Programmable logic controllers (PLCs) have become the most common choice for manufacturing controls. PLCs use ladder logic programming, which mimics relay logic wiring to make retraining easier. Ladder logic diagrams show inputs, outputs, and the logic that determines how inputs activate outputs. PLCs read ladder logic programs to operate digital and analog systems through input and output modules.
Programmable logic controllers (PLCs) are microprocessor-based devices used to monitor, control, and automate electromechanical processes. PLCs replaced hardwired relay panels and are programmed using ladder logic. A PLC consists of a central processing unit, input and output modules to interface with sensors and actuators, and a programming device. PLCs scan inputs, execute a user-written program, and update outputs to control machines and processes in a flexible, easy-to-program manner.
The document provides an introduction to programmable logic controllers (PLCs). It discusses how PLCs replaced electromechanical relays for control systems, allowing logic functions to be programmed rather than physically wired. The summary describes how PLCs work, including taking input from sensors and switches and outputting signals to devices. It also discusses how PLCs are programmed using ladder logic and can be reprogrammed to change control functions without rewiring. The document provides examples of programming PLCs to control lights and motors.
This Presentation provides detail description PLC basics, architecture, Programming with Case studies. Quizzes are also included in between the topics in order to measure the Learning.
The document provides an overview of programmable logic controllers (PLCs). It defines PLCs as digital electronic devices that use programmable memory to implement logic functions like sequencing and timing to control machines and processes. The document discusses the basic structure of PLCs including the CPU, memory, input/output interfaces, and power supply. It also covers programming methods like ladder logic and instruction lists. Additional topics include input/output addressing, timers, counters, and techniques like latching, internal relays, and sequencing using timers.
The document discusses the basics of programmable logic controller (PLC) programming including PLC architecture, memory organization, programming languages, ladder logic instructions, addressing schemes, and programming techniques. Specifically, it covers the processor memory being divided into program and data memory, the ladder logic programming language using relay-type instructions like examine if closed and examine if open, addressing I/O locations by module and bit, and programming concepts such as parallel and nested rungs, internal control relays, and adjustments for different scan patterns.
Ladder diagrams are a type of graphic language used to represent logic systems for industrial control. They resemble a ladder, with two vertical power rails and circuits connected horizontally in rungs. Standard symbols are used to represent components like switches, relays, and outputs. Logic functions like AND, OR, and NOT can be depicted using these symbols. Ladder diagrams are used to program PLCs and show control circuits for machines, depicting inputs, logic functions, and outputs.
Practical Programmable Logic Controllers (PLCs) for Automation and Process Co...Living Online
This workshop is designed to benefit you with practical up-to-date information on the application of PLCs for the automation and process control of plants and factories. It is suitable for people who have little or no exposure to PLCs, but expect to become involved in some or all aspects of PLC installation. It aims to give practical advice from experts in the field, to assist you to correctly plan, program and install a PLC with a shorter learning curve and more confidence. The inventible question is which PLC is being used. We present this course focusing on the generic PLC and use the open programming IEC 61131-3 standard.
For specific examples we use the Allen Bradley range, but are not selling Allen Bradley or for that matter any other PLC! While the workshop is ideal for electricians, technicians and engineers who are new to PLCs, much of the workshop and additional material in the extensive manual will be of value to those who already have some basic skills, but need a wider perspective for larger and more challenging tasks ahead. The accompanying manual includes contributions from a number of experts and will become a valuable reference in your work. The information contained in this workshop advances from the basics to challenge even the most experienced engineer in the industry today.
WHO SHOULD ATTEND?
Consulting engineers
Design engineers
DCS personnel
Electrical engineers
Engineering managers
Instrumentation and control engineers
Instrumentation technicians
Process control engineers
Process control operators
Shift electricians
Trades staff working with or near PLCs
MORE INFORMATION: http://www.idc-online.com/content/practical-programmable-logic-controllers-plcs-automation-and-process-control-39
The document discusses programmable logic controllers (PLCs). It describes PLCs as digital logic assemblies that make logical decisions and provide outputs. PLCs are programmed to interface between input sensors and output devices. The document outlines some advantages and disadvantages of PLCs. It also describes common PLC components, how PLCs operate in a cycle, and ladder logic programming.
Programmable logic controllers (PLCs) are microprocessor-based devices used to monitor, control, and automate electromechanical processes. PLCs replaced hardwired relay panels and are programmed using ladder logic. A PLC consists of a central processing unit, input and output modules to interface with sensors and actuators, and a programming device. PLCs scan inputs, execute a user-written program, and update outputs to control machines and processes in a flexible, easy-to-program manner.
The document provides an introduction to programmable logic controllers (PLCs). It discusses how PLCs replaced electromechanical relays for control systems, allowing logic functions to be programmed rather than physically wired. The summary describes how PLCs work, including taking input from sensors and switches and outputting signals to devices. It also discusses how PLCs are programmed using ladder logic and can be reprogrammed to change control functions without rewiring. The document provides examples of programming PLCs to control lights and motors.
This Presentation provides detail description PLC basics, architecture, Programming with Case studies. Quizzes are also included in between the topics in order to measure the Learning.
The document provides an overview of programmable logic controllers (PLCs). It defines PLCs as digital electronic devices that use programmable memory to implement logic functions like sequencing and timing to control machines and processes. The document discusses the basic structure of PLCs including the CPU, memory, input/output interfaces, and power supply. It also covers programming methods like ladder logic and instruction lists. Additional topics include input/output addressing, timers, counters, and techniques like latching, internal relays, and sequencing using timers.
The document discusses the basics of programmable logic controller (PLC) programming including PLC architecture, memory organization, programming languages, ladder logic instructions, addressing schemes, and programming techniques. Specifically, it covers the processor memory being divided into program and data memory, the ladder logic programming language using relay-type instructions like examine if closed and examine if open, addressing I/O locations by module and bit, and programming concepts such as parallel and nested rungs, internal control relays, and adjustments for different scan patterns.
Ladder diagrams are a type of graphic language used to represent logic systems for industrial control. They resemble a ladder, with two vertical power rails and circuits connected horizontally in rungs. Standard symbols are used to represent components like switches, relays, and outputs. Logic functions like AND, OR, and NOT can be depicted using these symbols. Ladder diagrams are used to program PLCs and show control circuits for machines, depicting inputs, logic functions, and outputs.
Practical Programmable Logic Controllers (PLCs) for Automation and Process Co...Living Online
This workshop is designed to benefit you with practical up-to-date information on the application of PLCs for the automation and process control of plants and factories. It is suitable for people who have little or no exposure to PLCs, but expect to become involved in some or all aspects of PLC installation. It aims to give practical advice from experts in the field, to assist you to correctly plan, program and install a PLC with a shorter learning curve and more confidence. The inventible question is which PLC is being used. We present this course focusing on the generic PLC and use the open programming IEC 61131-3 standard.
For specific examples we use the Allen Bradley range, but are not selling Allen Bradley or for that matter any other PLC! While the workshop is ideal for electricians, technicians and engineers who are new to PLCs, much of the workshop and additional material in the extensive manual will be of value to those who already have some basic skills, but need a wider perspective for larger and more challenging tasks ahead. The accompanying manual includes contributions from a number of experts and will become a valuable reference in your work. The information contained in this workshop advances from the basics to challenge even the most experienced engineer in the industry today.
