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PLC Programming for
Industrial Automation

Contents
Introduction
PLC Basics
Function of a PLC
Inputs and Outputs
PLC Architecture and Wiring Diagrams
Network Protocols
Questions
Ladder Programming
Conditional Logic
Ladder Diagrams
Normally closed contacts
Outputs and latches
Internal relays
Timers
The Pulse Generator
Counters
Questions and exercises
Sequential Programming
Introduction
A Simple Automation Sequence
Evolution of the Sequential Function Chart
Programming using the Sequential Function Chart
Entering the SFC program into the PLC
Modifying an SFC Program

Selective Branching
Parallel Branching
Appendix: Using the TriLogi software

Introduction
I have been teaching PLC programming for fifteen years and the question that
I hear most often from students is “Can you recommend a book on this?” In response
I have trotted out the titles of various standard text books but I have never come
across a book that really develops the skill of PLC programming instead of telling the
reader what PLCs are all about
...

“What sort of PLCs do you use?” is another popular question
...
I deliberately teach a generic style of
programming that allows the learner to switch between types of PLC as easily as
between different makes of electronic calculator
...
The students can load the software onto a computer
and practice the examples and exercises provided
...
Ladder logic is by far the most popular
programming language in use because of its resemblance to hard-wire control
diagrams
...
As the
automation task grows so the ladder program expands organically, until only the
original programmer can find his way through the tangle of inputs and outputs, relays
and function blocks
...
The solution
is to plan the program using a sequential function chart and then to enter it into the
PLC using ladder logic
...

The first two chapters of the book are used for programming basics
...

The examples used in the book have all been thoroughly tested and their
suitability for use in the classroom and in industry established
...
1 Function of a PLC
A PLC is a microprocessor-based controller with multiple inputs and outputs
...

The PLC performs the logic functions of relays, timers, counters and
sequencers
...
1 A programmable logic controller
1
...
These are typically switches and sensors
...

Input module circuits have opto-isolators to protect the internal PLC
circuitry from damage
...
2 An Opto-Isolator

The PLC outputs are connected directly or indirectly (e
...
through a relay) to
actuator controls
...

There are three main types of output module:
Relay (volt-free): The signal from the PLC operates a relay within the output
module connecting the control voltage to the output port and hence to the actuator
...
3 PLC Relay Output
Transistor: A transistor is used to switch on the output
...

Triac: This solid state device is used for switching alternating current
devices
...


1
...
4 PLC Connections

S1

S2

S3

X1

X2

X3

Y1

Y2

X4

Y3

Fig 1
...
4 shows a pictorial view of the PLC with its connections
...
5
...
4 Network Protocols
The wiring diagram in Fig 1
...
The devices shown are on/off or digital in nature but the
signal to the PLC is analog
...

With analog devices, a separate cable needs to be run between the end device
and the control system because only a single analog signal can be represented on the
circuit
...
Fieldbus is a multi-drop digital two-way communication link
between intelligent devices
...
Fieldbus comes in many varieties
depending on the manufacturer and application
...

A more recent trend is the development of Industrial Ethernet which has the
capacity to transport large quantities of data not only for process control but also to
integrate the process with management information systems
...
5 the programs are designed to receive data
from inputs and to send data to outputs regardless of the network system being used
...


Switches, proximity devices and sensors are generally used in what way in a plc
application?
Answer:

a
...
Software elements
c
...
Outputs

2
...
6
In the diagram Fig 1
...

Why would it be necessary to
connect a PC?

Answer:

a
...

b
...
To edit the plc program
...
To store the input values
...


Which option below
best describes the Answer:
action of an optocoupler?

a
...

b
...

c
...

d
...


Fig 1
...
7 and pick the correct statement about it
...
When the power is switched on to the plc all the n/o relay contacts
shown close
...
The 24 V supply shown is used to power the plc itself
...
The plc energises an output by closing the relevant relay contact
...
When the plc outputs are energised they are all latched on by the relay
contacts shown

5
...
"
The description above best describes what type of switch?

Answer:

a
...
push button
c
...
relay

Chapter 2
Ladder Programming
2
...
The logic state of the output depends on the input
conditions and so the term conditional logic is used
...
”
The conditions could be realised by a hard wire solution as shown in Fig 2
...

+V


...
1 Hard-Wire Conditional Logic Example
The two start switches are connected in parallel
...
The start switches are normally open
...

The three stop switches are connected in series
...
The stop switches are normally closed
...


