PLC Technical Terms

Glossary of PLC hardware and programming technical terms

ACsearch for term
See also: Alternating Current
Accumulatorsearch for term

A dedicated data register in memory that is used for temporary storage when doing calculations.

See also: Address
ADsearch for term
See also: ADC, Analog to Digital Converter
ADCsearch for term
See also: AD, Analog, Analog to Digital Converter
Addresssearch for term

A code or number used to reference an area of memory in a PLC.

See also: Accumulator
Algorithmsearch for term

A software procedure used to solve a particular problem.

Alternating Currentsearch for term

A current (and therefore voltage) that varies over time. Typical power supplies oscillate as sine waves at either 60Hz or 50Hz. Technically, at 60 Hz the current is reversing direction 60 times per second.

See also: AC, Direct Current
Analogsearch for term

A value (such as voltage or current) which can be at any level between certain limits. With PLCs this typically refers to signals that range between 0 and 10 volts or 4 to 20mA.

See also: ADC, Digital Synonyms: Analogue
Analog to Digital Convertersearch for term

Analog to digital converter. A circuit or device that inputs an analog signal and converts it to a digital value. The resolution refers to number of bits used to indicate the analog value.

See also: AD, ADC
ASCIIsearch for term

American Standard Code for Information Interchange. A code used to represent common numbers, letters, special symbols and control codes.

Related Links: ASCII Chart 

See also: Character
Assembly Languagesearch for term

Mnemonic commands (typically only three characters) that are one level above the machine code directly used by the processor. PLC programming software often has a mnemonic view that correlates with the ladder diagram.

See also: Mnemonic
Back Planesearch for term

The printed circuit board at the back of the PLC rack where the modules connect to a power supply and data bus.

See also: Bus
BASICsearch for term

A computer language developed by Kemeny & Kurtz in 1964 which stands for Beginners' All-purpose Symbolic Instruction Code. Some PLCs have coprocessor cards that use a form of BASIC to perform computional tasks without affecting the main CPU.

Battery Backupsearch for term

A battery that allows the memory to retain it's values when the PLC is turned off.

See also: Retentive Memory
Baudsearch for term

The number of times per second a communication transmission changes the carrier signal. A 2400-baud modem changes the signal 2400 times a second. Baud is technically not the same as bits per second (bps). For example a 1200 bit-per-second modem actually runs at 300 baud, but it moves 4 bits per baud (4 x 300= 1200 bits per second).

BCDsearch for term
See also: Binary Coded Decimal
Binarysearch for term

A base 2 numbering system meaning only the digits 0 and 1 are used. See: Binary People: Learning your 1s and 0s.

See also: Bit
Binary Coded Decimalsearch for term

A PLC coding system in which a word of 16 bits is divided up into four nibbles (4 bits) and each region used for the decimal range of 0 to 9. Thus a word will go from 0 to 9999 and can be easily deciphered. This technique is also used for devices like thumbwheel switches.

See also: BCD, Bit, Word
Bitsearch for term

One binary digit. A bit can be either a 1 or 0, either on or off. This is the smallest piece of information in the PLC. Typically eight bits make a byte and 16 bits make a word.

See also: Binary, Binary Coded Decimal, Byte, Word
Blocksearch for term

A group of bytes or words. A term often used when transmitting or moving areas of memory.

See also: Byte, Word
Block Diagramsearch for term

An electrical schematic that represents the exact layout of the components, connections and wiring.

See also: Ladder diagram, Logic diagram
Booleansearch for term

A system based on logical states such as AND, OR, NAND, NOR, NOT and XOR. The symbols can be used singly or in combination to form circuits that perform true or false operations.

Branchsearch for term

A parallel logic path on a ladder logic rung.

Broadcastsearch for term

A networking term indicating that the message should be sent to all slaves/clients.

Buffer Memorysearch for term

A register or group of registers used for temporary storage of data. Most often used in communication to compensate for different transmission rates and reception of data.

Bugsearch for term

Problems in software or hardware design that cause undesired behavior. See: The first actual software bug

Burnsearch for term

The process of writing information to PROM memory. The device that does the writing is called a burner.

Burn-insearch for term

A testing procedure used to run a device at high temperatures to increase the probability that any component weakness will be revealed. This ensures that the design is durabile and meets specifications.

Bussearch for term

In PLCs it is a set of parallel conductors used to deliver information at a high rate between electronic components. The term is also used for power distribution connectors.

See also: Back Plane
Bytesearch for term

A group of bits forming a piece of data. Usually a subset of a word consisting of eight bits.

See also: Bit, Block, Character, Word
CEsearch for term

An approval given to a product that conforms to the standards of the European Common Union.

Channelsearch for term

An input or output on a PLC card that requires more then one connection point. An example is a isolated current signal channel that requires a positive and negative wire into the PLC card.

See also: Points
Charactersearch for term

A single byte that holds a letter or punctuation.

