POGRAMMABLE LOGIC CONTROLLERS AND MICROCONTROLLERS
LIST OF CONTENTS
Unit1
Introduction to PLC, Concept of PLC - Building blocks of PLC - Functions of various blocks, limitations of relays - Advantages of PLCs over electromagnetic relays - Different programming
languages - PLC manufacturer etc.
Unit 2
Working of PLC, Basic operation and principles of PLC, Architectural details processor
Memory structures - I/O structure Programming terminal - Power supply
Unit 3
Instruction Set - Basic instructions bit.-Timer instruction like retentive timers, resetting of timers Counter instructions like up counter, down counter, resetting of counters.- Arithmetic Instructions
(ADD,SUB,DIV,MUL etc.)- MOV instruction - Comparison instructions like equal, not equal, greater,
greater than equal, less than, less than equal
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UNIT I
1.1 Introduction to PLC
The hydramatic Division of the General Motors Corporation specified the design criteria for the first
programmable controller come in 1968. Programmable logic controllers, also called programmable
controllers or PLCs, are solid-state members of the computer family, using integrated circuits instead of
electromechanical devices to implement control functions. They are capable of storing instructions, such
as sequencing, timing, counting, arithmetic, data manipulation, and communication, to control industrial
machines and processes
Purposes - Initially designed to replace relay logic boards, Sequence device actuation, Coordinate
activities accepts input from a series of switches, Sends output to devices or relays
What is PLC?
Programmable Logic Controller (PLC) is a digital computer used for the automation of various
electro-mechanical processes in industries. These controllers are specially designed to survive in harsh
situations and shielded from heat, cold, dust, and moisture etc. PLC consists of a microprocessor which
is programmed using the computer language.
The program is written on a computer and is downloaded to the PLC via cable. These loaded programs
are stored in non – volatile memory of the PLC. During the transition of relay control panels to PLC, the
hard wired relay logic was exchanged for the program fed by the user. A visual programming language
known as the Ladder Logic was created to program the PLC.
1.2 Advantages of PLCs over electromagnetic relays
• Ability to interface / communicate with computers but not with reley
• Simple programming is possible using ladder logic but not with relay
• Plc have High reliability
• High controllability due to the use of timers and counters.
• Easy maintenance
• Rugged construction - can operate in extremely harsh conditions
• Small size
• Easy expandability
• Economical in long term
1.3 Limitation of Relay•
•
•
•
It suffer from contact wear and reliability problems
Unsuitable for designing complex control circuit
Relay occupies large space
Relay takes large time to actuate
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1.4 Building blocks of PLC
1.5 Functions of various blocks
PLCs have input/output devices like switches, push buttons, limit switches,sensors,lamp
indicators,relays solenoids motors etc. block diagram of a PLC shown in above Fig. It has three major
units/sections.
• I/O (Input/Output) Modules.
• CPU (Central Processing Units).
• Programmer/Monitor.
The input section converts the field signals supplied by input devices/sensors to logic-level signals that
the PLC's CPU can read.
The Processor Section reads these inputs, Processes the signal, and prepares the output signals.
The output section converts the logic level output signals coming from processor section to high level
signals and used to actuate various output field devices.
The programmer/monitor is used to enter the user's program into memory and to monitor the execution
of the program.
I/O Section:The I/O section establishes the interfacing between physical devices in the real world outside the
PLC and the digital arena inside the PLC.
The input module has bank of terminals for physically connecting input devices, like push buttons, limit
switches etc. to a PLC. The role of an input module is to translate signals from input devices into a form
that the PLC's CPU can understand.
The Output module also has bank of terminals that physically connect output devices like solenoids,
motor starters, indicating lamps etc. to a PLC. The role of an output module is to translate signals from
the PLC's CPU into a form that the output device can use.
The tasks of the I/O section can be classified as:
• Conditioning
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• Isolation
• Termination
• Indication
An electronic system for connecting I/O modules to remotely located I/O devices can be added if
needed. The actual operating process under PLC Control can be thousands of feet from the CPU and its
I/O modules.
