Introduction to Tags for PLCs 265

Accumulated value

A value programmed into a counter or timer instruction that lists the current item counts or units of time that have accrued during a process. The accumulated value changes with each item counted or with each unit of time.

American Standard Code for Information Interchange

ASCII. An alphanumeric code that uses 8 bits to represent up to 128 different letters, numbers, and symbols. The American Standard Code for Information Interchange is used to transfer data to and from computers.

Analog

A continuously variable electrical signal within a given range of values. Analog signals communicate information about variable conditions, such as pressure or temperature.

Applications

A software program designed to perform specific tasks. Common computer applications include web browsers, writing software, and programming software.

Arrays

A complex data type that contains multiple tags of the same data type. Arrays allow operators to monitor and control multiple pieces of data with a single instruction.

Binary digits

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

Bit strings

Data types that contain lengths of binary data without pre-defined meanings. Bit string data is often used for analog inputs and outputs, communication protocol data, and groups of status bits.

Bits

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

Boolean

BOOL. A single-bit data type that uses 1 for true and 0 for false. Boolean data is used for many common PLC functions and for all Boolean logic operations.

Boolean logic

A branch of mathematics that deals with the use of logical operators that define relationships between two or more entities. Examples of Boolean functions include AND, OR, and Exclusive OR.

BYTE

An 8-bit data type that stores undefined binary code. BYTE is often used to store groups of status bits.

Cartesian coordinates

A set of numerical values that describe the location of an object along three linear axes. Cartesian coordinates are used to describe the position and orientation of robotic arms.

Celsius

A temperature measurement scale used in the metric system that is divided into degrees Celsius. On the Celsius scale, the freezing point of water is 0 degrees, and the boiling point is 100 degrees.

CHAR

Character. An 8-bit data type that store individual characters from the American Standard Code for Information Interchange (ASCII). CHAR from ASCII includes common numbers, symbols, and English language letters.

Character

CHAR. An 8-bit data type that store individual characters from the American Standard Code for Information Interchange (ASCII). Characters from ASCII include common numbers, symbols, and English language letters.

Communication protocol

A set of formal rules describing how to transmit or exchange data across a network. Communication protocol standards enable different pieces of equipment to efficiently communicate with one another.

Complex data types

Multi-part data types that are made up of groups of tags. Complex data types include structures and arrays.

Constant

A value or number that does not change. Constants in PLC programs cannot be changed by program instructions.

Conveyor belt

A moveable belt used in industry to transport materials over a distance. Conveyor belt motors can be controlled by PLCs.

Counters

A set of program instructions that allows a PLC to keep a tally of actions or events in a process. Counter instructions can be programmed to count up or down to a preset value.

Count-up counter

CTU. A counter instruction that increases the current count by one each time the input logic transitions from false to true on a rung. Count-up counters typically start counting from zero until the accumulated value matches the preset value.

Data type

A definition that specifies both the bit size of a data file and how the data is interpreted by the PLC. Common data types include BOOL, DINT, and WORD.

DATE

A 32-bit data type that stores the calendar date. DATE stores the current year, month, and day.

Date and time

DT. A 64-bit data type that stores the calendar date and the time of day. Date and time stores the current year, month, day, hour, minute, seconds, and milliseconds.

Decimal point

The period between whole numbers and parts of whole numbers. The higher the number of places to the right of the decimal point, the more precise the value.

Description field

A text box in a PLC tag that allows operators to enter additional information. Description fields are often used for extra details about a tag’s function within a program.

Discrete

An electrical signal that has only two states. Discrete signals are either on or off.

Done bit

Data in a counter or timer instruction that is set when the instruction meets the programmed values. Done bits function differently for counter instructions and timer instructions.

Double word

DWORD. A 32-bit data type that stores undefined binary code. Double word is often used for IP addresses and bit masks.

Drag-and-drop

The act of grabbing a digital item and moving it. To drag-and-drop, left-click on an object and move it while holding the button down.

Elementary data types

A set of basic data types for PLC tags described by the standard IEC 61131-3. Elementary data types are designed to store specific kinds of information, such as integers, characters, or bit strings.

Enable bit

Data in a timer instruction that sets whenever a timer instruction is active. An enable bit is set until the timer instruction reaches the programmed values.

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.

Field device

An outside component that connects to an input/output module on a PLC. Field devices send signals to and receive signals from PLCs.

Files

A named collection of digital data stored in a single location on a storage device. Common file types on personal computers include documents, pictures, videos, and programs.