The relay is a switch with multiple contacts that is operated when its coil is
energised
...
Large relays for motor starting are called contactors
...
2
...
2 Relay

2
...
1 using ladder logic we
connect the switches to a PLC as shown in Fig 2
...


Start 1

Start 2

X2

X1

Stop 1

X3

Stop 2

X4

Y1

Machine Relay
Fig 2
...
3 and remembering that the plc
scans each input and asks “Is it on or is it off?” The five switches shown are external
devices and the PLC knows nothing about them
...

I have written the ladder logic using the TriLogi software
...
4 PLC Ladder Diagram
It can be seen from the Fig 2
...
4 that the output machine will not be
energised until one of the inputs Start 1 or Start 2 is switched on
...
It is normal
practice to use normally closed push-button switches for stop buttons so that a failure
of control voltage supply has the same effect as the pressing of the stop button
...
3 Normally closed contacts

Fig 2
...

The contact Start 1 in Fig 2
...
Switching on the input opens the contact
and switches the output off
...


2
...

Fig 2
...
7
...
6

Fig 2
...
When the output DCV is off the green light is on
...
1
Write a PLC program to implement the conditional logic statements (a), (b)
and (c) below
...

(b) A PLC output is to switch on if any one of three inputs is switched on but
not two or more
...

Solution
(a)

(b)

Fig 2
...
9

This program can be read:
“The output switches on if Input 1 is on AND the other two are off, OR input 2 is on
AND the other two are off, OR input three is on AND the other two are off
...


Fig 2
...
A spring action reverts the switch to the
normal state as soon as the button or roller is released
...

The fact that the majority of control switches are not self-latching is not as
inconvenient as it sounds
...

Start

X1

Stop

X2

Y1

Fig 2
...
11 is pressed, the output Y1 is to switch
on and stay on until the stop button is pressed
...
12
When the output Y1 is energised we use a normally open contact of it in parallel with
the start button to hold (or latch) it on
...
Note that we have used a normally closed switch as a stop
button as explained in section 2
...


Start 1

Start 2

X2

X1

Stop 1

X3

Stop 2

X4

Stop 3

X5

Y1

Machine Relay
Fig 2
...
The
output Y1 in Fig 2
...
The required ladder diagram is shown in Fig
2
...


Fig 2
...
5 Internal relays
These have the same properties as outputs but they only exist in software
...
Fig 2
...
This is the inverse of the result of X1 AND X2
...


Fig 2
...
Throughout this book I use the
latch as described in section 2
...

2
...

For example, when a start push button is pressed, the pneumatic cylinder shown
in Fig 2
...
Draw the PLC
wiring diagram and the appropriate ladder logic
...
16

Start

X1

a+

X2

Y1

Fig 2
...
Any other components needed for the program can be
created in software
...
18
Pressing the start button latches on an internal relay called start_latch
...
The cylinder rod closes the limit switch a+ which starts the timer in
software
...


The timer set value in the TRiLOGI software is in units of 0
...
For a 5 s delay a
value of 50 is entered in the drop-down menu
...
19 Setting Timer preset Value
We can do another example using the same hardware with the addition of an alarm as
a second output:
Start

X1

Y1

a+

X2

Y2

Fig 2
...
An alarm sounds during the 5 s delay
...
21
When the start button is pressed the start_latch relay is energised
...
When the timer set value
has elapsed the alarm switches off and the solenoid Y1 is energised
...

The input to the delay-on timer must remain on for the duration of the timer set
value otherwise the timer will not operate
...
When the input to the timer switches off,
the timer contacts revert immediately to their normal states
...
22
...

In this book all timer function blocks are located at the right hand side of the ladder
diagram and their contacts, normally-open or normally closed, have the same label as
the timer
...
22
The delay-off timer causes a delay between its input switching off and its contacts
reverting to their normal states
...


2
...
This is best
illustrated by an example
...
23
Start

X1

Stop

X2

Y1

Fig 2
...
24 is pressed the cylinder in Fig 2
...


Fig 2
...
The flow chart in Fig 2
...


Start pressed

Stop
pressed?

N

Y
Timer 1 input on
Timer 1 delay
Output on
Timer 2 input on
Timer 2 delay
Timer 1 input off
Output off
Timer 2 input off

Fig 2
...
7 Counters
A counter allows a number of occurrences of input signals to be counted
...
A second input or software coil is provided to
reset the current value of the counter to zero
...
27
...
27
When a start button has been pressed the shaft is to make 10 revolutions and then
stop
...
The PLC wiring diagram is
shown in Fig 2
...