CPU Section:The Central Processing Unit, the brain of the system is the control portion of the PLC. It has three
Subparts.
• Memory System
• Processor
• Power Supply
Memory System:The memory is the area of the CPU in which data and information is stored and retrieved. The total
memory area can be subdivided into the following four Sections.
I/O Image Memory
The input image memory consists of memory locations used to hold the ON or OFF states of each input
field devices, in the input status file.
The output status file consists of memory locations that stores the ON or OFF states of hardware output
devices in the field. Data is stored in the output status file as a result of solving user program and is
waiting to be transferred to the output module's switching device.
Data Memory - It is used to store numerical data required in math calculation, bar code data etc.
User Memory - It contains user's application program.
Executive Memory - It is used to store an executive program or system software. An operating system of
the PLC is a special program that controls the action of CPU and consequently the execution of the
user's program. A PLC operating system s designed to scan image memory, interprets the instruction of
user's program stored in main memory, and executes the user's application program the operating system
is supplied by the PLC manufacturer and is permanently held in memory.
Processor:-The processor, the heart of CPU is the computerized part of the CPU in the form of
Microprocessor / Micro controller chip. It supervises all operation in the system and performs all tasks
necessary to fulfill the PLC function.
•





It reads the information i.e. status of externally connected input devices with input module.
It stores this information in memory for later use.
It carries out mathematical and logic operations as specified in application program.
After solving the user's program, it writes the result values in the memory.
It sends data out to external devices like output module, so as to actuate field hardware.
It performs peripheral and external device communication.
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Power Supply  The power supply provides power to memory system, processor and I/O Modules.
 It converts the higher level AC line Voltage to various operational DC values.
 For electronic circuitry.
 It filters and regulates the DC voltages to ensure proper computer operations.
Programmer/Monitor The Programmer/Monitor (PM) is a device used to communicate with the circuits of the
PLC. The programming unit allows the engineer/technicians to enter the edit the program to be
executed. In its simplest form it can be hand-held device with membrane keypad for program entry and a
display device (LED or LCD) for viewing program steps of functions. More advanced systems employ a
separate industrial terminal or personal computers with type-writer type keyboard and CRT monitors.
With the help of proprietary software, it allows programmer to write, view and edit the program and
download it into the PLC. It also allows user to monitor the PLC as it is running the program. With this
monitoring systems, such things as internal coils, registers, timers and other items not visible externally
can be monitored to determine proper operation. Also, internal register data can be altered, if required. to
fine tune program operation while debugging. Communication between PM and PLC is done via a cable
connected to a special programming port on PLC. Connection to the personal computer can be through a
serial port or from a dedicated card installed in the computer.
1.6 PLC Programming language - Using flowcharts, Using ladder logic and Using statement logics or
mnemonics
1.7 PLC manufacturer - Allen Bradley, Rockwell Automation, Mitsubishi, Omron, Siemens etc.
Programmable Logic Controllers Multiple Choice Questions
Q1. The acronym PLC stands for:
(A) Pressure Load Control
(B) Programmable Logic Controller
(C) Pneumatic Logic Capstan
(D) PID Loop Controller
(E) Pressure Loss Chamber
Q2. The PLC was invented by
A) Bill Gates
B) Dick Morley
C) Bill Landis
D) Tod Cunningham
Q3. Ladder logic programming consists primarily of:
(A) Virtual relay contacts and coils
(B) Logic gate symbols with connecting lines
(C) Function blocks with connecting lines
(D) Text-based code
(E) Hieroglyphics
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Q4. In a PLC, the scan time refers to the amount of time in which …..
