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Programmable Logic Controllers
( Definition according to NEMA standard ICS3-1978)
A digitally operating electronic apparatus which
uses a programming memory for the internal
storage of instructions for implementing specific
functions such as logic, sequencing, timing,
counting and arithmetic to control through
digital or analog modules, various types of
machines or process.
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INTRODUCTION TO PLCS
Advantages of PLCs
• Less wiring.
• Wiring between devices and relay contacts are
done in the PLC program.
• Easier and faster to make changes.
• Trouble shooting aids make programming easier
and reduce downtime.
• Reliable components make these likely to operate
for years before failure.
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Major Components of a Common PLC
POWER
SUPPLY
From
SENSORS
I M
N O
P D
U U
T L
E
PROCESSOR
Pushbuttons,
contacts,
limit switches,
etc.
PROGRAMMING
DEVICE
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O
U
T
P
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T
M
O
D
U
L
E
To
OUTPUT
Solenoids,
contactors,
alarms
etc.
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The Microprocessor or CPU Module
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Is the brain of a PLC system.
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It consists of the microprocessor, memory integrated circuits, and circuits necessary to
store and retrieve information from memory.
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It also includes communications ports to other peripherals, other PLC's or programming
terminals.
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Today's processors vary widely in their capabilities to control real world devices.
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Some control as few as 6 inputs and outputs (I/O) and others 40,000 or more.
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One processor can control more than one process or manufacturing line.
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Processors are often linked together in order to provided continuity throughout the
process.
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The number of inputs and outputs PLCs can control are limited by the overall capacity of
the PLC system hardware and memory capabilities.
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The job of the processor is to monitor status or state of input devices, scan and solve
the logic of a user program, and control on or off state of output devices.
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RAM
Random Access Memory is a volatile memory that would lose it's information if power
were removed.
This is why some processor units incorporate a battery back up.
The type of RAM normally used is CMOS or Complementary Metal Oxide
Semiconductor.
CMOS RAM is used for storage of the user's program (ladder logic diagrams) and
storage memory.
ROM or Read Only Memory is a non-volatile type of memory.
This means you don't need an external power source to keep information.
In this type of memory, information can be read, but not changed.
For this reason the manufacture sometimes calls this firmware.
It is placed there for the internal use and operation of processor units.
EEPROM or Electrically Erasable Programmable Read Only Memory
Is usually an add-on memory module that is used to back up the main program in
CMOS RAM of the processor.
In many cases, the processor can be programmed to load the EEPROM's program
to RAM if RAM is lost or corrupted.
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Input Module
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There are many types of input modules to choose from.
The type of input module used is dependent upon what real world input to the PLC is
desired.
Some examples of inputs are limit switches, electric eyes, and pushbuttons.
DC inputs, such as thumbwheel switches, can be used to enter integer values to be
manipulated by the PLC.
DC input cards are used for this application.
Since most industrial power systems are inherently noisy, electrical isolation is
provided between the input and the processor.
Electromagnetic interference (EMI) and Radio Frequency Interference (RFI) can
cause severe problems in most solid state control systems.
The component used most often to provide electrical isolation within I/O cards is
called an optical isolator or Opto-coupler.
The wiring of an input is not complex.
The object is to get a voltage at a particular point on the card.
Typically there are 8 to 32 input points on any one input module.
Each point will be assigned a unique address by the processor.
Analog input modules are special input cards that use analog to digital conversion
(A to D) to sense variables such as temperature, speed, pressure, and position.
The external device normally is connected to a controller (transducer) producing an
electrical signal the analog input card can interpret.
This signal is usually (4 to 20) mA or a (0 to 10) volt signal.
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Output Module
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Output modules:
can be used for ac or dc devices such as solenoids, relays, contractors, pilot
lamps, and LED readouts.
Output cards usually have from 6 to 32 output points on a single module.
The output device within the card provides the connection from the user
power supply to the load.
Usually silicon controlled rectifiers (SCR), triac, or dry contact relays are use
for this purpose.
Individual outputs are rated most often at 2 to 3 Amperes.
Output cards, like input cards have electrical isolation between the load
being connected and the PLC.
Analog output cards are a special type of output modules that use digital to
analog conversion (D to A).
The analog output module can take a value stored in a 12 bit file and
convert it to an analog signal.