Start

X1

d

X2

Y1

Motor
contactor
Fig 2
...
29
The pulse generator and counter can be combined as shown in this final
example
...
Pressing the conveyor stop button also resets the counter
...
30 and Fig 2
...

Start

X1

Stop

X2

Y1

Conveyor
Fig 2
...
31

Questions and Exercises
1
...
32
Answer:

a
...
Y1 switches on if X1 is on AND either X2 is off OR X3 is on
c
...
Y1 switches on if X1 is off AND either X2 is on OR X3 is off

2
...
33
Answer:

a
...

b
...
Y2 switches on if X1 is off AND X2 AND X3 are off AND X4 is on
...
Y2 switches on if Y1 AND X4 are both on

3
...
34
Answer:

a
...
NOR
c
...
NAND

4
...
35
Answer:

a
...
NAND
c
...
AND

The PLC diagram Fig 2
...


Fig 2
...


Fig 2
...
37
Answer:

a
...
When the alarm is on, so is the light
...
Closing S1 and S2 switches on the alarm
...
Closing switch S2 switches on the alarm

6
...
38
Pick the one incorrect statement below regarding the ladder diagram Fig 2
...


Answer:

a
...

b
...
The alarm is switched on when S1 or S2 or S3 is on
...
The alarm is started when S1 or S2 is switched on but not both
together

7
...
39
Pick the one correct statement below regarding the ladder diagram Fig 2
...
If switch S3 is closed the light will be on
b
...

c
...
The light is switched on by pressing S1 or S2

8
...
40

Pick the one incorrect statement below regarding the ladder diagram Fig 2
...
S2 can be used to switch off the light before the timer delay is
complete
...
The light output is latched on
...
When the timer delay is finished the light comes on
...
The light remains on for a time equal to the timer delay setting
...


Fig 2
...
41
Answer:

a
...

b
...

c
...

d
...


10
...
42
In the ladder diagram Fig 2
...
Choose the one incorrect
statement below
Answer:

a
...

b
...

c
...

d
...
A PLC is to be used to control a flood light
...

Draw the necessary PLC wiring diagram and the ladder logic to operate the system as
designed
...
A PLC is to be used to control the drive for a car window
...
If the switch is closed for
more than 1 second, the window contunues opening until fully open
...
Limit switches are provided to detect the
window fully open or fully closed positions
...

13
...
A sensor with a normally open
contact sees items passing on the conveyor
...
Draw the necessary PLC wiring diagram and the ladder logic to
operate the system as designed
...
1 Introduction

Chapter 3
Sequential Programming

Most machine operations are sequential in nature so it is necessary for the
PLC to switch outputs depending not only on the input combinations but also on the
current stage in the sequence
...

3
...
1
The two cylinders A and B in Fig 3
...
The 5 port 2 position
directional control valves (5/2 DCVs) are double-solenoid operated
...
2
...
2
Each event in a sequence is started by the completion of the previous event
...

We will write the program line-by-line on this basis
...
3
Pressing the Start button causes the cycle to execute once
...
This is shown in
Fig 3
...


Fig 3
...


A+ B+ B- A-

Fig 3
...
Everything
works fine until we get to the third rung of the program where the reed switch b+ is
supposed to energise solenoid Y4 to cause cylinder B to return
...
6
...
6
The fact that cylinder A is also extended means that reed switch a+ is closed
and therefore solenoid Y3 is energised, cylinder B therefore cannot return
...
It is characterised by having both solenoids of a
double solenoid directional control valve simultaneously energised and it prevents us
from programming many circuits in a simple sequential fashion
...

Trapped signals also occur in pneumatics and in electro-pneumatics and various
methods are employed to get over the problem
...

3
...

In 1975 a working group, drawn from the Association Française pour la
Cybernétique Economique et Technique, developed GRAFCET (GRAphe Foncionnel
de Commande, Etapes, Transitions) which has since formed the basis of the
Sequential Function Chart method of programming
...
The standard identifies 5 distinct programming
methods including ladder logic and the sequential function chart
...

Ladder Programming has become as one of the most popular graphical
languages for programming PLCs mainly because of its ressemblance to hard-wire
control circuits
...

The Sequential Function Chart (SFC) is an extremely effective graphical
language for expressing the high level sequential parts of a control program
...