See also: ASCII, Byte
Chassissearch for term
See also: Rack
Checksumsearch for term

Used in communications to ensure that the transmission has no errors. It is the numerical sum of all transmitted bytes and is placed at the end of the transmission.

Chipsearch for term

A term used for an integrated circuit typically memory.

Clearsearch for term

The process of removing values or settings from memory.

Closed Loopsearch for term

A system that measures the output and adjusts operation based on conditions. This is also known as feedback.

See also: Open Loop
Compilesearch for term

The process of translating a program into machine code for the computer to execute.

Complementsearch for term

A logical operation that inverts a signal or bit(s). The complement of 0 is 1 and the complement of 1 is 0.

CPUsearch for term

The Central Processing Unit also refered to as the Processor. The part of the PLC that runs and interprets the code.

CRCsearch for term

Cyclic Redundancy Check. Used in communications to ensure that the transmission has no errors. See for more detailed information.

Crosstalksearch for term

Signals being induced into other conductors possibly causing communication errors or false transmissions.

CRTsearch for term

Cathode Ray Tube. A display device that uses a phosphor coated screen and one or more electron guns to draw the screen image. Typically used in TVs and computer monitors but now being replaced with flat panel displays.

CTSsearch for term

Clear To Send. Used in serial communications to indicate that the device can now receive transmissions.

Current Loopsearch for term

Use of a 0-20mA or 4-20mA signal over two wires to transmit a value or state.

Current Sinksearch for term

A device which permits current to flow to ground when activated.

See also: Current Source
Current Sourcesearch for term

A device that supplies current when activated.

See also: Current Sink
DCsearch for term
See also: Direct Current
DCSsearch for term
See also: Distributed Control System
Debouncesearch for term

Switch and relay contacts bounce on the contacts a few times before settling down to make a stable electrical connection. Debouncing is the act of removing or ignoring these initial on/off states.

Debugsearch for term

The act of locating and eliminating malfunctioning code in a program.

Digitalsearch for term

A value which is discrete, that is, it can only be at certain defined levels. With PLCs this typically refers to an on or off state like a switch or relay.

See also: Analog, Points
DINTsearch for term
See also: Double Integer
Direct Currentsearch for term

A current (and therefore voltage) that does not vary over time. Typically a low voltage like 12 or 24 from a battery or power supply.

See also: Alternating Current, DC
Distributed Control Systemsearch for term

A system interconnecting sensors, controllers, operator terminals and actuators to a computer for process automation control. The term typically refers to proprietary interconnections in contrast to a Process Automation System.

See also: DCS, Process Automation System
Double Integersearch for term

A number that occupies two consecutive words in memory (32 bits). It varies from 0 to 4,294,967,295 (in hexadecimal nation from 0 to FFFFFFFF). A signed double integer goes from -2147483648 to +2147483647.

See also: DINT, Integer, Long Integer, Signed Integer, Unsigned Integer
Downloadsearch for term

Transfer of the program from the computer to the controller.

See also: Upload
Driversearch for term

A small program running in the background that defines the configuration settings for software to communicate with a hardware device.

EEPROMsearch for term

Electrically Erasable Programmable Read Only Memory is a second variation of ROM that can be reprogrammed. It is erased electrically and reprogrammed typically on board without any external device.

EPROMsearch for term

Erasable Programmable Read Only Memory is a variation of PROM that can be reprogrammed once the existing program is erased by ultraviolet light.

Fieldbussearch for term

A fieldbus is a system for serial data transfer in the field area, i.e. the installation,
sensor/actuator and controller level. Typicall fieldbuses include PROFIBUS, AS-Interface and InterBus.

See also: Profibus
Floating Pointsearch for term

Simply put, it's a way of representing numbers with a decimal point (e.g. 3.0 or 0.003). It is accomplished by using a double word which holds the sign bit, mantissa and exponent for scientific notation such as 3 X 10-3. The term Real Number and Floating Point are equivalent.

See also: Integer, Real Number
Full-Duplexsearch for term

A bidirectional mode of communication where data may be transmitted and received simultaneously.

See also: Half-Duplex
Half-Duplexsearch for term

A communication link in which data transmission is limited to one direction at a time.

See also: Full-Duplex
Incandescentsearch for term

Used technically to describe anything that glows with heat. Incandescent bulbs use a filament heated by electricity to produce light.

See also: Light Emitting Diode, Liquid Crystal Display
Integersearch for term

Used to indicate a whole number in memory. The term is used independant of the base or radix so it can be in octal, hex or decimal notation.

See also: Double Integer, Floating Point, Long Integer, Signed Integer, SINT, Unsigned Integer
Ladder diagramsearch for term

A circuit diagram used to display electrical control systems like relays and timers. They consist of power rails on either side with contacts and coils forming the "rungs of the ladder."