Normally this signal is (0 -10) volts dc or (4 to 20) m A.
This analog signal is often used in equipment such as motor operated
valves and pneumatic position control devices.
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NAND Gate
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NOT OPERATION
A
C
Rung
In the rung above, it can be seen that if input A is be true (1),
then the output C is true (0) or when A is (0), output C is 1.
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INPUTS
• These are the physical connections from the real world to the
PLC.
• They can be limit switches, push buttons, sensors, anything
that can "switch" a signal on or off.
• The voltage of these devices are usually, but not always, 24
Volt DC.
• PLC Manufacturers make inputs that can accept a wide range
of voltages both ac and dc.
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It should be remembered that an input will be ON, "status 1",
when the voltage is present at the input connection and OFF,
"status 0", when the voltage is no longer present at the input
connection.
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OUTPUTS
• These are the connections from the PLC to the
real world. They are used to switch solenoids,
lamps, contactors, etc. ON and OFF.
• Again they are usually 24 Volt DC, either relay
or transistor, but can also be 115/220 Volt AC.
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I/O Module
• The I/O interface section of a PLC connects it to
external field devices.
• The main purpose of the I/O interface is to condition
the various signals received from or sent to the external
input and output devices.
• Input modules converts signals from discrete or analog
input devices to logic levels acceptable to PLC’s
processor.
• Output modules converts signal from the processor to
levels capable of driving the connected discrete or
analog output devices.
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Common Industrial Hardware
Representing the Binary Concept
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I/O Module
DC INPUT MODULE
IS NEEDED TO:
 Prevent voltage
transients from
damaging the
processor.
Helps reduce the
effects of
electrical noise
USE TO
DROP THE
VOLTAGE
TO LOGIC
LEVEL
FROM
INPUT
DEVICE
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Current
Limiting
Resistor
OPTOISOLATOR
Buffer,
Filter,
hysteresis
Circuits
TO
PROCESSOR
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I/O Module
AC INPUT MODULE
IS NEEDED TO:
 Prevent voltage
transients from
damaging the
processor.
Helps reduce the
effects of
electrical noise
CONVERTS THE AC
INPUT TO DC AND
DROPS THE
VOLTAGE TO LOGIC
LEVEL
FROM
INPUT
DEVICE
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Rectifier,
Resistor
Network
OPTOISOLATOR
Buffer,
Filter,
Hysteresis
Circuits
TO
PROCESSOR
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OUTPUTS
INPUTS
MOTOR
CONTACTOR
LAMP
PUSHBUTTONS
PLC
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Allen-Bradley 1746-1A16
L2
L1
I= Input
Module
slot # in rack
I:2
0
P. B SWITCH
Module
Terminal #
Address I:2.0/0
LADDER PROGRAM
INPUT
MODULE
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WIRING
DIAGRAM
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Discrete Input
A discrete input also referred as digital input is an input that
is either ON or OFF are connected to the PLC digital input.
In the ON condition it is referred to as logic 1 or a logic high
and in the OFF condition maybe referred to as logic o or
logic low.
Normally Open
Pushbutton
Normally Closed
Pushbutton
Normally Open switch
Normally Closed switch
Normally Open contact
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Normally closed contact
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Analog Output
An analog output is an output signal that has a continuous
signal. Typical outputs may vary from 0 to 20mA, 4 to 20mA
or 0 to10V.
Electric to pneumatic
transducer
OUT
PLC
Analog
Output
Module
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0 to 10V
E
P
Supply air
Pneumatic control valve
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PLC Operation
Basic Function of a Typical PLC
Read all field input devices via the input interfaces,
execute the user program stored in application memory,
then, based on whatever control scheme has been
programmed by the user, turn the field output devices
on or off, or perform whatever control is necessary for
the process application.
This process of sequentially reading the inputs,
executing the program in memory, and updating the
outputs is known as scanning.
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While the PLC is running, the scanning process
includes the following four phases, which are repeated
continuously as individual cycles of operation:
PHASE 1
Read Inputs
Scan
PHASE 2
Program
Execution
PHASE 3
Diagnostics/
Comm
PHASE 4
Output
Scan
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PHASE 1 – Input Status scan
 A PLC scan cycle begins with the CPU reading the
status of its inputs.