Floating-point

A method for efficiently storing fractions of numbers as a series of bits. Floating-point data types can store numbers with significant digits after a decimal point.

Fractions

A number that indicates parts of a whole number. Fractions can be written as a decimal point followed by a series of numbers.

Global tags

Tags that can be used and edited by anyone working in a PLC system. Global tags are often used for sensor data and communication protocols.

HMI

Human-machine interface. A peripheral device connected to a controller that allows an operator to send commands and receive information from connected machines. An HMI usually includes a display screen and one or more input devices.

Human-machine interfaces

HMIs. A peripheral device connected to a controller that allows an operator to send commands and receive information from connected machines. An HMI usually includes a display screen and one or more input devices.

IEC 61131-3

A standard of the International Electrotechnical Commission that defines software for PLCs. IEC 61131-3 describes a set of elementary data types for tags.

Inputs

Data or signals received by computer processors. Inputs for PLC programs may come from input devices or from the PLC memory.

Integers

A whole number or its negative equivalent. Integer data types can store whole numbers but not fractions of numbers.

International Electrotechnical Commission

IEC. An international organization that prepares and publishes all standards for electrical, electronic, and related technologies. The International Electrotechnical Commission develops standards for PLC hardware and software.

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.

Local tags

Tags that can be used and edited only by employees working on a specific PLC program. Local tags are often used for timers, status bits, and mathematical functions.

Logs

A record of system or machine activities. Logs store data that can help to troubleshoot problems and verify activity.

Long real

LREAL. A 64-bit data type that can store up to 15 significant digits after a decimal point. Long real data uses large amounts of storage space, but it is very precise.

Long time

LTIME. A 64-bit data type that stores measurements of time elapsed. Long time stores lengths of time as a number of nanoseconds.

Long word

LWORD. A 64-bit data type that stores undefined binary code. Long word is very large analog inputs and specialized communications.

Mathematical functions

A set of instructions that tells a PLC to perform calculations with numbers. Mathematical functions include add, subtract, multiply, and divide.

Millionth

A fraction that is one million times smaller than a whole number. One millionth can be written as 0.000001.

Milliseconds

A unit of time equal to one thousandth of a second. Milliseconds are used to measure the operation time of many machines.

Naming conventions

The standardized rules for naming tags and files. Naming conventions vary from workplace to workplace.

Nanoseconds

A unit used to measure time that is equal to one billionth of a second. Nanoseconds are used to measure the operation time of lasers and the frequency of electromagnetic waves.

OTE

Output energize. 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 OTE instruction that is true will energize the associated output device.

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.

Outputs

Data or signals produced by computer processors. Outputs for PLC programs may be sent to output devices and/or stored in the PLC memory.

Personal computer

PC. A processor-driven device that runs multiple programs and processes simultaneously. Personal computers use files to organize data, similar to how PLCs use tags.

PLC

Programmable logic controller. A processor-driven device that uses logic-based software to control machines and processes. PLCs are used in industrial automation.

Preset value

A value programmed into a counter or timer instruction that lists a target number for the instruction to reach. Preset values for counters list the number of items to be counted, while preset values for timers list a unit of time that must elapse.

Programmable logic controllers

PLC. A processor-driven device that uses logic-based software to control machines and processes. Programmable logic controllers are used in industrial automation.

Programming language

A set of symbols and rules used to present information to a processor. Programming languages for PLCs include ladder logic, structured text, function block diagrams, and sequential function charts.

Proximity sensor

A device that uses an electronic sensing field to detect the presence of an object. Proximity sensors are common discrete input devices.

Quadrillionth

A fraction that is one quadrillion times smaller than a whole number. One quadrillionth can be written as 0.000000000000001.

Quintillion

A number equal to 1 followed by 18 zeros. Quintillion can also be written 1,000,000,000,000,000,000.

Reads

To retrieve data from a memory location or input address. Tags tell PLCs where to read the input data for programs.

REAL

A 32-bit data type that can store up to 6 significant digits after a decimal point. REAL is a signed data type that can store positive or negative numbers.

Revolutions per minute

rpm. The number of times a component rotates in a complete circle in one minute. Revolutions per minute is a measurement of speed.

Robotic arm

A programmable or remote-controlled mechanical device that simulates the movement of a human arm. Robotic arms are used in a variety of assembly and manufacturing applications.

