Introduction to PLCs 201
Introduction to PLCs provides an overview of programmable logic controllers used in manufacturing. This class introduces the components of PLCs and their functions, provides basic information on the ladder logic programming language used in PLCs, and also gives an overview of common internal relay instructions used in PLC programs.
Manufacturers use PLCs to control automated processes and machines. As Industry 4.0 and smart manufacturing are gaining widespread use, PLCs are more important than ever. Having a foundational knowledge of the basic functions of a PLC helps to increase productivity and efficiency.
Number of Lessons 21
- PLC Basics
- PLCs in Manufacturing
- PLC Components
- Types of PLCs
- Reviewing PLC Basics
- Central Processing Unit
- I/O Module
- I/O Devices
- CPU Operating Cycle
- Reviewing PLC I/O and Operations
- PLC Standardization
- PLC Ladder Diagrams
- PLC Ladder Diagram Symbols
- Logic Gates
- Logic Gates in PLC Ladder Diagrams
- Reviewing PLC Logic
- Internal Relays
- Final Review
- Describe PLCs.
- Describe why PLCs are used in manufacturing.
- Describe the main components of a PLC.
- Distinguish between types of PLCs.
- Describe the central processing unit.
- Describe the I/O module.
- Describe I/O devices.
- List the steps in a CPU operating cycle.
- Explain PLC standardization.
- Describe PLC ladder diagrams.
- Identify PLC ladder diagram symbols.
- Distinguish between logic gates.
- Identify PLC ladder diagram symbols.
- Describe internal relays in a PLC program.
- Describe counter instructions in a PLC program.
- Describe timer instructions in a PLC program.
- Describe sequencer instructions in a PLC program.
Alternating current. A flow of electricity that reverses direction at regularly recurring intervals of time. AC switches direction 60 times per second, or 60 hertz.
A number programmed into a PLC counter or timer instruction that lists the current units of time or item counts that have accrued during a process. When the accumulated value equals the preset value, the counter or timer must reset before they begin counting again.
An output instruction in a PLC ladder diagram. An action is the result of a condition being met in a PLC program.
The unique identification of the location of the data for an input device, output device, or other saved data in the CPU's memory. Addresses are listed alongside associated devices or data in a PLC ladder diagram.
AC. A flow of electricity that reverses direction at regularly recurring intervals of time. Alternating current switches direction 60 times per second, or 60 hertz.
A continuously variable electrical signal. Analog signals communicate variable conditions, such as pressure or temperature.
A basic logic gate that acts as a logical conjunction. AND requires all inputs to be true for the output to be true.
The smallest unit of information on a computer. Bits, or binary digits, consist of 1s and 0s.
A logical and parallel path coming off a rung of a PLC ladder diagram. Branches contain additional program instructions for a PLC.
central processing unit
CPU. The microprocessor device inside a computer that controls system activities. The central processing unit executes a PLC program and performs all logic activities for the system.
An enclosure with slots in it that is used to connect the multiple units of a modular PLC. A chassis typically allows room for additional units.
A channel that connects two or more devices on a network. A communication link may be a physical medium or a logical connection.
An input instruction in a PLC ladder diagram. A condition must be met by PLC components in order to result in an action.
A programmed value for a sequencer instruction that lists the area of memory in the CPU that stores sequencer data. The control file value is shown in the ladder diagram with other programmed values for sequencer instructions.
A series of process control operations. A control loop contains all the necessary physical components and control functions necessary for a process.
count down counter
CTD. A counter instruction that decreases the current value by a specific increment every time an input transitions from false to true. Count down counters should always start counting at the preset value.
count up counter
CTU. A counter instruction that increases the current value by a specific increment every time an input transitions from false to true. Count up counters start from zero and continue counting until the accumulated value matches the preset value.
A set of program instructions that allows a PLC to keep track of actions or events in a process and to respond when a specific count is reached. Counters can be programmed to count up to or down from a preset value.
Central processing unit. The microprocessor device inside a computer that controls system activities. The CPU executes a PLC program and performs all logic activities for the system.
Digital information about an application or process. Data is transmitted and processed in a PLC as 1s or 0s.
The area of a CPU's memory that contains the status of inputs, outputs, and program instructions for sequencers, timers, and counters. A data file for a sequencer load instruction stores sequencer information.
Direct current. A flow of electricity that forms when electrons move in one continuous direction. DC is used in many PLC circuits.
A programmed value in a sequencer instruction that specifies the address of the output data. A destination value is used for sequencer output instructions only.
An electrical signal that has only two states: on or off. Digital signals are also known as discrete signals.
DC. A flow of electricity that forms when electrons move in one continuous direction. Direct current is used in many PLC circuits.