WHO SHOULD ATTEND?
Consulting engineers
Design engineers
DCS personnel
Electrical engineers
Engineering managers
Instrumentation and control engineers
Instrumentation technicians
Process control engineers
Process control operators
Shift electricians
Trades staff working with or near PLCs
MORE INFORMATION: http://www.idc-online.com/content/practical-programmable-logic-controllers-plcs-automation-and-process-control-39
The document discusses programmable logic controllers (PLCs). It describes PLCs as digital logic assemblies that make logical decisions and provide outputs. PLCs are programmed to interface between input sensors and output devices. The document outlines some advantages and disadvantages of PLCs. It also describes common PLC components, how PLCs operate in a cycle, and ladder logic programming.
This document provides an overview of programmable logic controllers (PLCs). It defines a PLC as a digital computer used for industrial control systems. The document outlines the history of PLCs, their major components including the processor, power supply, and I/O modules. It describes the operational sequence of a PLC including input scan, logic solve, and output scan. Programming methods like ladder logic and functional block diagrams are discussed. Examples are provided of ladder logic programs for starting and stopping a motor. Advantages of PLCs include reliability, flexibility, and cost effectiveness. The document concludes by listing some common industrial applications of PLCs.
The document discusses programmable logic controllers (PLCs). It outlines the general objectives of explaining PLC concepts, including the structure of PLC systems and their components, programming languages, logic gates, counters, shift registers, and programming applications. It then defines PLCs, describes their advantages, functions, basic control systems, hardware and software components, programming formats, logic gates, timers, counters, internal relays, shift registers, and provides examples of PLC programming applications for machines.
This document outlines a training course on programmable logic controllers (PLCs) using the Siemens S7-1200 PLC and TIA Portal software. The course consists of 9 modules that cover topics such as PLC hardware components, programming basics, function blocks, timers and counters, math operations, diagnostics, closed-loop control, networking, and human-machine interfaces. The introduction module describes the major PLC components, relay ladder logic, and provides an overview of the S7-1200 PLC and TIA Portal software. The course objectives are to teach students how to program and configure the S7-1200 PLC to automate various industrial processes and systems.
A PLC is a programmable device that was invented to replace sequential relay circuits for machine control. It scans its inputs and outputs and executes a user-programmed logic program to control outputs based on inputs. Early PLCs were programmed using ladder logic to resemble relay circuits. Modern PLCs can be programmed in various languages and communicate with other devices. A PLC consists of a CPU, memory, and input and output modules to interface with the outside world.
PLC stands for programmable logic controller, which is an electronic device used to automate industrial processes. It uses a programming memory to store instructions to control machines and processes. PLCs are commonly used in manufacturing, food processing, metals, power, mining, and petrochemical industries. A PLC has a processor, power supply, input and output modules, and a programming device. PLCs were developed in the 1960s to replace relay-based control systems and became popular in the 1980s due to lower costs. PLCs use ladder logic programming to control inputs like sensors and outputs like motors.
The document discusses input and output devices used with programmable logic controllers (PLCs). It describes how input modules convert signals from devices like sensors into logic levels for the PLC processor, while output modules convert signals from the processor into levels that can drive devices like motors and valves. A variety of sensors are described that provide digital input signals to PLCs, including switches, encoders, temperature sensors, and smart sensors with integrated circuitry. Common output devices controlled by PLCs include relays, DC motors, solenoid valves, and stepper motors. Examples of PLC applications include conveyor belts, liquid level monitoring, lifts, and robot control systems.
A Programmable Logic Controller (PLC) or Programmable Controller is an electronic device used for Automation of industrial processes, such as control of machinery on factory assembly lines.
This document provides an introduction to Programmable Logic Controllers (PLCs). It discusses the history and evolution of control systems from humans to relays to PLCs. The key components of a PLC including the input/output modules, central processing unit, memory, and programming terminal are described. Programming methods for PLCs such as ladder logic, Boolean expressions, and mnemonics are also introduced. Examples of using timers, counters, and internal relays in PLC programs are provided.
This document provides an overview of programmable logic controller (PLC) architecture. It discusses PLC components like the memory unit and input/output modules. It describes different PLC types including fixed, modular, and rack PLCs. The document also covers the PLC scan cycle involving input scanning, program execution, and output scanning. Common PLC programming methods like ladder logic and structured text are introduced. Key concepts such as latching and unlatching in PLC programs are defined.
This document provides an overview of programmable logic controllers (PLCs) and their history and applications. It discusses how the first PLC was developed by General Motors in 1968 to replace a relay-based system. It also covers PLC components like the CPU, I/O modules, memory organization, and programming environments. Additional topics include sensors, discrete and analog I/O, addressing schemes, programming instructions like moves, comparisons, and counters. The purpose is to introduce students to basic PLC programming concepts.
This document provides an overview of a basic PLC training course. It describes the major components of a PLC including the processor, memory, I/O modules, and programming device. It also outlines the course contents which will cover the history of PLCs, programming concepts, applications, and troubleshooting. The objectives are for participants to understand PLC components, programming, applications, and basic troubleshooting.
Programmable logic controllers (PLCs) have evolved from relay controllers and semiconductor logic controllers. PLCs are commonly used in industrial applications for on-off control, sequential control, feedback control, and motion control. PLCs have input and output modules that interface with field devices, a processor that runs the user-defined control program, and memory. Leading PLC manufacturers include Allen-Bradley, Siemens, and Omron. PLCs offer advantages over traditional controllers like easy reprogramming, real-time operation, low cost, and minimal maintenance.
This is a small project on Siemens PLC Step 7 models. The project required lot of lateral thinking and logical decision making in order to develop programs for the traffic light management for the entire chandigarh city. The project is known as Total Traffic Security & Management (TTSM)
This document discusses programmable logic controllers (PLCs). It provides a brief history of PLCs, describing their origins in the 1960s as replacements for relay logic systems. The document outlines the typical components of a PLC system, including the processor, memory unit, power supply, input/output modules, and programming device. It also describes ladder logic, a common programming language for PLCs using a logic diagram format. Finally, some applications of PLCs in industries like manufacturing and materials handling are listed.
Deep Tech System No1 Institute for providing plc training in Delhi NCR. We offer PLC programming training a wide scope to gain good knowledge in automation industrial training with extensive lab facilities.
This document provides an introduction to PLC programming and ladder logic. It discusses the most common programming languages for PLCs, with ladder logic being the dominant method as it was developed to mimic relay logic. Ladder logic uses graphic symbols of rungs and contacts to represent circuit diagrams. The document also briefly outlines other programming methods for PLCs such as sequential function charts, structured text, and function block diagrams.
The document provides an overview of programmable logic controllers (PLCs). It discusses that PLCs were invented in 1968 to replace relay-based control systems. The document then describes the basic architecture of a PLC including its main components like the power supply, input/output modules, processor, and programming device. It also discusses the different types of PLC programming including ladder logic and the use of timers and counters. Some key applications and advantages of PLCs are that they provide reliable, flexible and cost-effective control of complex automation systems.
