Introduction to PLCs 201

AC

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.

accumulated value

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.

action

An output instruction in a PLC ladder diagram. An action is the result of a condition being met in a PLC program.

address

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.

alternating current

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.

analog

A continuously variable electrical signal. Analog signals communicate variable conditions, such as pressure or temperature.

AND

A basic logic gate that acts as a logical conjunction. AND requires all inputs to be true for the output to be true.

binary digits

The smallest unit of information on a computer. Binary digits, also known as bits, are formed using 1s and 0s.

binary digits

The smallest unit of information on a computer. Binary digits, or bits, consist of 1s and 0s.

bits

The smallest unit of information on a computer. Bits, or binary digits, consist of 1s and 0s.

branches

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.

chassis

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.

communication links

A channel that connects two or more devices on a network. A communication link may be a physical medium or a logical connection.

condition

An input instruction in a PLC ladder diagram. A condition must be met by PLC components in order to result in an action.

control file

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.

control loops

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.

counter

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.

CPU

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.

data

Digital information about an application or process. Data is transmitted and processed in a PLC as 1s or 0s.

data file

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.

DC

Direct current. A flow of electricity that forms when electrons move in one continuous direction. DC is used in many PLC circuits.

destination

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.

digital

An electrical signal that has only two states: on or off. Digital signals are also known as discrete signals.

direct current

DC. A flow of electricity that forms when electrons move in one continuous direction. Direct current is used in many PLC circuits.

discrete

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.

DN

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.

done bit

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.

electrical signals

A current that carries data from one device to another. Electrical signals are either analog or discrete.

EN

Enable bit. A status bit that activates PLC functions when the referenced bit is on. The EN is used with various PLC program instructions.

enable bit

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.

external relays

An electrical switch that opens and closes a circuit. External relays are replicated by PLC software.

false

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.

field devices

A component that connects to an input/output module on a PLC. Each field device, or input/output device, has its own electrical requirement.

file number

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.

fixed PLC

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.

flow sensor

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.

graphical languages

A type of programming language that uses symbols and figures to develop a program. Graphical languages include ladder logic and functional block diagrams.

handheld programmer

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.

hardware

The physical components that form a programmable logic controller system. Hardware in a PLC includes the central processing unit and the power supply.

I/O devices

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.

I/O modules

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.

I/O points

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.

IEC 61131-3

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.

Industry 4.0

A trend of automation and data exchange in manufacturing technologies. Industry 4.0 is seen as the next phase in the digitization of manufacturing.

input devices

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.

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.

input signals

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.

input/output devices

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.

input/output modules

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.

input/output points

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.

instruction lists

A low-level, textual PLC programming language that uses compact code. Instruction lists optimize a PLC's memory space.

integrated circuits

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.

internal relays

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.

isolates

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.

ladder logic

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.

latches

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.

length

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.

limit switch

A type of mechanical contact that detects the presence or absence of an object. Limit switches are input devices that send discrete signals.

logic gates

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.

mask

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.

memory

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.

micro PLCs

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.

microprocessor

A computer processor on a small circuit. A microprocessor completes most of the functions of the CPU.

milliampere

mA. A unit of current equal to one thousandth of an ampere. Milliamperes may be used to measure current.

modular PLC

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.

motor starter

A device that starts or stops a motor circuit. Motor starters are common discrete output devices.

NAND

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.

networked

A group of hardware devices connected through a communication link. Networking allows multiple devices to communicate and share information.

NOR

A basic logic gate that triggers a true output only if both inputs are false. A NOR operation is OR followed by NOT.

NOT

A basic logic gate that reverses the true/false condition of an input. NOT logic requires only one input.

operating cycle

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.

operating modes

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.

OR

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.

output devices

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.

output energize

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.

output latch

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.

output module

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.

output signal

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.

output unlatch

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.

PC

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.

peripheral devices

A device connected to a computer or PLC that performs an auxiliary function. Peripheral devices include printers and programming devices.

personal computer

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.

PLCs

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.

ports

An interface on a computer that links the device with another device. Ports can connect computers to devices such as monitors or printers.

position

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.

power supply

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.

preset value

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.

pressure transducer

A device that converts variations in pressure into an analog electrical signal. Pressure transducers are a type of analog input device.

process control

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.

program

The instructions entered into a central processing unit that direct the PLC to control input/output devices. A program contains lines of instruction.

program mode

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.

program scan

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.

programming device

The physical device used to enter a program into a PLC's memory. Programming devices include personal computers and handheld programmers.

programming language

A set of symbols and rules used to present information to a processor. Programming languages are used to create programs for a PLC.

proportional valve

A valve that can change its output in response to an input value. A proportional valve is an analog output device.

protocols

A set of rules used by networked devices for communication. Protocols allow connected devices to exchange information.

push button

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.

rails

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.

real-time responses

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.

retentive timer

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.

run mode

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.

rungs

A horizontal line in a PLC ladder diagram that contains program instructions. A rung is read from left to right.

sequencer

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.

sequencer input

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.

sequencer load

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.

sequencer output

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.

smart factories

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.

software

The programs and instructions that a processor-driven device carries out. Software in a PLC replaces many physical components, such as hard-wired relays.

solenoid

A device that converts electrical energy into linear mechanical force. Solenoids are common discrete output devices.

source

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.

status bits

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.

status LEDs

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.

structured text

A textual PLC programming language that is block-structured. Structured text can be used with other programming languages within a PLC program.

textual languages

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.

timer

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.

timer-timing

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.

true

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.

truth table

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.

user interfaces

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.

volts

V. A unit of measurement for electromagnetic force, or pressure. Volts are used to measure the strength of electrical current.

XIC

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.

XIO

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.