is, therefore, very important not to make the mistake of

assuming a machine's operation without close reference to the manufacturer's

manual.

004

MR000

004

KR000

005

Non-retentive through power failure

Retentive through power failure

SET LINE

RESET LINE

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________________________________________________________________________________________

TIMERS ________________________________________________________________________________________

For many years the ability to introduce some form of timing or time delay into

the operation of a circuit has proved to be a very valuable asset to control

technology. Timers of various descriptions have been used in the past for a

variety of applications. At one time a manual operative would be employed to

switch a load on or off at a given time of the day. This practice is now

considered costly and unacceptable. Time switches, synchronised to the

frequency of the a.c. mains supply, are used to maintain switching at preset

times of the day or night. Such devices are ideal for switching on/off mains

loads at the same time each day (central heating boilers, security lights etc.).

Unfortunately, because they cover a 24 hour period, the settings may not be

very exact and the smallest timed period (often limited by the mechanical

construction) would be just too large for many applications. Certain other

applications may require very short time periods which are not necessarily

related to the time of day.

Consider, for example, the STAR/DELTA starting requirements of a three-

phase induction motor. At one time (and occasionally even now), motors of

this type used to be started manually. The operator would move the starting

lever to the STAR position for starting, then wait for a short period to allow the

rotor to pick up speed before switching over to the DELTA (run) position to

allow the machine to attain its normal running speed. The introduction of a

mechanical timing device (similar to clockwork) allowed the same motor to be

started and run up to speed remotely from the operator.

In pneumatic control, similar delay timing can be achieved by allowing

compressed air to enter a chamber via a restriction. The air pressure within the

chamber gradually increases until it is sufficient to operate an electrical

pressure switch. In this case the switch operation is delayed by a period of

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time determined by the air pressure of the supply line and the nature and size

of the restriction.

More recently, and certainly since the introduction of integrated circuits,

electronic timers have become more widely available. For industrial purposes

these can be obtained in self-contained modules which clip onto DIN rail and

have the electrical connections provided by terminals at the top and bottom.

Quite often the time setting is adjustable over a given range by turning a dial so

that a pointer indicates on a scale the time setting chosen. When settings

outside of the range are required the timer unit must be replaced by a different

unit.

Obviously in the few types of timing devices mentioned the number and type

of electrical switching contacts provided are very limited.

The provision of timers within programmable logic controllers is considered to

be a basic facility even though a timer may not be considered to be a logic

device in the same way as an AND gate, OR gate etc. The need to purchase

separate timers is therefore avoided. A PLC timer facility may well rely upon

the operation of an electronic hardware timer I.C. fitted as part of the

microcomputer hardware. The use of the timers is, however, completely

arranged by the programming within the ladder diagram or program listing.

As with relays etc. timers can have any number of contacts, unlike the time

switches and other timers previously mentioned.

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________________________________________________________________________________________

TIMING OPERATIONS ________________________________________________________________________________________

When incorporating timers into ladder diagrams one of two modes of operation

is required.

DELAY IN SWITCHING ON

In this mode of operation a timer may be "enabled" to allow it to begin timing.

After a preset time period has elapsed the timer output will switch on. Any

contacts carrying the same identification as the timer will then operate i.e. N/O

timer contacts will close and N/C timer contacts will open. In this way a delay

exists between the timer being "enabled" and the timer output coming on. In

most cases, timers are preloaded by the programmer with a value representing

the required time period. As specified time intervals pass, the preset value is

decremented (i.e. reduced) by one. When the value is reduced to zero the timer

output will be switched on. Typically, the time intervals used are 0.1 second or

1 second.

If, for example, the timer has a 1 second timing interval and it is loaded with

the value 5 then, when it is enabled, the value 5 is reduced to 4 after one

second, further reduced to 3 after the next second and so on until its value is

zero. The timer output will then switch on. If, however, during the timing

sequence, the timer loses its enable signal then the preloaded value (5 in this

case) is immediately reinstated into the timer and the timer does not begin

decrementing again until it is next enabled. It is not uncommon for some

manufacturers to provide timers having different time intervals i.e. some with

1 second increments and others with 0.1 second increments. At least one

manufacturer leaves it to the user to select the timer interval by providing a

link wire which can be connected to one of a number of terminals inside the

machine so as to define a specific time increment. The user obviously needs to

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know what time interval is being provided. Here again assumptions should not

be made – consult the manual.

The ladder diagram of FIGURE 27 illustrates the arrangement for a typical

delay-in-on timer operation. In this case a timer identified as 06 is enabled by

the closing of the input contacts X003.

PROGRAM LISTING

STEP OP CODE OPERAND

019 LD X003

020 TIM 06 005

021 LD TIM 06

022 OUT Y000

FIG. 27

If input contacts X003 close, timer 06 begins to decrement. If X003 remains

closed continually for 5 seconds the timer output switches on. In rung 2 the

N/O contacts identified as TIM06 will close, thus switching on the physical

output Y000. Timers themselves are not physical outputs and so they must be

used to drive a physical output if an external effect from the timer is to be

obtained.

TIM 06

Rung 1

Rung 2

Y000

5 seconds

X003

TIM 06

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FIGURE 28 is a timing diagram which can be used to show the state of each

element in the ladder diagram with the passage of time.

FIG. 28

The timing diagram is read from left to right along the time axis. The timing

diagram shows that the output Y000 does not switch on until 5 seconds after

TIM 06 has been enabled by the closing of input X003. The timer output will

remain switched on until its enable signal is lost i.e. in this case until input

contacts X003 are opened.

DELAY IN SWITCHING OFF

The second mode of operation to be considered occurs when a timer is used to

produce a delay before switching something off. With this mode an output

may be switched on by the occurrence of a defined condition but the output

will only remain on for a specified period of time before being switched off.