- Programmable logic controllers (PLCs) were developed in the late 1960s to replace relay-based control systems used in manufacturing.
- The first PLC was created by General Motors in 1968 to meet their need for a programmable, reliable, and durable controller that could operate in industrial environments.
- PLCs became popular in the 1980s as their costs dropped. They are now commonly used in industrial automation to control machinery and processes.
The document provides an overview of programmable logic controllers (PLCs) and their programming. It discusses what a PLC is, its basic components like the CPU, I/O modules, and power supply. It also covers PLC programming languages and common types of programs like ladder logic. The document then presents examples of interfacing sensors and output devices to PLC I/O modules. It discusses digital and analog I/O modules and how devices are connected via sinking and sourcing configurations. Finally, it provides guidance on setting up a basic PLC system by selecting components based on I/O counts and power requirements.
Ch 2 Ladder Basics 1
Chapter 2 Ladder Basics
Understanding Electrical Ladder Drawings
Before discussing or understanding the Programmable Logic Controller, we must first
understand the devices the PLC was invented to replace. That was the ladder diagram. The
ladder diagram or electrical schematic or elementary diagram can be divided into two distinct
portions. The first is the power portion and the second is the control. Showing flow of power to
a motor or other device in a factory environment is the primary focus of the power portion.
Showing control of that motor is the focus of the control portion. While power flow is
important, the focus is to dissect the control portion. Items such as fuses and disconnect devices
may appear in both power and control circuits. The use of symbols is important for both power
and control. The types of symbols vary and are somewhat arbitrary based on where you are
located and, in some cases, the company you work for. This is contrary to the symbols used in
the PLC which must be fixed or uniform. The use of symbols is a descriptive description of the
device being used. A list of some symbols and the function of the device is given below:
SPST SPDT
DPST DPDT
Spring
return
Spring
return
Pushbutton
make
Pushbutton
break
Pushbutton
2-circuit
Selector
switch
Shorting
selector
Proximity
limit switch
Time delay
make
Time delay
break
Safety
interlock
Flow
actuated
Liquid level
actuated
Pressure
activated
Temperature
activated
Inertia
switch
Relay
contact
NO
Relay
coil
Relay
contact
NC
Pilot
light
Fig. 2-1a Examples of American
Symbols used in Electrical Diagrams
Ch 2 Ladder Basics 2
Make
contact
Break
contact
Two way
contact
Passing
make-contact
Spring
return
Circuit
breaker
Limit
switch
Stay put
Time delay
make
Time delay
break
Liquid level
actuated
Gas flow
actuated
Temperature
switch
Fuse
Some of the symbols are used in examples on the following pages. Many devices follow the
pattern of how the device actually works. For example, notice how the foot switch resembles an
actual foot switch or how a flow or sail switch resembles a sail on a sailboat. Also, note that
different combinations of open and closed contacts can be furnished on the same device to give
multiple combinations of functions to the control circuit either through hard-wired circuits or to
the PLC and then through its logic to the appropriate output.
Many times, the input devices are said to be either normally open (NO) or normally closed (NC).
The normally open or closed status refers to the shelf state of the device. If a device is normally
open, a resistance check of the device with a digital multi-meter will give a reading of OL. If the
device is normally closed, a resistance check will give a reading of 0.0.
The normally open and normally closed state of the de ...
This document provides an introduction to programming programmable logic controllers (PLCs) using ladder diagrams, functional block diagrams, and statement list languages. It describes the basic techniques for developing ladder and function block programs representing logic functions like AND, OR, XOR, NAND and NOR. Specific topics covered include ladder diagram conventions, logic gates and truth tables, and latching circuits to hold an output energized even after the input ceases. Examples are provided of drawing ladder diagrams for logic functions and latching circuits to control devices like motors.
This document provides an overview of programmable logic controllers (PLCs). It defines a PLC as a digital computer used for industrial control systems. The document outlines the history of PLCs, their major components including the processor, power supply, and I/O modules. It describes the operational sequence of a PLC including input scan, logic solve, and output scan. Programming methods like ladder logic and functional block diagrams are discussed. Examples are provided of ladder logic programs for starting and stopping a motor. Advantages of PLCs include reliability, flexibility, and cost effectiveness. The document concludes by listing some common industrial applications of PLCs.
The document discusses programmable logic controllers (PLCs). It outlines the general objectives of explaining PLC concepts, including the structure of PLC systems and their components, programming languages, logic gates, counters, shift registers, and programming applications. It then defines PLCs, describes their advantages, functions, basic control systems, hardware and software components, programming formats, logic gates, timers, counters, internal relays, shift registers, and provides examples of PLC programming applications for machines.
This document outlines a training course on programmable logic controllers (PLCs) using the Siemens S7-1200 PLC and TIA Portal software. The course consists of 9 modules that cover topics such as PLC hardware components, programming basics, function blocks, timers and counters, math operations, diagnostics, closed-loop control, networking, and human-machine interfaces. The introduction module describes the major PLC components, relay ladder logic, and provides an overview of the S7-1200 PLC and TIA Portal software. The course objectives are to teach students how to program and configure the S7-1200 PLC to automate various industrial processes and systems.
A PLC is a programmable device that was invented to replace sequential relay circuits for machine control. It scans its inputs and outputs and executes a user-programmed logic program to control outputs based on inputs. Early PLCs were programmed using ladder logic to resemble relay circuits. Modern PLCs can be programmed in various languages and communicate with other devices. A PLC consists of a CPU, memory, and input and output modules to interface with the outside world.
PLC stands for programmable logic controller, which is an electronic device used to automate industrial processes. It uses a programming memory to store instructions to control machines and processes. PLCs are commonly used in manufacturing, food processing, metals, power, mining, and petrochemical industries. A PLC has a processor, power supply, input and output modules, and a programming device. PLCs were developed in the 1960s to replace relay-based control systems and became popular in the 1980s due to lower costs. PLCs use ladder logic programming to control inputs like sensors and outputs like motors.
The document discusses input and output devices used with programmable logic controllers (PLCs). It describes how input modules convert signals from devices like sensors into logic levels for the PLC processor, while output modules convert signals from the processor into levels that can drive devices like motors and valves. A variety of sensors are described that provide digital input signals to PLCs, including switches, encoders, temperature sensors, and smart sensors with integrated circuitry. Common output devices controlled by PLCs include relays, DC motors, solenoid valves, and stepper motors. Examples of PLC applications include conveyor belts, liquid level monitoring, lifts, and robot control systems.
A Programmable Logic Controller (PLC) or Programmable Controller is an electronic device used for Automation of industrial processes, such as control of machinery on factory assembly lines.
This document provides an introduction to Programmable Logic Controllers (PLCs). It discusses the history and evolution of control systems from humans to relays to PLCs. The key components of a PLC including the input/output modules, central processing unit, memory, and programming terminal are described. Programming methods for PLCs such as ladder logic, Boolean expressions, and mnemonics are also introduced. Examples of using timers, counters, and internal relays in PLC programs are provided.
This document provides an overview of programmable logic controller (PLC) architecture. It discusses PLC components like the memory unit and input/output modules. It describes different PLC types including fixed, modular, and rack PLCs. The document also covers the PLC scan cycle involving input scanning, program execution, and output scanning. Common PLC programming methods like ladder logic and structured text are introduced. Key concepts such as latching and unlatching in PLC programs are defined.