(A) the technician enters the program
(B) timers and counters are indexed by
(C) one “rung” of ladder logic takes to complete
(D) the entire program takes to execute
(E) transmitted data communications must finish
Q5. Which one of the following is not a PLC manufacturer
a. Siemens
b. Mitsubishi
c. Microsoft
d. ABB
Short/long answer types questions
Q1 Define PLC
Q2 What are the limitations of relay over PLC
Q3 Explain building blocks of PLC in detail
Q4 List the names of PLC manufacturer.
UNIT 2
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2. Working of PLC (Programmable Logic Controller)
The Programmable logic controller functions in four steps.

Input scan: The state of the input is scanned which is connected externally. The inputs include
switches, pushbuttons, and proximity sensors, limit switches, pressure switches. Ideally, they are
transformers and not relays.

Program scan: The loaded program is executed to carry out the function appropriately.

Output scan: The input sources have a control over the output ports to energize or de-energize
them. The outputs include solenoids, valves, motors, actuator, and pumps. Depending on the
model of PLC, these relays can be transistors.
2.1 PLC ARCHITECTURE
• The input sources convert the real time analog electric signals to suitable digital electric signals
and these signals are applied to the PLC through the connector rails.
• These input signals are stored in the PLC external image memory in locations known as bits.
This is done by the CPU
• The control logic or the program instructions are written onto the programming device through
symbols or through mnemonics and stored in the user memory.
• The CPU fetches these instructions from the user memory and executes the input signals by
manipulating, computing, processing them to control the output devices.
• The execution results are then stored in the external image memory which controls the output
drives.
• The CPU also keeps a check on the output signals and keeps updating the contents of the input
image memory according to the changes in the output memory.
• The CPU also performs internal programming functioning like setting and resetting of the timer,
checking the user memory.
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Program
Loader
PC
Switches
Printer
Processor
I/O
Modules
Memory
Power
Supply
Machines
Cassette
Loader
EPROM
Loader
Peripherals
External Devices
2.2 PLC Hardware
•
The hardware components of a PLC system are CPU, Memory, Input/Output, Power supply unit,
•
and programming device. Below is a diagram of the system overview of PLC.
CPU – Keeps checking the PLC controller to avoid errors. They perform functions including
logic operations, arithmetic operations, computer interface and many more.
•
Memory – Fixed data is used by the CPU. System (ROM) stores the data permanently for the
operating system. RAM stores the information of the status of input and output devices, and the
values of timers, counters and other internal devices.
•
I/O section – Input keeps a track on field devices which includes sensors, switches.
•
O/P Section - Output has a control over the other devices which includes motors, pumps, lights
and solenoids. The I/O ports are based on Reduced Instruction Set Computer (RISC).
•
Power supply – Certain PLCs have an isolated power supply. But, most of the PLCs work at
•
220VAC or 24VDC.
Programming device – This device is used to feed the program into the memory of the
processor. The program is first fed to the programming device and later it is transmitted to the
PLC’s memory.
System Buses – Buses are the paths through which the digital signal flows internally of the PLC.
The four system buses are:
•
Data bus is used by the CPU to transfer data among different elements.
•
•
Control bus transfers signals related to the action that are controlled internally.
Address bus sends the location’s addresses to access the data.
•
System bus helps the I/O port and I/O unit to communicate with each other
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2.3 The memory space can be divided into two broad categories:
Program and Data Memory:
Advanced ladder logic functions allow controllers to perform calculations, make decisions and do other
complex tasks. Timers and counters are examples of ladder logic functions. They are more complex than
basic inputs contacts and output coils and relay heavily upon data stored in the memory of the PLC.
The user program will account for most of the memory of a PLC system. Program files contain the logic
controlling machine operation. This logic consists of instructions that are programmed in a ladder logic
format.
The data file portion of memory stores input and output status, processor status, the status of various bits
and numerical data.