PHASE 2– Logic Solve/Program Execution
 The application program is executed using the status
of the inputs
PHASE 3– Logic Solve/Program Execution
 Once the program is executed, the CPU performs
diagnostics and communication tasks
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PHASE 4 - Output Status Scan
•An output status scan is then performed, whereby the
stored output values are sent to actuators and other
field output devices. The cycle ends by updating the
outputs.
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Selecting a PLC
Criteria
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Number of logical inputs and outputs.
Memory
Number of special I/O modules
Scan Time
Communications
Software
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A Detailed Design Process
1. Understand the process
2. Hardware/software selection
3. Develop ladder logic
4. Determine scan times and memory requirements
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Specifications
OUTPUT-PORT POWER RATINGS
Each output port should be capable of supplying
sufficient voltage and current to drive the output
peripheral connected to it.
SCAN TIME
This is the speed at which the controller executes the
relay-ladder logic program. This variable is usually
specified as the scan time per 1000 logic nodes and
typically ranges from 1 to 200 milliseconds.
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PLC Status Indicators
•Power On
•Run Mode
•Programming Mode
•Fault
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Examples of PLC Programming Software:
1. Allen-Bradley – Rockwell Software RSLogix500
2. Modicon - Modsoft
3. Omron - Syswin
4. GE-Fanuc Series 6 – LogicMaster6
5. Square D- PowerLogic
6. Texas Instruments – Simatic
6. Telemecanique – Modicon TSX Micro
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What is a PLC?
A special computer for logic controls
using ladder logic programming.
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PLC a Real World Relay
Replacement
Real world Parts:
• Switch
• Bell
• Relay
Switch controlling AC
circuit or bell
Relay
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Example of a Motor Control
START
STOP
output
EQUIVALENT DIAGRAMS
START
STOP
output
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Normally
Normally
Open Contact Closed Contact
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How to program PLCs?
• Using a specialized Ladder Logic Software
– Allen-Bradley uses RsLogix 500 software to program
SLC, Micrologix and so on
– Omron uses Cx Programmer software for its
SYSMAC PLCs
• Other manufacturers has their own software
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Ladder Logic
• Ladder logic is a method of drawing electrical
logic schematics
• A graphical language popular for PLCs
Controllers
• Invented to describe logic made from relays
• Language resemble ladders, with two vertical
"rails" and a series of horizontal "rungs" between
them
• Rule-based language, rather than a procedural
language
• Executed sequentially by software, in a loop
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Examples
Relay Logic
Ladder Logic
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Binary Data Representation
• We communicate to others using groups
of letters arranged into words.
• The PLC uses groups of bits called words.
• Different bit patterns represent different
information.
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PLC Words
• One measure of a computer’s capabilities
is the length of the data words on which
it can operate.
• Current PLCs use 16-bit words.
• Newer PLCs use 32-bit words.
• SLC 500 family PLCs are 16-bit
computers.
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Information Represented
as Combinations of Bits
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Parts of a 16-Bit Word
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Bytes, Nibbles, and Bits
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16-Point Module’s
I/O Points Represented in a
Word
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Physical Input Conditions and
the Corresponding Input Data
Word
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8-Point Input Module
Represented in a Word
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24-Point I/O Module
Represented in Two Words
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Two Words Representing
Inputs for a 32-Bit Module
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Data Table Format
• Words are 16 bits.
– Bits 0 through bit 15
• First word or bit is always 0.
• SLC 500 data tables can contain up to 256
words (0 to 255).
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Words Arranged in a Data Table
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Input Data File (1 of 2)
• Each input screw terminal has one
memory location to store on or off status.
• Input data is stored in the input data file.
– Also called the input status file
• Input status file holds input status
information, which is used to solve ladder
program.
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Input Data File (2 of 2)
• Identified as an I-type data file
• Only one input status file allowed per
project
• Only has words created for actual modules
in system
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Output Data File (1 of 3)
• Each output screw terminal has one
memory location to store on or off status.
• Output data is stored in the output data
file, also called the output status file.
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Output Data File (2 of 3)
• Output status file holds output status
information to update outputs
• Is the result of solving the ladder program.
• Output data sent to modules during output
update portion of scan
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Output Data File (3 of 3)
• Identified as an O-type data file
• Only one output status file allowed per
project
• Only has words created for actual modules
in system
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