This document provides an overview of programmable logic controllers (PLCs) and their history and applications. It discusses how the first PLC was developed by General Motors in 1968 to replace a relay-based system. It also covers PLC components like the CPU, I/O modules, memory organization, and programming environments. Additional topics include sensors, discrete and analog I/O, addressing schemes, programming instructions like moves, comparisons, and counters. The purpose is to introduce students to basic PLC programming concepts.
This document provides an overview of a basic PLC training course. It describes the major components of a PLC including the processor, memory, I/O modules, and programming device. It also outlines the course contents which will cover the history of PLCs, programming concepts, applications, and troubleshooting. The objectives are for participants to understand PLC components, programming, applications, and basic troubleshooting.
Programmable logic controllers (PLCs) have evolved from relay controllers and semiconductor logic controllers. PLCs are commonly used in industrial applications for on-off control, sequential control, feedback control, and motion control. PLCs have input and output modules that interface with field devices, a processor that runs the user-defined control program, and memory. Leading PLC manufacturers include Allen-Bradley, Siemens, and Omron. PLCs offer advantages over traditional controllers like easy reprogramming, real-time operation, low cost, and minimal maintenance.
This is a small project on Siemens PLC Step 7 models. The project required lot of lateral thinking and logical decision making in order to develop programs for the traffic light management for the entire chandigarh city. The project is known as Total Traffic Security & Management (TTSM)
This document discusses programmable logic controllers (PLCs). It provides a brief history of PLCs, describing their origins in the 1960s as replacements for relay logic systems. The document outlines the typical components of a PLC system, including the processor, memory unit, power supply, input/output modules, and programming device. It also describes ladder logic, a common programming language for PLCs using a logic diagram format. Finally, some applications of PLCs in industries like manufacturing and materials handling are listed.
Deep Tech System No1 Institute for providing plc training in Delhi NCR. We offer PLC programming training a wide scope to gain good knowledge in automation industrial training with extensive lab facilities.
This document provides an introduction to PLC programming and ladder logic. It discusses the most common programming languages for PLCs, with ladder logic being the dominant method as it was developed to mimic relay logic. Ladder logic uses graphic symbols of rungs and contacts to represent circuit diagrams. The document also briefly outlines other programming methods for PLCs such as sequential function charts, structured text, and function block diagrams.
The document provides an overview of programmable logic controllers (PLCs). It discusses that PLCs were invented in 1968 to replace relay-based control systems. The document then describes the basic architecture of a PLC including its main components like the power supply, input/output modules, processor, and programming device. It also discusses the different types of PLC programming including ladder logic and the use of timers and counters. Some key applications and advantages of PLCs are that they provide reliable, flexible and cost-effective control of complex automation systems.
- Programmable logic controllers (PLCs) were developed in the late 1960s to replace relay-based control systems used in manufacturing.
- The first PLC was created by General Motors in 1968 to meet their need for a programmable, reliable, and durable controller that could operate in industrial environments.
- PLCs became popular in the 1980s as their costs dropped. They are now commonly used in industrial automation to control machinery and processes.
The document provides an overview of programmable logic controllers (PLCs) and their programming. It discusses what a PLC is, its basic components like the CPU, I/O modules, and power supply. It also covers PLC programming languages and common types of programs like ladder logic. The document then presents examples of interfacing sensors and output devices to PLC I/O modules. It discusses digital and analog I/O modules and how devices are connected via sinking and sourcing configurations. Finally, it provides guidance on setting up a basic PLC system by selecting components based on I/O counts and power requirements.
Ch 2 Ladder Basics 1
Chapter 2 Ladder Basics
Understanding Electrical Ladder Drawings
Before discussing or understanding the Programmable Logic Controller, we must first
understand the devices the PLC was invented to replace. That was the ladder diagram. The
ladder diagram or electrical schematic or elementary diagram can be divided into two distinct
portions. The first is the power portion and the second is the control. Showing flow of power to
a motor or other device in a factory environment is the primary focus of the power portion.
Showing control of that motor is the focus of the control portion. While power flow is
important, the focus is to dissect the control portion. Items such as fuses and disconnect devices
may appear in both power and control circuits. The use of symbols is important for both power
and control. The types of symbols vary and are somewhat arbitrary based on where you are
located and, in some cases, the company you work for. This is contrary to the symbols used in
the PLC which must be fixed or uniform. The use of symbols is a descriptive description of the
device being used. A list of some symbols and the function of the device is given below:
SPST SPDT
DPST DPDT
Spring
return
Spring
return
Pushbutton
make
Pushbutton
break
Pushbutton
2-circuit
Selector
switch
Shorting
selector
Proximity
limit switch
Time delay
make
Time delay
break
Safety
interlock
Flow
actuated
Liquid level
actuated
Pressure
activated
Temperature
activated
Inertia
switch
Relay
contact
NO
Relay
coil
Relay
contact
NC
Pilot
light
Fig. 2-1a Examples of American
Symbols used in Electrical Diagrams
Ch 2 Ladder Basics 2
Make
contact
Break
contact
Two way
contact
Passing
make-contact
Spring
return
Circuit
breaker
Limit
switch
Stay put
Time delay
make
Time delay
break
Liquid level
actuated
Gas flow
actuated
Temperature
switch
Fuse
Some of the symbols are used in examples on the following pages. Many devices follow the
pattern of how the device actually works. For example, notice how the foot switch resembles an
actual foot switch or how a flow or sail switch resembles a sail on a sailboat. Also, note that
different combinations of open and closed contacts can be furnished on the same device to give
multiple combinations of functions to the control circuit either through hard-wired circuits or to
the PLC and then through its logic to the appropriate output.
Many times, the input devices are said to be either normally open (NO) or normally closed (NC).
The normally open or closed status refers to the shelf state of the device. If a device is normally
open, a resistance check of the device with a digital multi-meter will give a reading of OL. If the
device is normally closed, a resistance check will give a reading of 0.0.
The normally open and normally closed state of the de ...
This document provides an introduction to programming programmable logic controllers (PLCs) using ladder diagrams, functional block diagrams, and statement list languages. It describes the basic techniques for developing ladder and function block programs representing logic functions like AND, OR, XOR, NAND and NOR. Specific topics covered include ladder diagram conventions, logic gates and truth tables, and latching circuits to hold an output energized even after the input ceases. Examples are provided of drawing ladder diagrams for logic functions and latching circuits to control devices like motors.
The document provides an overview of logic gates and their functions. It defines logic gates as basic building blocks of digital circuits that take binary inputs and produce binary outputs. The five basic logic gates are described as AND, OR, NOT, NAND, and NOR gates. Truth tables and circuit diagrams are given to illustrate the input-output relationships for each gate. Examples of applications of logic gates in digital systems like alarms and automobiles are also presented.
SCAN CHAINS TESTING FOR LATCHES TO REDUCE AREA AND THE POWER CONSUMPTIONcscpconf
During the test mode of flip flop in a chip, a set of input vectors are sent through the flip-flop, it
consumes more power consumption than in the normal functional mode. In this paper, we
propose a latch with bi -stable element which reduces area as well as the power consumed. The
latch proposed consists of simple basic gates involving two inverters back to back which acts as
a bi-stable element and a transmission gate with the clock signal used to enable and disable the
rest of the circuit with impact on running the latch on Static Timing Analysis. The input test
vectors are either given by Automatic Test Pattern Generation (ATPG) or many other methods.