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Multiple Choice Questions
Q.1 one of the following is an input device
a. Motor
b. Light
c. Valve
d. Sensor
Q.2 Solenoids, lamps, motors are connected to:
a. Analog output
b. Digital output
c. Analog input
d. Digital input
Q.3 The type of memory which is fast and temporarily stores the data which are immediately
required for use is called as______.
a. HDD
b. ROM
c. RAM
d. SSD
Q.4 how is the speed of operation of conventional relay system as compared to digital controllers?
a. very slow
b. very fast
c. same
d. almost similar
Q.5 The PLC is used in _______.
a. machine tools
b. automated assembly equipment
c. moulding and extrusion machines
d. all of the above
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UNIT 3 INSTRUCTION SETS
3. PLC Instructions
3.1 Bit instructions
3.2 Timer and Counter instructions
3.3 Program Control Instructions
3.4 Compare Instructions
3.5 Math Instructions
3.1 Bit Instructions
Mnemonic
Name
Symbol
Description
XIC
Examine If
Closed
Examines a bit for an On (set, high)
condition.
XIO
Examine If
Open
Examines a bit for an Off (cleared, low)
condition.
OTE
Output
Energize
When rung conditions are true, the OTE
will either set or clear the data bit.
OTL
Output Latch
When enabled, the instruction signals to
the controller to turn on the addressed
bit. The bit remains on, regardless of the
rung condition.
OTU
Output
Unlatch
When enabled, it clears (unlatches) the
data bit. The bit remains Off, regardless
of rung condition.
3.2 Timer and Counter Instructions
Mnemonic
Name
TON
Timer On
Delay
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Symbol
Description
A non-retentive timer that accumulates
time when the instruction is enabled.
The accumulated value is reset when
rung conditions go false.
TOF
Timer Off
Delay
A non-retentive timer that accumulates
time when the rung makes a true-tofalse transition.
RTO
Retentive
Timer On
A retentive timer that accumulates time
when the instruction is enabled. Retains
its accumulated value when rung
conditions become false.
TON
Count Up
An instruction that counts false-to-true
rung transitions. It counts upward and
the accumulated value is incremented
by one count on each of these
transitions.
TOF
Count Down
This instruction counts downward on
each false-to-true rung transition. The
accumulated value is decremented by
one count on each of these transitions.
RTO
Retentive
Timer On
A retentive timer that accumulates time
when the instruction is enabled. Retains
its accumulated value when rung
conditions become false.
CTU
Count Up
An instruction that counts false-to-true
rung transitions. It counts upward and
the accumulated value is incremented
by one count on each of these
transitions.
CTD
Count Down
This instruction counts downward on
each false-to-true rung transition. The
accumulated value is decremented by
one count on each of these transitions.
RES
Reset
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This instruction is used to reset a timer,
counter or control structure. The
accumulated value of these instructions
are cleared when the RES instruction is
enabled.
3.3 Move/Logical Instructions
Mnemonic
Name
MOV
Move
MVM
Masked Move
CLR
Clear
DTR
Data
Transitional
Passes the Source value through a Mask
and compares the result with the Reference
value.
SWPB
Swap Byte
Instruction
Rearranges the bytes of a tag and stores the
bytes in the new order.
SWP
Swap Byte
Instruction
Swaps the lower and higher bytes of a
specified number of words.
BTD
Bit Field
Distribute
Copies the specified bits from the Source,
shifts the bits to the appropriate position,
and writes the bits into the Destination.
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Symbol
Description
Copies the Source (which remains
unchanged) to the Destination.
Copies the Source to a Destination and
allows segments of the data to be masked.
Clears all the bits of the Destination.
AND
Bitwise AND
Performs a logical AND operation by using
the bits in Source A and Source B and
placing the result in the Destination.
OR
Bitwise OR
Performs a logical AND operation by using
the bits in Source A and Source B and
placing the result in the Destination.
XOR
Bitwise
Exclusive OR
Performs a bitwise XOR operation using
the bits in Source A and Source B and
stores the result in the Destination.