Scope

An option in a PLC tag that defines who can use and edit the tag. Scope helps to keep tags organized and prevents employees from accidentally using the wrong tags.

Signed

Using the leftmost bit in a data type to indicate whether a number is positive or negative. Signed data types include SINT, DINT, and REAL.

Signed long integer

LINT. A 64-bit data types that can store positive and negative whole numbers. Signed long integer data has a range from negative 9 quintillion to positive 9 quintillion.

Signed short integer

SINT. An 8-bit data type that can store positive and negative whole numbers. Signed short integer data has a range of -128 to 127.

Significant digits

The number of digits to the right of a decimal point. Numbers with more significant digits often represent more precise and accurate measurements.

SINT

Signed short integer. An 8-bit data type that can store positive and negative whole numbers. SINT data has a range of -128 to 127.

Status bits

A binary digit that represents the true (1) or false (0) condition of a PLC program instruction. Status bits are used to represent the current state of internal processes.

STRING

A data type of variable length that stores multiple characters from the American Standard Code for Information Interchange (ASCII). STRING is often used to display text on an HMI.

Structures

A complex data type that contains multiple tags of different data types. Common structures include timers and counters.

Tag list

A display of all the tags in a PLC program. The tag list, also known as the tag table, shows key information about each tag, such as name, scope, data type, description, and constant or variable status.

Tag table

A display of all the tags in a PLC program. The tag table, also known as the tag list, shows key information about each tag, such as name, scope, data type, description, and constant or variable status.

Tag-based

An addressing method for PLC data files in modern PLC software. Tag-based systems allow users to create tags for data files that include a name, data type, and other information.

Tags

A user-defined name and description assigned to memory locations in tag-based PLC systems. Tags replace the numeric file addresses of older PLC software.

Target position

The end position of a robot. The target position of a robot is the end position of its movement path.

TIME

A 32-bit data type that stores measurements of time elapsed. TIME stores lengths of time as a number of milliseconds.

Time of day

TOD. A 32-bit data type that stores the time of day. Time of day stores the current hour, minute, seconds, and milliseconds.

Time stamps

The recorded date and time an event occurs. Time stamps are typically tied to a specific machine or employee ID number for traceability.

Timer off delay

TOF. A timer instruction that delays turning off an output device a specific amount of time after an input device turns off. Timer off delays are often used for motion-activated devices like lights.

Timer on delay

TON. A timer instruction that waits to turn on an output device after receiving an on signal from an input. A timer on delay instruction may be used as a safety feature.

Timers

A PLC function block that measures elapsed time and sets a bit when a preset value is reached. Timers can delay an output for a preset amount of time.

Timer-timing bit

Data in a timer instruction that sets when a timer instruction is timing and has not reached the programmed values yet. The timer-timing bit and the enable bit may be active at the same time.

Unicode

An international encoding standard that uses 16 bits in various combinations to represent up to 65,536 different characters. Unicode is designed to be used with different languages and platforms.

Unsigned

Only able to store positive numbers in a data type. Unsigned data types lack a sign bit and include USINT, UINT, and UDINT.

Unsigned long integer

ULINT. A 64-bit data type that can store only positive whole numbers. Unsigned short integer data has a range of 0 to over 18 quintillion.

Unsigned short integer

USINT. An 8-bit data type that can store only positive whole numbers. Unsigned short integer data has a range of 0 to 255.

Variable

A value or number that changes. Variables for PLC math functions may come from input devices or other programming functions.

Variable frequency drive

VFD. A device that converts incoming 60 Hz AC power into other desired frequencies. Variable frequency drives can be used to control the speed of AC motors.

Wide character

WCHAR. A 16-bit data type that stores individual Unicode characters. Wide character data from Unicode includes common English language characters and characters from multiple other languages.

Wide string

WSTRING. A data type of variable length that stores multiple characters from Unicode. Wide string is often used to display text on an HMI.

WORD

A 16-bit data type that stores undefined binary code. WORD is often used for analog inputs and Modbus communication.

Writes

To send data to a memory location or output address. Tags tell PLCs where to write the output data created by programs.

X axis

The linear axis that describes side-to-side movement. The X axis is one of the two horizontal axes in the Cartesian coordinate system.

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.

Y axis

The linear axis that describes front-to-back movement. The Y axis is one of the two horizontal axes in the Cartesian coordinate system.

Z axis

The linear axis that describes up-and-down movement. The Z axis is the single vertical axis in the Cartesian coordinate system.