We model this using T-Simulation Program with Integrated Circuit Emphasis (T-SPICE) and see the power consumed.
Here is the code for an open loop speed controller:
#include <PWM.h>
#define ENABLE 5
#define DIR1 3
#define DIR2 4
void setup() {
pinMode(ENABLE, OUTPUT);
pinMode(DIR1, OUTPUT);
pinMode(DIR2, OUTPUT);
PWM.begin();
PWM.setPeriod(500); // Period in microseconds
}
void loop() {
// Run motor forward at half speed
digitalWrite(DIR1, HIGH);
digitalWrite(DIR2, LOW);
PWM.setDuty(ENABLE, 50); // Duty cycle in percent
delay(2000); // Run for 2 seconds
This document discusses digital logic gates and integrated circuits. It describes the NAND gate and how it is constructed from an AND gate and inverter. It explains that integrated circuit chips contain networks of transistors to perform logic functions and discusses the common 7400 series chips. It then gives details on an experiment to design and simulate OR, NAND, AND, and NOR logic gates using integrated circuits, LEDs, resistors, and a breadboard. Truth tables are provided to summarize the output for each gate based on the input values.
This document discusses digital logic gates and integrated circuits. It describes the NAND gate and how it is constructed from an AND gate and inverter. It explains that integrated circuit chips contain networks of transistors to perform logic functions and discusses the common 7400 series chips. It then gives details on an experiment to design and simulate OR, NAND, AND logic gate circuits using integrated circuits, resistors, LEDs, and a breadboard. Truth tables are provided to summarize the output for each gate based on the input values.
This document describes a study that designed a fuzzy logic controller for a boost DC-DC converter using MATLAB/Simulink software. The objective was to develop a fuzzy logic algorithm to control the output voltage of the boost converter in steady state conditions. Simulation results showed that the fuzzy logic controller was able to maintain the output voltage with no overshoot, unlike the open loop converter which had 80% overshoot. In conclusion, the fuzzy logic controller improved the dynamic performance and stability of the boost converter compared to an open loop design.
counter using 4 master slave flip-flops ZunAib Ali
This document describes how to design a 4-bit counter using master-slave JK flip-flops. It begins by explaining what a flip-flop is and describing common flip-flop types like the SR, JK, and master-slave JK flip-flop. It then shows how to connect 4 master-slave JK flip-flops in a ring configuration to form a counter that will count from 0 to 15 (hexadecimal F). The document concludes by presenting the circuit design of a 4-bit counter created using DSCH simulation software, along with output waveforms and a timing diagram verifying the counter operates as intended.
This document discusses programmable logic controllers (PLCs), including their basic structure, input/output processing, and programming. It notes that PLCs process inputs and outputs through programming, with ladder programming being a common technique. Ladder diagrams follow certain conventions, such as each rung defining one operation and starting with an input and ending with an output. Inputs and outputs are identified by their addresses in the PLC's memory.
This document discusses relay logic diagrams and how to create them. Relay logic diagrams illustrate how industrial control systems operate by showing the logical relationships between devices. They are created by first defining components like rails, rungs, relay coils and contacts. Examples are then provided of basic circuits and rules for drawing relay logic diagrams are outlined, such as placing input devices on the left, a single output on the right, and ensuring current flows left to right. The document provides information on the purpose and process of creating relay logic diagrams for industrial control systems.
EEN 1200 L – Digital FundamentalsMerrimack CollegeLaborato.docxSALU18
EEN 1200 L – Digital Fundamentals
Merrimack College
Laboratory 2 – Introduction to 7404 Hex Inverter & Basic Gates & Timing Diagrams
NAME _____________________________
The purpose of this laboratory experiment is to:
I. Get practice in wiring integrated circuits on the trainers
II. Generate “Truth Tables” for basic gates.
III. Use these logic elements to generate a high level logic element.
IV. Generate the “Truth Table”, and identify the logic element.
V. Generate timing diagrams for the basic gates.
Part A: Introduction to 7404 Hex Inverter (IC)
Hook up the 7404 Hex Inverter integrated circuit, and look at the input and output of ONE of the inverters
(pins 1 and 2). Note: There are 6 inverters in the same package, hence the term Hex. See pin-outs of the 7404 Hex Inverter at the last page.
A
B
Z
A
B
Z
A
B
Z
A
B
C
Z
Z
A
B
Logic 1
Logic 1
A
B
Z
A-1) Using the built in signal generator (RED Trainer), place a 1 kHz and TTL input to the inverter. [Be sure to connect the IC (pin 14) to 5VDC and (pin 7) to the ground (GND)].
Look at the input and output together on the scope using 2 Volts DC per division. Draw what you see:
Output Waveform
Input Waveform
Note: Label the x-axis and y-axis
A-2) What is the period? ________________milli-seconds
A-3) Use 5.0 VDC per division on the scope, then read the following on the scope. Draw what you see below.
Voltage amplitude of input=____________
Voltage amplitude of output=____________
Frequency of input=____________
Frequency of output=____________
Period of input=____________
Period of output=____________
Note: Label the x-axis and y-axis
Part B: Basic Gates & Timing Diagrams
A
B
Z
A Truth Table consists of all possible logic inputs and the output that corresponds to each logic input.
In the following, an AND Gate with logic inputs A & B with a logic output Z is shown:
A input
B input
Z output
0
0
0
1
0
0
0
1
0
1
1
1
The timing diagram is discussed at length in Chapter 3. The basic timing diagrams for the logic gates we are using in this lab allow for a visual reading of how the logic functions. By viewing the timing diagram, the reader can see what outputs result from the different possible combinations of inputs. The truth table can easily be determined by looking at the basic timing diagram.
B-1) Follow steps of part A-1 to hook up the 7408 of Quad 2-Input AND gate, then connect input A to SW7 and input B to SW6 then generate a truth table below by measuring output Z using voltmeter. Connect a wire from the output pin to any of the 8 bits LED display located on the upper right corner of the function generator. Note: There are 4 AND gates in the same package.
A
B
C
Z
Input A
Input B
output Z
0
0
1
0
0
1
1
1
B ...
Automation could be achieved with the aid of Industrial Controller PLC. PLC basic Programming are discussed in this presentation.Case studies are available and solutions for those questions will be updated in next presentation.
This document provides an introduction to arithmetic logic units (ALUs), combinational circuits, and sequential circuits. It defines what an ALU is, its basic components and that it is the fundamental unit of any computing system. It then describes the differences between combinational and sequential circuits, listing examples of each type including common gates, adders and flip-flops. The document outlines the procedures for designing, analyzing and implementing both types of digital circuits.
This document discusses the design and analysis of a Single-Ended Primary-Inductor Converter (SEPIC) circuit. It provides an overview of SEPIC converters and how they allow the output voltage to be greater than, less than, or equal to the input voltage. The document then describes the methodology for analyzing a SEPIC circuit operating in continuous mode. It includes calculations for determining the output voltage, inductor and capacitor values, voltage ripple, current stresses, and MOSFET selection. Simulation results are presented and disadvantages of SEPIC converters are noted.