3
...
7 Sequential Function Chart
The system passes through successive states during which events take place
...


a-

a+

Y1
Fig 3
...
1 When a start push button is pressed the cylinder in Fig 3
...
When the system is
powered up and ready a green light Y2 is on and while the cylinder is oscillating, a
red light Y3 is on
...

Start

X1

Y1

Stop

a+

X3

X2

a-

X4

Y2

Fig 3
...
10 Completed SFC

States: S1, S2 and S3 are respectively States 1, 2 and 3
...

e
...
S2 (A+)•(red light) means that when state 2 is active cylinder A extends and
(the dot • means and) the red light is on
...
The
origin and destination states are indicated by the transition label
...
The label
(start )•(stop) under T1-2 means that the start button has been pressed and the stop
button has not been pressed
...
”
3
...


Each state and transition is assigned an internal relay
...
We write down the states and transitions
with their associated relays in an assignment list
...
1 Assignment List
Now we’re ready to enter the program
...


Initial Conditions
Transitions
States
Outputs
Timers
Counters
The ladder diagram is shown below broken into sections for clarity (Fig 3
...
15)
...

Initial Conditions
The only initial condition for this program is the latching on of the start button
by an internal relay
...


Fig 3
...
The relevant
state must be active and the input (or timer contact) that indicates that the event is
completed must be on
...
1 shows the transition conditions for this program
...
2
Now we can enter the transitions as ladder logic
...
12 Transitions
States
State 1 is the initial state
...
To do this we use the fact that no internal relays are
energised when the PLC is powered up
...
” Fig 3
...


Fig 3
...

The transition relays are often only briefly energised so the states are latched on
...
14 shows the ladder logic for states 2 and 3
...
14 States 2 and 3
Outputs
Each output is switched on during the relevant state or states
...
15
...
15 Outputs
It seems like an awful lot of ladder logic for a simple program (and it is) but
extra lines of software come free of charge and, as the programming tasks get more
complex, the merits of sequential function chart programming quickly become
apparent
...
2
...
16

Start

Stop

X1

Y1

a-

X2

X3

Y2

a

b-

b+

X4

X5

X6

Y3

Y4

Fig 3
...
18 and the assignment list in Table 3
...


S1
T1-2
(start)•(stop)
S2
(A+)
T2-3
(a+)
S3
(B+)

S4
(B-)
S5
(A-)

T3-4
(b+)

T4-5
(b-)
T5-0
(a-)

Fig 3
...
3

R1
R2
R3
R4
R5
R6
R7
R8
R9
R10

Now we can enter the ladder logic, remember the order

Initial Conditions

Transitions

Initial Conditions
Transitions
States
Outputs
Timers
Counters

Fig 3
...
20

States

Outputs

Fig 3
...
22

3
...

The modified pneumatic diagram is shown in Fig 3
...
24

Fig 3
...
24
The cylinder sequence remains unchanged so cylinder A still extends during
State 2 and must stay extended during states 3 and 4
...

This is shown in Fig 3
...


Fig 3
...

3
...
1 and 3
...
Sometimes timers or counters are used
instead of inputs
...

We’ll start with an example that includes a timer
...
1
a-

a+

Y1
Fig 3
...
27
Example 3
...
26 is to extend, remain extended for 3
seconds and then return
...
The SF chart is shown in Fig 3
...


S1
T0-1
(start )•(stop)
S2
(A+)
T2-3
(a+)
S3
(3s)
delay)

S4
(A-)

Fig 3
...
4

S1
S2
S3
S4
T1-2
T2-3
T3-4
T4-0
Initial Conditions:

Transitions:

Table 3
...
28

Fig 3
...
30

Fig 3
...
32
So when State 3 becomes active the timer starts (Fig 3
...
When the delay has
finished a normally open contact of the timer is used to activate Transition 3-4 (Fig
3
...


Now we’ll do a counter example
...
4
An indexing conveyor carries empty oil containers for filling and capping as
shown in Fig 3
...
The conveyor is indexed one pitch by three revolutions of the cam
on the drive shaft as shown in Fig 3
...


Fig 3
...
34
Write a PLC program that indexes the conveyor by one pitch when a start
button is pressed
...