An electrical signal that has only two states: on or off. Discrete signals are also known as digital signals.
distributed control system
A control system consisting of smaller programmable logic controllers placed strategically throughout a manufacturing facility. Distributed control systems require a high-speed communication network.
Done bit. A status bit that either sets or turns off when the accumulated value is equal to the preset value. DN status bits function differently for counter instructions and timer instructions.
DN. A status bit that either sets or turns off when the accumulated value is equal to the preset value. Done bits function differently for counter instructions and timer instructions.
electrical circuit schematic
A graphical representation of the components in an electrical circuit. Electrical circuit schematics and PLC ladder diagrams look similar, but an electrical circuit schematic shows the electrical connection between devices.
A current that carries data from one device to another. Electrical signals are either analog or discrete.
Enable bit. A status bit that activates PLC functions when the referenced bit is on. The EN is used with various PLC program instructions.
EN. A status bit that activates PLC functions when the referenced bit is on. The enable bit is used with various PLC program instructions.
examine if closed
XIC. An input instruction that prompts a CPU to look for an on, or true, condition in a referenced input device. The examine if closed instruction is logically true if the associated input device is on.
examine if open
XIO. An input instruction that prompts a CPU to look for an off, or false, condition in a referenced input device. The examine if open instruction is logically true if the associated input device is off.
An electrical switch that opens and closes a circuit. External relays are replicated by PLC software.
A logical condition that exists when the associated device is off. A false condition fails to provide a continuous logical path on a ladder rung.
A component that connects to an input/output module on a PLC. Each field device, or input/output device, has its own electrical requirement.
The specific address assigned to a sequencer file. The file number for a sequencer instruction allows the data to be easily found within the ladder logic program.
A type of PLC in which the central processing unit, input/output module, and the power supply are housed in a self-contained unit. A fixed PLC is used for applications requiring basic process control.
A device that measures the speed or quantity of a flowing liquid or gas. Flow sensors can convert physical quantities into analog electrical signals.
functional block diagrams
A graphical PLC programming language that uses a box format to display instructions. Functional block diagrams allow for both sequential and parallel processing of data.
A type of programming language that uses symbols and figures to develop a program. Graphical languages include ladder logic and functional block diagrams.
A device with a screen and keypad that employees use to enter sets of operating instructions for a programmable logic controller. Handheld programmers can also be used for troubleshooting the PLC system.
The physical components that form a programmable logic controller system. Hardware in a PLC includes the central processing unit and the power supply.
Input/output device. A component that connects to an input/output module on a PLC. Each I/O device, or field device, has its own electrical requirement.
Input/output module. The portion of a PLC that contains connection points for input/output devices and circuitry to convert electrical current. An I/O module may be input only, output only, or a combination.
Input/output points. A connection point on an input/output module for a field device wire. I/O points allow an electrical connection between field devices, or input/output devices, and the input/output module.
An international standard for machine control programming tools and associated programming software. The IEC 61131-3 supports five programming languages with standard commands and data structures.
A trend of automation and data exchange in manufacturing technologies. Industry 4.0 is seen as the next phase in the digitization of manufacturing.
A field device on a PLC that responds to a condition in a process and sends a signal to the input module. Input devices connect to the input points on an input module.
The portion of a PLC that contains input connection points that allow an input device to connect to the PLC. An input module sends an input signal to the central processing unit.
An electrical signal from an input device. Input signals enter the input module and are then sent to the central processing unit.
input status file
The memory area in the CPU that stores the on/off status of input points. Input status files can be viewed from a programming device when monitoring signals and modules.
I/O devices. A component that connects to an input/output module on a PLC. Each input/output device, or field device, has its own electrical requirement.
I/O modules. The portion of a PLC that contains connection points for input/output devices and circuitry to convert electrical current. An input/output module may be input only, output only, or a combination.
I/O points. A connection point on an input/output module for a field device wire. Input/output points allow an electrical connection between field devices, or input/output devices, and the input/output module.
A low-level, textual PLC programming language that uses compact code. Instruction lists optimize a PLC's memory space.
A complete electronic circuit within a single component. An integrated circuit in a central processing unit stores program data and allows for communication with I/O devices.
Software instructions used within a PLC program that simulate the physical connections of relay circuits. Internal relays have an address in the program memory.
International Electrotechnical Commission
IEC. An organization that provides conformity assessment for government and consumer organizations for all electrical, electronic, and related technologies. The International Electrotechnical Commission developed a standard related to programming software for PLCs.
To separate a circuit or equipment from a source of electrical power. An input/output module contains power conversion components that allow it to isolate an electrical signal.
A graphical PLC programming language that uses simple symbols to indicate the status of input and output devices. Ladder logic displays program instructions in a ladder diagram.