1. A bus is a communication system that transfers data between components inside a computer or between computers using both parallel and serial connections.
2. An Arithmetic Logic Unit (ALU) performs arithmetic and logical operations and was a core component of the earliest computer architectures proposed by John Von Neumann in 1945.
3. Logic gates are the basic building blocks of digital circuits and perform logical operations like AND, OR, and NOT on binary inputs to produce binary outputs. Common logic gates include AND, OR, NOT, NAND, NOR, XOR, and XNOR.
The document discusses NOR gates and their universality. NOR gates are universal gates, meaning they can be combined to form any other logic gate. For example, an AND gate can be implemented using four NOR gates in series, and an OR gate can be implemented using three NOR gates. The document describes an experiment using NOR gates with LED outputs to verify the truth table for a NOR gate circuit. It summarizes that the LED outputs matched the expected outputs in the truth table, demonstrating the functionality of the NOR gate.
বাংলাদেশের অর্থনৈতিক সমীক্ষা ২০২৪ [Bangladesh Economic Review 2024 Bangla.pdf] কম্পিউটার , ট্যাব ও স্মার্ট ফোন ভার্সন সহ সম্পূর্ণ বাংলা ই-বুক বা pdf বই " সুচিপত্র ...বুকমার্ক মেনু 🔖 ও হাইপার লিংক মেনু 📝👆 যুক্ত ..
আমাদের সবার জন্য খুব খুব গুরুত্বপূর্ণ একটি বই ..বিসিএস, ব্যাংক, ইউনিভার্সিটি ভর্তি ও যে কোন প্রতিযোগিতা মূলক পরীক্ষার জন্য এর খুব ইম্পরট্যান্ট একটি বিষয় ...তাছাড়া বাংলাদেশের সাম্প্রতিক যে কোন ডাটা বা তথ্য এই বইতে পাবেন ...
তাই একজন নাগরিক হিসাবে এই তথ্য গুলো আপনার জানা প্রয়োজন ...।
বিসিএস ও ব্যাংক এর লিখিত পরীক্ষা ...+এছাড়া মাধ্যমিক ও উচ্চমাধ্যমিকের স্টুডেন্টদের জন্য অনেক কাজে আসবে ...
How to Build a Module in Odoo 17 Using the Scaffold MethodCeline George
Odoo provides an option for creating a module by using a single line command. By using this command the user can make a whole structure of a module. It is very easy for a beginner to make a module. There is no need to make each file manually. This slide will show how to create a module using the scaffold method.
Walmart Business+ and Spark Good for Nonprofits.pdfTechSoup
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Answers about how you can do more with Walmart!"
The simplified electron and muon model, Oscillating Spacetime: The Foundation...RitikBhardwaj56
Discover the Simplified Electron and Muon Model: A New Wave-Based Approach to Understanding Particles delves into a groundbreaking theory that presents electrons and muons as rotating soliton waves within oscillating spacetime. Geared towards students, researchers, and science buffs, this book breaks down complex ideas into simple explanations. It covers topics such as electron waves, temporal dynamics, and the implications of this model on particle physics. With clear illustrations and easy-to-follow explanations, readers will gain a new outlook on the universe's fundamental nature.
This presentation was provided by Steph Pollock of The American Psychological Association’s Journals Program, and Damita Snow, of The American Society of Civil Engineers (ASCE), for the initial session of NISO's 2024 Training Series "DEIA in the Scholarly Landscape." Session One: 'Setting Expectations: a DEIA Primer,' was held June 6, 2024.
Executive Directors Chat Leveraging AI for Diversity, Equity, and InclusionTechSoup
Let’s explore the intersection of technology and equity in the final session of our DEI series. Discover how AI tools, like ChatGPT, can be used to support and enhance your nonprofit's DEI initiatives. Participants will gain insights into practical AI applications and get tips for leveraging technology to advance their DEI goals.
ISO/IEC 27001, ISO/IEC 42001, and GDPR: Best Practices for Implementation and...PECB
Denis is a dynamic and results-driven Chief Information Officer (CIO) with a distinguished career spanning information systems analysis and technical project management. With a proven track record of spearheading the design and delivery of cutting-edge Information Management solutions, he has consistently elevated business operations, streamlined reporting functions, and maximized process efficiency.
Certified as an ISO/IEC 27001: Information Security Management Systems (ISMS) Lead Implementer, Data Protection Officer, and Cyber Risks Analyst, Denis brings a heightened focus on data security, privacy, and cyber resilience to every endeavor.
His expertise extends across a diverse spectrum of reporting, database, and web development applications, underpinned by an exceptional grasp of data storage and virtualization technologies. His proficiency in application testing, database administration, and data cleansing ensures seamless execution of complex projects.
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Date: May 29, 2024
Tags: Information Security, ISO/IEC 27001, ISO/IEC 42001, Artificial Intelligence, GDPR
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A workshop hosted by the South African Journal of Science aimed at postgraduate students and early career researchers with little or no experience in writing and publishing journal articles.
it describes the bony anatomy including the femoral head , acetabulum, labrum . also discusses the capsule , ligaments . muscle that act on the hip joint and the range of motion are outlined. factors affecting hip joint stability and weight transmission through the joint are summarized.
Exploiting Artificial Intelligence for Empowering Researchers and Faculty, In...Dr. Vinod Kumar Kanvaria
Exploiting Artificial Intelligence for Empowering Researchers and Faculty,
International FDP on Fundamentals of Research in Social Sciences
at Integral University, Lucknow, 06.06.2024
By Dr. Vinod Kumar Kanvaria
Film vocab for eal 3 students: Australia the movie
Plc intro
1. 5
2. PROGRAMMABLE LOGIC CONTROLLERS
Topics:
• PLC History
• Ladder Logic and Relays
• PLC Programming
• PLC Operation
• An Example
Objectives:
• Know general PLC issues
• To be able to write simple ladder logic programs
• Understand the operation of a PLC
2.1 INTRODUCTION
Control engineering has evolved over time. In the past humans were the main method for con-
trolling a system. More recently electricity has been used for control and early electrical control was
based on relays. These relays allow power to be switched on and off without a mechanical switch. It is
common to use relays to make simple logical control decisions. The development of low cost computer
has brought the most recent revolution, the Programmable Logic Controller (PLC). The advent of the
PLC began in the 1970s, and has become the most common choice for manufacturing controls.
PLCs have been gaining popularity on the factory floor and will probably remain predominant
for some time to come. Most of this is because of the advantages they offer.
• Cost effective for controlling complex systems.
• Flexible and can be reapplied to control other systems quickly and easily.
• Computational abilities allow more sophisticated control.
• Trouble shooting aids make programming easier and reduce downtime.
• Reliable components make these likely to operate for years before failure.
2.1.1 Ladder Logic
Ladder logic is the main programming method used for PLCs. As mentioned before, ladder
logic has been developed to mimic relay logic. The decision to use the relay logic diagrams was a stra-
tegic one. By selecting ladder logic as the main programming method, the amount of retraining needed
for engineers and tradespeople was greatly reduced.