NOT
Bitwise NOT
Executes a bitwise NOT operation by using
the bits in the Source and placing the result
in the Destination
3.4 Program Control Instructions
Mnemonic
Name
JSR
Jump to
Subroutine
This instruction jumps execution to a
specific routine and initiates the execution
of this routine, called a subroutine.
SBR
Subroutine
Stores recurring sections of program logic.
RET
Return
Used to return to the instruction following
the a JSR operation.
JMP
Jump to Label
LBL
Label
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Symbol
Description
Skips sections of ladder logic.
Target of the JMP instruction with the same
label name.
MCR
Master Cont.
Res.
Used in pairs to create a program zone that
can disable all rungs between the MCR
instructions.
AFI
Always False
Disables all instructions on a rung.
NOP
No Operation
This instruction functions as a placeholder.
TND
Temporary End
When enabled, it lets the controller execute
logic only up to this instruction.
IOT
Immediate
Output
This instruction immediately updates the
specified output data.
3.5 Compare Instructions
Mnemonic
Name
EQU
Equal
GEQ
Greater Than
or Equal To
Determines whether Source A is greater than
or equal to Source B. If the value at Source A
is greater than or equal to the value at Source
B, then the instruction is true.
GRT
Greater Than
This instruction is used to test whether one
value (Source A) is greater than another value
(Source B).
LEQ
Less Than or
Equal To
Determines whether one value (Source A) is
less than or equal to another (Source B).
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Symbol
Description
This instruction is used to test whether two
values are equal. If Source A is equal to
Source B, the instruction is logically true.
LES
Less Than
This instruction determines whether Source A
is less than Source B.
LIM
Limit
This instruction is used to test for values
within the range of the Low Limit to the High
Limit.
MEQ
Mask Equal
To
NEQ
Not Equal
To
Passes the Source and Compare values
through a Mask and compares the results.
This instruction tests whether Source A is not
equal to Source B.
3.6 Math Instructions
Mnemonic
Name
ADD
Add
SUB
Subtract
Subtracts Source B from Source A and
places the result in the Destination.
MUL
Multiply
Multiplies Source A by Source B and stores
the result in the destination.
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Symbol
Description
Adds Source A to Source B and stores the
result in the Destination.
DIV
Divide
Divides Source A by Source B and places
the result in the Destination.
MOD
Modulo
Divides Source A by Source B and stores
the remainder in the Destination.
SQR
Square Root
NEG
Negate
ABS
Absolute
Calculates the square root of the source and
places the integer result in the Destination.
Changes the sign (+, -) of the Source and
stores the result in the Destination.
Takes the absolute value of the Source and
places the result in the Destination.
Multiple Choice Questions
Question 1: Examine-on instruction in PLC language is symbolically represented by what shape?
A. -] [B. -( )C. -]/[D. None of these
Question 2: The symbol -(L)- represent what instruction in the PLC
language?
A. OUT output unlatch instruction.
B. OTL output latch instruction.
C. Examine off instruction.
D. Output energizes instruction.
Question 3: The address T4:6.ACC is used to address what?
A. Accumulator for timer 4 in file 6.
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B. Preset value of timer 6 in file 4.
C. Accumulator for timer 6 in file 4.
D. Accumulator for counter 6 in file 4.
Q4: To reset the time for a PLC what condition must be true?
A. Reset rung of TON must be true.
B. Reset rung of TON must be false.
C. RST instruction with timer address must be true.
D. RST instruction with associated timer address must be false.
Question 5: To increase the number of inputs and outputs of the PLC, one can use expansion modules.
A. True.
B. False.
C. None of the above.
Question 6: Solenoids, lamps, motors are connected to:
A. Analog output.
B. Digital output.
C. Analog input.
D. Digital input
Question 7: In a counter instruction if the accumulator value becomes greater than or equal to the preset
value then which of the following is true?
A. CU bit goes on.
B. DN bit goes on.
C. Both CU and DN bits go on.
D. None of the above.
Short / long Answer type Questions
Q1 Explain time instructions in detail
Q2 Explain Counter instructions in detail
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