S1
(wait)
T1-2
(start )
S2
(Conveyor
Drive)
T2-3
(C1)

Fig 3
...
36 PLC Wiring Diagram

S1
S2
T1-2
T4-0

R1
R2
R5
R8

Table 3
...
37
States:

Fig 3
...
39
Counter

Fig 3
...
39) and the closing of
the cam switch increments the counter
...
40)
...


Fig 3
...
42
The dcv and cylinder in Fig 3
...
43
...
Y1 in states 1 and 3, Y2 in state 2
would be energised?
b
...
Y1 in state 0, Y2 in state 3
d
...


Fig 3
...
43
...
A latch for state 0 is missing in the diagram
...
State 0 is energised by transitions
c
...
State 0 is switched on when all the other states are on

3
...
44
Select the only incorrect statement below regarding the diagram Fig 3
...


Answer:

a
...

b
...
A transition is enabled by an input or timer contact
...
TA, TB and TC are plc inputs

4
...
45

Fig 3
...
45 which is used to control the
pneumatic cylinder
A in Fig 3
...

Answer:
a
...
No output is switched on during state 3
c
...

d
...


5
...
47
Select the only incorrect statement below regarding Fig 3
...


Answer:

a
...
State 1 is switched on by Transition T0-1
c
...

d
...


Fig 3
...
49
Which option below correctly identifies the fault with the ladder conditions shown in Fig
3
...
49?
Answer:

a
...

b
...

c
...

d
...


Select the only incorrect statement below regarding Fig 3
...
State 2 is active for 15 seconds
...
When the plc is first powered up state 0 becomes active
...
It is not possible to mix transitions enabled by inputs and transitions
enabled by timers in the same program
...
When trans 3-0 becomes active state 3 switches off
...


8
...
51
Select the only correct statement below regarding Fig 3
...
Input 1 only switches the light off
...
Input 1 only switches the light on
...
There can be no default state in such a sequence
...
Input 1 alternately switches the light on and off
...

It is necessary to automate the drilling of 2 holes in the clamp body shown in Fig
3
...
Both holes are at an angle of 45º to the vertical
...
53
Two views of the drill assembly are shown in Fig 3
...
Each double acting
pneumatic cylinder is operated by a double solenoid 5/2 directional control valve
...

When the start button is pressed the sequence is as follows
1
...

2
...

3
...

4
...

5
...

6
...
The jig is rotated back through 90º (cylinder R)
...
The jig is clamped (cylinder I)
...

(a) Draw the appropriate state/transition diagram
...

(c) Draw the lines of ladder logic to operate the system as designed
...
52
...
53 Rotation Jig

Fig 3
...


Fig 3
...
55 shows a quick approach circuit for a drill
...
When the limit switch b1 is reached the drill continues slowly until the
end-of-stroke limit switch b2 is closed
...
The hydraulic pump may be assumed to run continuously
...

(b) Draw a suitable state/ transition diagram
...


4
...
E
...
if a product is good the process goes on to the next
operation, if it is bad the product is rejected
...

Example 4
...
1
When a Start pushbutton is pressed and the gate is closed the cylinder in
Fig 4
...
If the start button is pressed but the
gate is not shut, an alarm sounds until the gate is closed
...


The PLC wiring diagram is shown in Fig 4
...
3
Start

Stop

a-

a+

guard

plc

Y1

cycle on

alarm

Fig 4
...
3

S5
(alrm

)

T1-5
(strt•grd
shut)

T5-0
(grd
shut)

If the guard switch is closed and the start button is pressed then the transition 12 is activated and the cylinder cycle starts
...

The assignment list is shown in Table 4
...


Table 4
...
4

R1
R2
R3
R4
R5
R6
R7
R8
R9
R10
R11

Transitions:

States:

Fig 4
...
6

Outputs:

Fig 4
...
8
It is important to remember that the decision criteria whether to choose one
branch or another must be mutually exclusive
...
This is a suitable criterion because the switch cannot be simultaneously
open and closed
...
2
Here is another example that uses the selective branching technique
...
9
The cylinders above are to execute the sequence A+ B+ B- A- continuously
until a stop button is pressed when both cylinders will stop at the end of the
sequence
...

(a) Draw a plc wiring diagram for the system
...

(d) Draw the ladder logic to operate the system as designed
...
10

a1

b0

b1

(b)

S1

R1

T1-2

R9

S2

R2

T2-3

R10

S3

R3

T3-4

R11

S4

R4

T4-5

R12

S5

R5

T5-1

R13

S6

R6

T1-6

R14

S7

R7

R15

S8

R8

T6-7
T7-8
T7-8
T8-1

R17

Table 4
...
11

(d)
Initial Conditions:

Fig 4
...
13

States:

Outputs:

Fig 4
...
15

Timer:

Fig 4
...
3
Here is a slightly more complicated example of a process that uses selective
branching
...