Modern control systems still include relays, but these are rarely used for logic. A relay is a sim-
ple device that uses a magnetic field to control a switch, as pictured in Figure 2. When a voltage is
2. 6
applied to the input coil, the resulting current creates a magnetic field. The magnetic field pulls a metal
switch (or reed) towards it and the contacts touch, closing the switch. The contact that closes when the
coil is energized is called normally open. The normally closed contacts touch when the input coil is not
energized. Relays are normally drawn in schematic form using a circle to represent the input coil. The
output contacts are shown with two parallel lines. Normally open contacts are shown as two lines, and
will be open (non-conducting) when the input is not energized. Normally closed contacts are shown
with two lines with a diagonal line through them. When the input coil is not energized the normally
closed contacts will be closed (conducting).
input coil
OR
normally
closed normally
open
OR
Figure 2 Simple Relay Layouts and Schematics
Relays are used to let one power source close a switch for another (often high current) power
source, while keeping them isolated. An example of a relay in a simple control application is shown in
Figure 3. In this system the first relay on the left is used as normally closed, and will allow current to
flow until a voltage is applied to the input A. The second relay is normally open and will not allow cur-
rent to flow until a voltage is applied to the input B. If current is flowing through the first two relays
3. 7
then current will flow through the coil in the third relay, and close the switch for output C. This circuit
would normally be drawn in the ladder logic form. This can be read logically as C will be on if A is off
and B is on.
115VAC
wall plug
relay logic
output C
input A input B
(normally open) (normally open)
(normally closed)
A B C
ladder logic
Figure 3 A Simple Relay Controller
The example in Figure 3 does not show the entire control system, but only the logic. When we
consider a PLC there are inputs, outputs, and the logic. Figure 4 shows a more complete representation
of the PLC. Here there are two inputs from push buttons. We can imagine the inputs as activating 24V
DC relay coils in the PLC. This in turn drives an output relay that switches 115V AC, that will turn on a
light. Note, in actual PLCs inputs are never relays, but outputs are often relays. The ladder logic in the
PLC is actually a computer program that the user can enter and change. Notice that both of the input
push buttons are normally open, but the ladder logic inside the PLC has one normally open contact, and
one normally closed contact. Do not think that the ladder logic in the PLC needs to match the inputs or
outputs. Many beginners will get caught trying to make the ladder logic match the input types.
4. 8
push buttons
power
supply
+24V
com.
PLC
inputs
ladder
logic
A B C
outputs
115Vac light
AC power
neut.
Figure 4 A PLC Illustrated With Relays
Many relays also have multiple outputs (throws) and this allows an output relay to also be an
input simultaneously. The circuit shown in Figure 5 is an example of this, it is called a seal in circuit. In
this circuit the current can flow through either branch of the circuit, through the contacts labelled A or
B. The input B will only be on when the output B is on. If B is off, and A is energized, then B will turn
on. If B turns on then the input B will turn on, and keep output B on even if input A goes off. After B is
turned on the output B will not turn off.
5. 9
A B
B
Note: When A is pushed, the output B will turn on, and
the input B will also turn on and keep B on perma-
nently - until power is removed.
Note: The line on the right is being left off intentionally
and is implied in these diagrams.
Figure 5 A Seal-in Circuit
2.1.2 Programming
The first PLCs were programmed with a technique that was based on relay logic wiring sche-
matics. This eliminated the need to teach the electricians, technicians and engineers how to program a
computer - but, this method has stuck and it is the most common technique for programming PLCs
today. An example of ladder logic can be seen in Figure 6. To interpret this diagram imagine that the
power is on the vertical line on the left hand side, we call this the hot rail. On the right hand side is the
neutral rail. In the figure there are two rungs, and on each rung there are combinations of inputs (two
vertical lines) and outputs (circles). If the inputs are opened or closed in the right combination the
power can flow from the hot rail, through the inputs, to power the outputs, and finally to the neutral rail.
An input can come from a sensor, switch, or any other type of sensor. An output will be some device
outside the PLC that is switched on or off, such as lights or motors. In the top rung the contacts are nor-
mally open and normally closed. Which means if input A is on and input B is off, then power will flow
through the output and activate it. Any other combination of input values will result in the output X
being off.
6. 10
HOT NEUTRAL
A B X
C D G Y
E F H
INPUTS OUTPUTS
Note: Power needs to flow through some combination of the inputs
(A,B,C,D,E,F,G,H) to turn on outputs (X,Y).
Figure 6 A Simple Ladder Logic Diagram
The second rung of Figure 6 is more complex, there are actually multiple combinations of
inputs that will result in the output Y turning on. On the left most part of the rung, power could flow
through the top if C is off and D is on. Power could also (and simultaneously) flow through the bottom
if both E and F are true. This would get power half way across the rung, and then if G or H is true the
power will be delivered to output Y. In later chapters we will examine how to interpret and construct
these diagrams.
There are other methods for programming PLCs. One of the earliest techniques involved mne-
monic instructions. These instructions can be derived directly from the ladder logic diagrams and
entered into the PLC through a simple programming terminal. An example of mnemonics is shown in
Figure 7. In this example the instructions are read one line at a time from top to bottom. The first line
00000 has the instruction LDN (input load and not) for input A. This will examine the input to the PLC
and if it is off it will remember a 1 (or true), if it is on it will remember a 0 (or false). The next line uses
an LD (input load) statement to look at the input. If the input is off it remembers a 0, if the input is on it
remembers a 1 (note: this is the reverse of the LDN). The AND statement recalls the last two numbers
remembered and if the are both true the result is a 1, otherwise the result is a 0. This result now replaces
the two numbers that were recalled, and there is only one number remembered. The process is repeated
for lines 00003 and 00004, but when these are done there are now three numbers remembered. The old-
est number is from the AND, the newer numbers are from the two LD instructions. The AND in line
00005 combines the results from the last LD instructions and now there are two numbers remembered.
The OR instruction takes the two numbers now remaining and if either one is a 1 the result is a 1, other-
wise the result is a 0. This result replaces the two numbers, and there is now a single number there. The
last instruction is the ST (store output) that will look at the last value stored and if it is 1, the output will
be turned on, if it is 0 the output will be turned off.
7. 11
00000 LDN A
00001 LD B
00002 AND the mnemonic code is equivalent to
00003 LD C the ladder logic below
00004 LD D
00005 AND
00006 OR
00007 ST X
00008 END A B X
C D
END
Note: The notation shown above is SOR
not standard Allen-Bradley BST
notation. The program to the XIC A
right would be the A-B equiva- XIO B
lent. NXB
XIO C
XIO D
BND
OTE X
EOR
END
Figure 7 An Example of a Mnemonic Program and Equivalent Ladder Logic
The ladder logic program in Figure 7, is equivalent to the mnemonic program. Even if you have
programmed a PLC with ladder logic, it will be converted to mnemonic form before being used by the
PLC. In the past mnemonic programming was the most common, but now it is uncommon for users to
even see mnemonic programs.