Closes Opens

X003

TIM06

Y000

TIM06 output turns off

Y000 turns off

Time axis5 second delay

between X003 coming on and Y000 coming on

Off

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The diagram of FIGURE 29 can be used as an example of this operation.

PROGRAM LISTING

STEP OP CODE OPERAND

016 LD X001

017 AND X003

018 AND NOT TIM 04

019 OUT Y002

020 LD X001

021 AND X003

022 TIM 04 007

FIG. 29

The switching logic of rung 1 is such that Y002 will be switched on if inputs

X001 AND X003 are both closed AND TIM 04 is not switched on.

i.e. Y002 = X001 AND X003 AND NOT TIM 04

Y002 = X001.X003.

The switching logic of rung 2 is such that TIM04 will be enabled to begin

timing if inputs X001 AND X003 are both closed. If they remain closed for a

period in excess of 7 seconds then, after 7 seconds, the TIM04 output will

switch on.

TIM 04

Y002

Rung 1

Rung 2

TIM 04

7 seconds

X001 X003 TIM 04

X001 X003

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The operation of the ladder diagram can now be assessed. When X001 and

X003 are both closed then the output Y002 will switch on and for the same

reason TIM 04 will be enabled to begin timing. If X001 and X003 remain

closed for 7 seconds then the timer will time out and TIM 04 output will come

on. All contacts with TIM 04 identification will change state. Therefore, in

the next program pass the N/C TIM04 contacts in rung 1 will open and switch

off the Y002 output. Y002 was switched on by the closing of X001 and X003

but only remained on for a delay of 7 seconds (the preset time of TIM 04).

The timing diagram of FIGURE 30 indicates this operation.

FIG. 30

The TIM 04 output will be disabled if either or both of X001 or X003 are

opened.

Y002

TIM04

Off

Time axis

X001

X003

X001 AND X003 AND NOT TIM04

7 second period

Timer 04 enabled

Timer 04 output on

Timer 04 output disabled by the

loss of X001

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Carefully examine the timing diagram to ensure that you are familiar with the

circuit operation.

You should now attempt the Self-Assessment Questions given on page 32.

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________________________________________________________________________________________

ANSWERS TO LESSON QUESTIONS ________________________________________________________________________________________

FROM PAGE 5

Expression for the circuit of FIGURE 6.

OUT 24 = (003 OR 005) AND 004

OUT 24 = (003 + 005).004

Expression for the circuit of FIGURE 7.

OUT 24 = 004 AND (003 OR 005)

OUT 24 = 004.(003 + 005)

FROM PAGE 7

Expression for the circuit of FIGURE 9.

OUT 24 = (004 AND 003) OR 005

OUT 24 = (004.003) + 005

FROM PAGE 11

Program listing for the ladder diagram of FIGURE 13.

STEP OP CODE OPERAND

001 LD 002

002 OR 003

003 OR 004

004 AND 001

005 OUT 20

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________________________________________________________________________________________

SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________

1. Draw a ladder diagram for each of the five expressions given below.

(a) OUT 22 = (002 AND 004) OR (003 AND 006)

(b) OUT 24 = 003 AND (004 OR 002 OR 007)

(d) Y007 = (X005 AND ) OR R006

(e) Y000 = ((X001 OR R000) AND X003) OR R004

2. Explain the difference between retentive and non-retentive relays/devices.

3. FIGURE 31 shows a ladder diagram which incorporates:

X devices as physical inputs

R devices as memory relays

Y devices as physical outputs

and TIM devices as timers.

X003

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FIG. 31

The X001 input is connected to a push-to-make switch similar to a bell

push.

If this switch is momentarily pressed and then released at the point in time

indicated on the timing diagram of FIGURE 32 (when the condition of

the circuit elements is as indicated) then complete the remainder of the

timing diagram.

X001

R001

R002

R001

TIM04

TIM03

TIM04

Y003

R001

Y002

TIM04

R002

TIM03

2 seconds

4 seconds

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Timing diagram for Question 3:

FIG. 32

1 sec

time intervals

TIM04

X001

R001

R002

TIM03

Y002

Y003

Off

Time when X001 closes

Time axis

Level of the ON state

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________________________________________________________________________________________

NOTES ________________________________________________________________________________________

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

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________________________________________________________________________________________

ANSWERS TO SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________

1. The ladder diagrams of FIGURE 33 show the implementation of the

expressions.

FIG. 33

(a)

(b)

002 004

003 006

002

004

007

003

002 003

004 005

X005 X003

R006

X001

R000

X003

R004

OUT 22

OUT 24

OUT 23

Y007

Y000

(c)

(d)

(e)

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2. In most respects retentive and non-retentive relays/devices behave in the

same way. One special difference, however, is in their behaviour during

and after a power-down condition.

Non-retentive devices are served by non battery-backed RAM which

means that the condition of such devices (whether on or off) is lost during

a power failure event and cannot be expected to be the same after the

power is restored. Retentive devices, on the other hand, will be served by

battery-backed RAM which means that the on/off condition before a

power failure will be retained and reinstated at the point in time when

power is restored.

3. Compare your timing diagram (FIGURE 32) with that given as FIGURE 34

below.

FIG. 34

TIM04

X001

R001

R002

TIM03

Y002

Y003

Off

4 seconds 2 seconds Time when X001 closes

Teesside University Open Learning (Engineering)

________________________________________________________________________________________

SUMMARY ________________________________________________________________________________________

In this lesson we have covered additional work on ladder diagrams and

completed basic switching logic. The new elements introduced were retentive

and non-retentive latching relays and PLC timer facilities. The next lesson in

this topic will cover a little more work on timers and their application within

practical problems.

Teesside University Open Learning (Engineering)

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