A process liquid is prepared in a tank as shown in Fig 4
...
The tank is filled
with liquid from pump A until the high level switch is reached, then a small amount
of liquid is added by running pump B for 10 seconds
...

Additive pump
B
stirrer

HL

inflow pump A

heater
LL

Fig 4
...
10

S1
(wait)

T1-2
(start enabled
& LL off)

S2
(pump A)

T1-5
(start
enabled &
LL on )

T6-1
(LL off)

T5-6
(start pump
C)

S3
(pump B
10sec)

T2-3
(HL on)

T3-4
(tmr=10s)
S4
(heater &
stirrer)

T4-5
(T=60º C)

S5
(wait)

T5-4
(T=50º C )

S6
(pump C)
T6-5
( LL on)
Fig 4
...

We will first consider the case that the start button has been pressed and the
tank is completely empty
...
During this state Pump A pumps liquid into the tank
...


When the 10 seconds are up the contents of the tank are ready to be mixed and
heated; this happens during State 4
...

When Pump C is switched on (manually) State 6 becomes active
...
If
however pump C is switched off while there is still liquid left then State 5 becomes
active again
...
State 4 then becomes active and the mixture is heated and stirred until the
temperature once again reaches 60 º C
...
This occurs is there is liquid
in the tank at system start-up
...

4
...


The PLC wiring diagram is shown in Fig 4
...
19

heater

S1
S2
S3
S4
S5
S6
T1-2
T2-3
T3-4
T4-5
T5-6
T6-1
T1-5
T5-4
T6-5

R1
R2
R3
R4
R5
R6
R7
R8
R9
R10
R11
R12
R13
R14
R15

Table 4
...
The company safety officer might suggest the inclusion of panel
lights or alarms, the process operator will have experience of practicalities which the
designer may not have considered, the quality assurance manager may introduce
measurement requirements etc
...

Initial Conditions:

Fig 4
...
21

States:

Fig 4
...

Stated another way, State 4 can be switched on by T3-4 or T5-4 and is switched off by
T4-5
...
If you are in any doupt about this refer
back to section 2
...


Outputs:

Fig 4
...


Fig 4
...
25
for state 2 of the state/transition diagram shown in Fig 4
...
26
Answer:

a
...

b
...

c
...

d
...


2
...
27
The ladder diagram in Fig 4
...
27
...
28
Select the one correct statement below regarding both
...
It is wrong to have a state contact on the left and over on the ri ght as
well
...
State 0 is the default state so it shouldn't be specified in terms of
transitions
...
Trans 2-0 should be shown as a n/c contact not n/o
...
Trans 0-1 and Trans 0-2 n/c contacts should be in parallel, not in series
as shown
...


Fig 4
...
29
...
Trans 2-0 is unnecessary because state 0 is the default state
...
It is not possible to skip a state in selective branching
...
State 2 cannot work because there are two routes into it but only one
route out
...
The input conditions for trans 0-1 and trans 0-2 should be mutually
exclusive

4
...
30

Fig 4
...
31 is part of the program for the state/transition diagram Fig
4
...
Select the only correct statement below regarding both
...
The input 1 contact should be shown n/c on the first line of ladder logic
...
Trans 0 is the default transition
...
State 1 cannot be a part-condition for two transitions as shown in the
ladder diagram
...
The ladder logic for for state 1 shows that you can't have a transition
out of and back into the same state

5
...
32
Select the only incorrect statement below regarding the state/transition diagram Fig 4
...

Answer:

a
...

b
...

c
...

d
...


6
...
33
Select the only incorrect statement below regarding the diagram Fig 4
...
When a box is full state 7 becomes active
...
When a part has been added to a box trans 6-5 becomes active
whether or not the box is full
...
States 6 and 7 will never be simultaneously active
...
Trans 5-7 and trans 5-6 are not mutually exclusive
...


Fig 4
...

Answer:

a
...
When state 0 is active and input e is on and input a is off, state 3
becomes active
...
State 3 is entered directly from state 4
d
...


The sequence for a set of traffic lights is to be plc controlled
...