When a relay maintains its on or off condition until it receives its next instruction. Latching relays are achieved with output latch and output unlatch instructions.
A programmed value in a sequencer instruction that tells the PLC how many steps it must take to complete the sequence. The length value is shown in the ladder diagram with other programmed values for sequencer instructions.
A type of mechanical contact that detects the presence or absence of an object. Limit switches are input devices that send discrete signals.
A basic building block of a PLC program that instructs the CPU to perform a logical operation on a true or false input to produce a true or false output. Logic gates use a binary digits to express true and false conditions.
A programmed value in a sequencer instruction that filters out selected data bits. The mask value stops selected input/output data from being transferred from a source to a destination.
The portion of a computer where data is stored. Memory storage can be built into the central processing unit of a PLC or can be in the form of a removable device.
A type of PLC with 32 or fewer input/output points. Micro PLCs are small, but they are still able to offer process control capabilities.
A computer processor on a small circuit. A microprocessor completes most of the functions of the CPU.
mA. A unit of current equal to one thousandth of an ampere. Milliamperes may be used to measure current.
A type of PLC that has removable and interchangeable units, including the central processing unit, power supply, and input/output modules. A modular PLC can process a larger variety of input/output signal levels to meet the needs of a more complex application.
A device that starts or stops a motor circuit. Motor starters are common discrete output devices.
A basic logic gate that requires two input devices to be true in order to trigger a false output. NAND is an AND operation followed by a NOT operation.
A group of hardware devices connected through a communication link. Networking allows multiple devices to communicate and share information.
A basic logic gate that triggers a true output only if both inputs are false. A NOR operation is OR followed by NOT.
A basic logic gate that reverses the true/false condition of an input. NOT logic requires only one input.
A series of steps that the central processing unit of a PLC performs during run mode. The operating cycle repeats as many times as necessary to complete the program.
A state of operation in the central processing unit of a PLC. Operating modes allow the CPU to complete actions, such as running a program and accepting program instructions.
A basic logic gate that excludes one of two or more entities among a group of entities based on the conditions of the logical statement. OR logic requires one or more inputs to be true in order for the output to be true.
A field device on a PLC that performs a desired action after receiving a signal from the output module. Output devices connect to the output points on an output module.
OTE. An output instruction that is true if all input instructions on a rung are true and will be false if any input instructions on the rung are false. An output energize instruction that is true will energize the associated output device.
OTL. An output instruction that turns on an output device and allows the device to remain on even if the rung changes to false. The output latch instruction can turn a device on, but it cannot turn the device off.
The portion of a PLC that contains output connection points that allow output devices to connect to the PLC and receive signals. An output module sends signals to output devices.
An electrical signal that the CPU sends to an output module. Output signals are sent to the output device to cause it to turn on or off.
output status file
The memory area in the CPU that stores the on/off status of output points. The output status file contains the results of the program scan in the CPU's operating cycle.
OTU. An output instruction that turns off the device that was turned on by an output latch instruction. The output unlatch instruction can turn a device off, but it cannot turn the device on.
Personal computer. A processor-driven device for an individual user and commercial software. A PC may be used as a programming device for a PLC.
A device connected to a computer or PLC that performs an auxiliary function. Peripheral devices include printers and programming devices.
PC. A processor-driven device for an individual user and commercial software. A personal computer may be used as a programming device for a PLC.
PLC ladder diagram
A representation of a ladder logic program that uses symbols to represent the on or off status of input/output devices. PLC ladder diagrams also contain internal program instructions, such as counter, timer, or sequencer instructions.
Programmable logic controllers. A processor-driven device that uses logic-based software to provide electrical control to a machine or process. A PLC is a solid-state device.
An interface on a computer that links the device with another device. Ports can connect computers to devices such as monitors or printers.
A value that may be programmed into a sequencer instruction that tells the CPU what step of the sequencer instructions is currently running. The position value may not be required for all sequencer instructions or PLCs.
The component that provides power to the central processing unit and input/output modules on a PLC. The power supply converts incoming current into a lower current for the CPU.
A number programmed into a PLC counter or timer instruction that lists a target number. Preset values for counters list a specific number of items to be counted, while preset values for timers list a specific unit of time that must elapse.
A device that converts variations in pressure into an analog electrical signal. Pressure transducers are a type of analog input device.
An approach to manufacturing that attempts to closely monitor and control all aspects of the process. Process control methods include collecting and analyzing data to help reduce errors.
The instructions entered into a central processing unit that direct the PLC to control input/output devices. A program contains lines of instruction.
An operating mode on the CPU that is active when loading new program instructions, monitoring a program, or making changes to a program. During program mode, the CPU does not perform the operating cycle.
