2. PROGRAMMABLE LOGIC
CONTROLLER
A PROGRAMMABLE LOGIC
CONTROLLER (PLC) is an industrial
computer control system that continuously
monitors the state of input devices and
makes decisions based upon a custom
program to control the state of output
devices.
3. Block diagram of PLC
PROCESSOR
POWER
SUPPLY
I M
N O
P D
U U
T L
E
O M
U O
T D
P U
U L
T E
MEMORY
From
SENSORS
Pushbuttons,
contacts,
limit switches,
etc.
To
OUTPUT
Solenoids,
contactors,
alarms
etc.
7. STEPS INVOLVED IN INPUT / OUTPUT PROCESSING
The sequence followed by a PLC
when carrying out a program can
be as follows:
• Scan the inputs associated with
one rung of the ladder program
• Solve the logic operation involving
those inputs.
• Set / Reset the outputs for that
rung
• Move on the next rung and repeat
the operations 1, 2, 3
9. INTERNAL RELAY
• An internal relay behaves like relays
with their associated contacts, buy they
are not actual relays whose simulations
are controlled by the PLC software.
• Internal relays can be very useful in the
implementation of switching
sequences.
• They are often used when there are
programs with multiple input
conditions.
• They are also known as Auxiliary relays
or markers.
10. • TYPES OF COUNTERS:
• Up Counters
• Down Counters
• UP COUNTERS:
• Up counters count up from the zero to pre – set value
• The events are added until the pre – set value is reached
• When the counter reaches the set value, its contacts
change state
• DOWN COUNTERS:
• Down counters count down from the pre – set value to zero
• The events are subtracted until the pre – set value is
reached
• When the counter reaches the Zero value, its contacts
change state
11. TIMER
• A timer is a special counter ladder function that allows
the PLC to perform timing operations based on a
precise internal clock
Types of Timers:
• Delay ON Timers or ON delay timers
• Delay OFF Timers or OFF delay timers
• Pulse Timers
• Cascaded Timers
• ON-OFF Cycle Timers
• One Shot Timers
12. Delay ON Timers:
• The term delay is used to indicate that this timer
burns on, after waiting for a fixed time delay period.
• When there is an input, the timer is energised and
starts timing, after some pre-set value, the timer
contacts are closed to output.
• TON is used to denote ON-delay.
13. Delay OFF Timers:
• OFF delay timers are maintained as ON for a
fixed time of delay period before turning off.
• TOF is used to denote OFF-delay.
14. SHIFT REGISTER
A shift register is an electronic storage
device that allows the stored bits of one relay
to get shifted into another relay.
15. AND Logic Function
• AND logic circuit represents
series circuit
• AND gate is composed with
two inputs and one output.
• AND gate produce output
when both the inputs are
HIGH state.
INPUT A INPUT B OUTPUT
0 0 0
0 1 0
1 0 0
1 1 1
16. OR Logic Function
• OR logic circuit represents the
parallel circuit.
• OR Gate is composed of two or more
inputs and one output.
• OR operation is like addition of
binary numbers.
• OR gate produce output when any
one input are HIGH state.
INPUT A INPUT B OUTPUT
0 0 0
0 1 1
1 0 1
1 1 1
17. NOT Logic Function
• NOT function is also known as Inverter.
• NOT gate is composed of single input and a
single output.
• The bubble, or circle, at the output is the
standard symbol used to represent
inversion.
• In NOT gate, there is an output, when there
is no input and no output when there is an
input
INPUT A OUTPUT
0 1
1 0
18. NAND Logic Function
• NAND is a combination of AND and NOT gates.
• Arrangement shows AND gate is followed by NOT
gate. Hence it is called NOT AND
• gate.
• Both the inputs A and B have to be at LOW state
to get the output at HIGH state.
• NAND Gate is composed of two or more input
with a single output.
• Any one input is in LOW state also output will be
HIGH state
INPUT A INPUT B OUTPUT
0 0 1
0 1 1
1 0 1
1 1 0
19. NOR Logic Function
• NOR is a combination of OR and NOT gates.
• Arrangement shows OR gate is followed by
NOT gate. Hence it is called NOT OR gate.
• Both the inputs A and B have to be at LOW
state to get the output at HIGH state.
• NOR Gate is composed of two or more input
with a single output.
• Any one input is in HIGH state also output
will be LOW state
INPUT A INPUT B OUTPUT
0 0 1
0 1 0
1 0 0
1 1 0
20. Exclusive OR (XOR) Logic Function
• When both the inputs are at LOW
state the output will be at LOW
state
• When both the inputs are at
HIGH state the output will be at
LOW state
• When any one input is HIGH state
the output will be at HIGH state
INPUT A INPUT B OUTPUT
0 0 0
0 1 1
1 0 1
1 1 0
21. SELECTION OF PLCS
The selection process of PLC for a particular
task depends on the following factors.
• Capacity of Input and Output
• No. of Inputs and Outputs
• Types of Inputs and Outputs
• Size of memory required
• Speed and Power required of the CPU
22. PLC applications
• Continuous bottle filling system
• Automation industry
• Speed control of dc motor
• 3 stage air conditioning system
• Control of machines