Select the only incorrect statement below regarding the program
...
The transitions from one state to the next will be activated by timers
...
Selective branching will be necessary for the red and amber part of the
sequence
...
The lights will be plc outputs
...
Two outputs will be on during the red and amber state
...


Fig 4
...
35
...
There can be no default state in such a sequence
...
Input 1 only switches the light on
...
Input 1 only switches the light off
...
Input 1 alternately switches the light on and off
...


Fig 4
...
36
...
If the plc count is less than 10, state 0 will always follow state 2
b
...
Provision must be made in the system to reset the counter
...
The counter present value is the deciding factor between trans 2-0 and
trans 2-1

11
...

Motor
LS3

Y1

Y2

LS4

LS1 LS2

Start

Clamp

Stop
Workpiece

Y3

Fig 4
...
When the
start button is pressed the piece is clamped, drilled and then released
...

(a) Draw a PLC wiring diagram for the system
...

(c) Draw the necessary lines of ladder logic to operate the system as designed
...
1 Introduction

Chapter 5
Parallel Branching

It is sometimes desirable in sequential systems to carry out certain
operations simultaneously
...
The SF chart technique known as parallel branching allows for some states
to be active simultaneously and so the events run in parallel
...
1
5
...
1 shows a SF chart with parallel branching
...
The parallel section of the sequence is indicated
by the double lines below S1 and below S4 and S6
...
The parallel section of
the program only finishes when both branches are complete
...
When both waiting states are active T(4,6)-1 occurs and the
sequence returns to S1
...
2

Motor

LS1
Example 5
...
2 extends and retracts
once
...

(a) Draw a PLC wiring diagram for the system
...

(c) Write out an assignment list
...


Solution
(a)

Start

Stop

a-

plc

Y1

Motor

Fig 5
...
4

(c)

S1
S2
S3
S4
S5
S6
T1-(2,5)
T2-3
T3-4
T5-6
T(4,6)-1
Start
Enbl
Table 5
...
5

R1
R2
R3
R4
R5
R6
R7
R8
R9
R10
R11
R50

Transitions:
Note that the condition for T(4,6)-1 to be energised is that states 4 and 6 are
simultaneously on
...
6

Two states (2 and 5) are switched on by T1-(2,5) and two states (4 and 6) are
switched off by T(4,6)-1

Fig 5
...
8

While the motor is running, during state 5, the counter is incremented by the
proximity switch LS1
...


Fig 5
...

Example 5
...
The conveyor is
indexed by a third cylinder extending and retracting
...
The installation is shown in Fig
5
...

b-

c-

b+

c+

a-

a+

Fig 5
...
The task of inclusion of clamping has been added as an exercise at
the end of the chapter
...
11 and
Fig 5
...
The assignment list is shown in Table 5
...


start

stop

a-

a+

b-

b
+

c-

Y6

Drill

c+

PLC
PLCPLC

Y1

Y2

Y3

Y4
Fig 5
...
12

T7-8
(c+)
T8-9
(c-)

S1
S2
S3
S4
S5
S6
S7
S8
S9
T1-2
T2-3
T3-(4,7)
T4-5
T5-6
T7-8
T8-9
T(6,9)-1
Start
Enbl
Table 5
...
13

Fig 5
...
15

Outputs:

Fig 5
...
3:
It has been decided to modify the drill and countersink process in Example 5
...
Two extra proximity switches are fitted to detect
the presence of a part under the drill and under the countersink
...
17
...
u
...
u
...
u
...
17

T11-9
(p
...
csk)
T7-8
(c+)
T8-9
(c-)

Two extra waiting states S10 and S11 have been introduced because it is not
possible for a the sequence to simultaneously enter a selective and a parallel branch
...

The PLC wiring diagram with the two additional inputs is shown in Fig 5
...
3
...
18

Y6

Drill

c/sink

p
...

drill

p
...

csk

S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
S11
T1-2
T2-3
T3-(10,11)
T4-5
T5-6
T7-8
T8-9
T(6,9)-1
T10-4
T10-6
T11-7
T11-9
Start Enbl
Table 5
...

The software can be downloaded from http://www
...
com/trilogi
...


Open the plc Program Editor

Click on the arrow to get the toolbar

Select normally open (n/o) input symbol

Click on the bar to get the
Define Label Name dialog box

 Call the input Start_Button or Start Button and press return
...


Now select an output and call it light

Highlight Start_Button again and select the parallel contact button
...

Click on the Start_Button input to switch it on
...




---