A step in the operating cycle of a central processing unit. The program scan requires the CPU to execute the program and save the on/off condition of the output points.
programmable logic controller
PLC. A processor-driven device that uses logic-based software to provide electrical control to a machine or process. A programmable logic controller is a solid-state device.
The physical device used to enter a program into a PLC's memory. Programming devices include personal computers and handheld programmers.
A set of symbols and rules used to present information to a processor. Programming languages are used to create programs for a PLC.
A valve that can change its output in response to an input value. A proportional valve is an analog output device.
A set of rules used by networked devices for communication. Protocols allow connected devices to exchange information.
A type of mechanical contact that controls some aspect of a machine or process. A push button is a common type of discrete input device.
A vertical line in a PLC ladder diagram that represents a power supply line in the circuit. Rails in a PLC ladder diagram are on the left and right side.
A characteristic of a PLC that allows it to control a process without significant delay. Real-time responses are often in the order of milliseconds and microseconds.
A type of timer instruction that keeps track of how much time has passed and stores the value even if the timer stops timing. Retentive timers store values during power loss or CPU operating mode changes.
An operating mode on the CPU that allows the PLC to execute the steps of a program and complete an operating cycle. During run mode, no changes can be made to a program.
A horizontal line in a PLC ladder diagram that contains program instructions. A rung is read from left to right.
A set of program instructions that determines the order in which a PLC performs a group of processes. Sequencer instructions can be time-driven or event-driven.
SQI. A sequencer instruction in which input data is compared for equality with data that has been stored in memory. A sequencer input is typically used to monitor or troubleshoot machines.
SQL. A sequencer instruction that loads the current data into the sequencer data file in the CPU. A sequencer load instruction does not use a programmed mask value.
SQO. A sequencer instruction that determines the output conditions for the next step of the process. A sequencer output instruction is only enabled once certain conditions are met.
sequential function charts
A graphical programming language used for PLCs. Sequential function chart language can be used for processes that can be split into steps.
A flexible system that can autonomously run an entire production process. A smart factory can connect to a global network of similar production systems and to the digital supply network.
The programs and instructions that a processor-driven device carries out. Software in a PLC replaces many physical components, such as hard-wired relays.
A device that converts electrical energy into linear mechanical force. Solenoids are common discrete output devices.
A programmed value in a sequencer instruction that specifies the address of the input data. A source value is used for sequencer input instructions only.
A two-letter abbreviation that indicates the current state or process of an internal relay in a program. Status bits are listed on ladder rungs.
A light made of a semiconductive material that flashes to indicate the operating condition or channel of a device or module. Status LEDs may indicate the status of input/output devices or the operating mode of a central processing unit.
A textual PLC programming language that is block-structured. Structured text can be used with other programming languages within a PLC program.
A type of programming language that is typed using a keyboard and stored as text files. Textual languages include structured text and instruction lists.
time base value
A value programmed into a PLC timer instruction that specifies the rate at which the timer increases. A time base value specifies units of time.
A type of program instruction that allows a PLC to control and monitor time-dependent aspects of machines and processes. A timer instruction can delay an output from turning off or on for a preset amount of time.
timer off delay
TOF. A timer instruction that waits to turn off an output device after receiving an off signal from an input. A timer off delay delays sending an output signal.
timer on delay
TON. A timer instruction that waits to turn on an output device after receiving an input signal. A timer on delay instruction may be used as a safety feature.
TT. A status bit used with timer instructions in a PLC program. A timer-timing status bit stores the accumulated time when a timer is in operation.
A logical condition that exists when the associated device is on. A true condition in an input device triggers an on condition in an output device.
A display showing all the possible true and false combinations for a logic gate and the corresponding output for each combination. Truth tables use binary digits to express true/false conditions.
UI. A device that allows a user to control a software application or hardware device. User interfaces vary according to manufacturer, but they generally include a display screen and controls.
variable frequency drive
A device that converts incoming AC power into other desired frequencies to allow for AC motor speed control. A variable frequency drive adjusts the speed of a motor to match a required load.
V. A unit of measurement for electromagnetic force, or pressure. Volts are used to measure the strength of electrical current.
Examine if closed. An input instruction that prompts a CPU to look for an on, or true, condition in a referenced input device. The XIC instruction is logically true if the associated input device is on.
Examine if open. An input instruction that prompts a CPU to look for an off, or false, condition in a referenced input device. The XIO instruction is logically true if the associated input device is off.
The smallest unit of information on a computer. Binary digits, also known as bits, are formed using 1s and 0s.
The smallest unit of information on a computer. Binary digits, or bits, consist of 1s and 0s.