Sequential Function Charts (SFCs) have been developed to accommodate the programming of
more advanced systems. These are similar to flowcharts, but much more powerful. The example seen in
Figure 8 is doing two different things. To read the chart, start at the top where is says start. Below this
there is the double horizontal line that says follow both paths. As a result the PLC will start to follow
the branch on the left and right hand sides separately and simultaneously. On the left there are two func-
tions the first one is the power up function. This function will run until it decides it is done, and the
power down function will come after. On the right hand side is the flash function, this will run until it is
done. These functions look unexplained, but each function, such as power up will be a small ladder
8. 12
logic program. This method is much different from flowcharts because it does not have to follow a sin-
gle path through the flowchart.
Start
power up Execution follows
multiple paths
flash
power down
End
Figure 8 An Example of a Sequential Function Chart
Structured Text programming has been developed as a more modern programming language. It
is quite similar to languages such as BASIC. A simple example is shown in Figure 9. This example uses
a PLC memory location i. This memory location is for an integer, as will be explained later in the book.
The first line of the program sets the value to 0. The next line begins a loop, and will be where the loop
returns to. The next line recalls the value in location i, adds 1 to it and returns it to the same location.
The next line checks to see if the loop should quit. If i is greater than or equal to 10, then the loop will
quit, otherwise the computer will go back up to the REPEAT statement continue from there. Each time
the program goes through this loop i will increase by 1 until the value reaches 10.
i := 0;
REPEAT
i := i + 1;
UNTIL i >= 10
END_REPEAT;
Figure 9 An Example of a Structured Text Program
2.1.3 PLC Connections
When a process is controlled by a PLC it uses inputs from sensors to make decisions and update
9. 13
outputs to drive actuators, as shown in Figure 10. The process is a real process that will change over
time. Actuators will drive the system to new states (or modes of operation). This means that the control-
ler is limited by the sensors available, if an input is not available, the controller will have no way to
detect a condition.
PROCESS
Feedback from Connections to
sensors/switches actuators
PLC
Figure 10 The Separation of Controller and Process
The control loop is a continuous cycle of the PLC reading inputs, solving the ladder logic, and
then changing the outputs. Like any computer this does not happen instantly. Figure 11 shows the basic
operation cycle of a PLC. When power is turned on initially the PLC does a quick sanity check to
ensure that the hardware is working properly. If there is a problem the PLC will halt and indicate there
is an error. For example, if the PLC power is dropping and about to go off this will result in one type of
fault. If the PLC passes the sanity check it will then scan (read) all the inputs. After the inputs values are
stored in memory the ladder logic will be scanned (solved) using the stored values - not the current val-
ues. This is done to prevent logic problems when inputs change during the ladder logic scan. When the
ladder logic scan is complete the outputs will be scanned (the output values will be changed). After this
the system goes back to do a sanity check, and the loop continues indefinitely. Unlike normal comput-
ers, the entire program will be run every scan. Typical times for each of the stages is in the order of mil-
liseconds.
PLC program changes outputs
by examining inputs Set new outputs
THE Power turned on
CONTROL
LOOP
Process changes and PLC pauses
Read inputs while it checks its own operation
Figure 11 The Scan Cycle of a PLC
10. 14
2.1.4 Ladder Logic Inputs
PLC inputs are easily represented in ladder logic. In Figure 12 there are three types of inputs
shown. The first two are normally open and normally closed inputs, discussed previously. The IIT
(Immediate InpuT) function allows inputs to be read after the input scan, while the ladder logic is being
scanned. This allows ladder logic to examine input values more often than once every cycle. (Note:
This instruction is not available on the ControlLogix processors, but is still available on older models.)
x
Normally open, an active input x will close the contact
and allow power to flow.
x
Normally closed, power flows when the input x is not open.
x
IIT
immediate inputs will take current values, not those from
the previous input scan. (Note: this instruction is actually
an output that will update the input table with the current
input values. Other input contacts can now be used to
examine the new values.)
Figure 12 Ladder Logic Inputs
2.1.5 Ladder Logic Outputs
In ladder logic there are multiple types of outputs, but these are not consistently available on all
PLCs. Some of the outputs will be externally connected to devices outside the PLC, but it is also possi-
ble to use internal memory locations in the PLC. Six types of outputs are shown in Figure 13. The first
is a normal output, when energized the output will turn on, and energize an output. The circle with a
diagonal line through is a normally on output. When energized the output will turn off. This type of out-
put is not available on all PLC types. When initially energized the OSR (One Shot Relay) instruction
will turn on for one scan, but then be off for all scans after, until it is turned off. The L (latch) and U
(unlatch) instructions can be used to lock outputs on. When an L output is energized the output will turn
on indefinitely, even when the output coil is deenergized. The output can only be turned off using a U
output. The last instruction is the IOT (Immediate OutpuT) that will allow outputs to be updated with-
out having to wait for the ladder logic scan to be completed.
11. 15
When power is applied (on) the output x is activated for the left output, but turned
off for the output on the right.
x x
An input transition on will cause the output x to go on for one scan
(this is also known as a one shot relay)
x
OSR
When the L coil is energized, x will be toggled on, it will stay on until the U coil
is energized. This is like a flip-flop and stays set even when the PLC is turned off.
x x
L
U
Some PLCs will allow immediate outputs that do not wait for the program scan to
end before setting an output. (Note: This instruction will only update the outputs using
the output table, other instruction must change the individual outputs.)
x
IOT
Note: Outputs are also commonly shown using parentheses -( )- instead of
the circle. This is because many of the programming systems are text
based and circles cannot be drawn.
Figure 13 Ladder Logic Outputs
2.2 A CASE STUDY
Problem: Try to develop (without looking at the solution) a relay based controller that will allow
three switches in a room to control a single light.
12. 16
Solution: There are two possible approaches to this problem. The first assumes that any
one of the switches on will turn on the light, but all three switches must be off for the
light to be off.
switch 1
light
switch 2
switch 3
The second solution assumes that each switch can turn the light on or off, regardless of
the states of the other switches. This method is more complex and involves thinking
through all of the possible combinations of switch positions. You might recognize
this problem as an exclusive or problem.
switch 1 switch 2 switch 3
light
switch 1 switch 2 switch 3
switch 1 switch 2 switch 3
switch 1 switch 2 switch 3
Note: It is important to get a clear understanding of how the controls are expected to
work. In this example two radically different solutions were obtained based upon a
simple difference in the operation.
2.3 SUMMARY
• Normally open and closed contacts.
• Relays and their relationship to ladder logic.
• PLC outputs can be inputs, as shown by the seal in circuit.
• Programming can be done with ladder logic, mnemonics, SFCs, and structured text.
• There are multiple ways to write a PLC program.
2.4 PRACTICE PROBLEMS
1. Give an example of where a PLC could be used.
13. 17
2. Why would relays be used in place of PLCs?
3. Give a concise description of a PLC.
4. List the advantages of a PLC over relays.
5. A PLC can effectively replace a number of components. Give examples and discuss some good and bad
applications of PLCs.
6. Explain why ladder logic outputs are coils?
7. In the figure below, will the power for the output on the first rung normally be on or off? Would the output
on the second rung normally be on or off?
8. Write the mnemonic program for the Ladder Logic below.
A Y
B
2.5 ASSIGNMENT PROBLEMS
1. Explain the trade-offs between relays and PLCs for contro applications.
2. Develop a simple ladder logic program that will turn on an output X if inputs A and B, or input C is on.