See also: Block Diagram, Logic diagram
LCDsearch for term
See also: Liquid Crystal Display
LEDsearch for term
See also: Light Emitting Diode
Light Emitting Diodesearch for term

A semiconductor diode that emits light when biased in the forward direction. They are typically small domes or square shapped with the most popular colors being red, green and yellow. They draw less power then regular incandescent lights but more the LCDs. Some diplays are made of a matrix of LEDs but they usually are used for indicator lights.

See also: Incandescent, LED, Liquid Crystal Display
Liquid Crystal Displaysearch for term

A type of display used typically for text messages which is low cost and low power but needs a backlight. The technology uses a fluid between glass which is polarized to block light.

See also: Incandescent, LCD, Light Emitting Diode
Logic diagramsearch for term

An electrical schematic showing the components in a ladder diagram or using AND,OR and NOT symbols to represent the logical flow of the controls rather then their real world physical connections.

See also: Block Diagram, Ladder diagram
Long Integersearch for term

A number that occupies two consecutive words in memory. It varies from 0 to FFFFFFFFFFFFFFFF in hexadecimal notation.

See also: Double Integer, Integer, Signed Integer, Unsigned Integer
LSBsearch for term

Least significat bit is the rightmost bit or lowest order of a binary number.

mAsearch for term

Abbreviation for milliAmps. Milli is a standard metric designation for 1/1000th. Therefore 1 mA = 0.001 Amps.

Mnemonicsearch for term

A mnemonic is a code, usually from 1 to 5 letters, that represents a machine code used directly by the processor for commands. Examples are ADD (add), MUL (multiply), MOV (move).

See also: Assembly Language
Modbussearch for term

A communication protocol used to share data between controllers. It was originally designed for Modicon PLCs but was adopted by many other manufacturers since it is an open architecture and thus has become an industry standard.

Related links:
History | Specifications |
Modbus Organization

MSBsearch for term

Most significant bit is the leftmost bit or highest order of a binary number.

Multitaskingsearch for term

The ability of a system to perform two or more actions simultaneously. True multitasking requires multiple processors.

See also: Pre-emptive Multitasking
OPCsearch for term

OPC is a set of standards that specify the communication of data between software applications and automation hardware. The acronym OPC originally stood for OLE for Process Control because it was based upon the Microsoft OLE (Object Linking and Embedding) technology. In fact, it was meant as the automation industries version of OLE but with the changes in Microsoft technology the term OLE has been dropped from the acronym. OPC now stands as a phrase to describe a set of standards for open connectivity in automation and process control.

Open Loopsearch for term

A system that does not monitor the output to adjust control. This is typically used on well behaved systems.

See also: Closed Loop
PASsearch for term
See also: Process Automation System
PIDsearch for term

Acronym for Proportional, Integral, Derivative. Control algorithm that is used to closely control processes such as temperature, mixture, position, and velocity. The proportional portion takes care of the magnitude of the error. The derivative compensates for the rate of error change. The integral takes care of small errors over time.

Platinum Resistance Thermometerssearch for term

Platinum Resistance Thermometers. RTDs made of platinum.

See also: PRT, Resistance Temperature Detector
PLCsearch for term

Programmable Logic Controller

See also: Points, Programmable Controller
Pointssearch for term

The number of possible connections on a PLC card. Usually in reference to digital inputs and/or outputs. Common numbers of points are 8,16,32 and 64.

See also: Channel, Digital, PLC
Pre-emptive Multitaskingsearch for term

The ability of a processor to perform multiple tasks which have priority ratings. The processor performs one task at any given time, but can be interrupted by the calling of a higher priority task. When the higher priority task is completed, the processor returns to the lower priority task that was interrupted.

See also: Multitasking
Process Automation Systemsearch for term

A system interconnecting sensors, controllers, operator terminals and actuators to a computer for process automation control. A PAS is based on open standards in contrast to a DCS (distributed control system), which is traditionally proprietary.

See also: Distributed Control System, PAS
Profibussearch for term

A fieldbus standard developed in a government assisted joint research project in
1987. In 1991, DIN 19245 was adopted, whereby the Profibus became one of the
first standardised fieldbus systems. This is an open fieldbus standard covering a
wide area within process and building automation.

See also: Fieldbus, Profibus-DP, Profibus-FMS, Profibus-PA
Profibus-DPsearch for term

Profibus Decentralised Peripherals is tailored to the requirements for a quick, efficient data exchange between automation equipment and decentralised devices such as binary or analogue input/output modules and drives for time-critical applications.

See also: Profibus, Profibus-PA
Profibus-FMSsearch for term

Profibus Fieldbus Message Specification is used for messaging (telegrams) between higher level controls such as PLCs and PCs where timing is not critical.

See also: Profibus
Profibus-PAsearch for term

Profibus Process Automation is a Profibus variant which provides power on the bus, is intrinsically safe and inter-operable with Profibus-DP.

See also: Profibus, Profibus-DP
Programmable Controllersearch for term

An electronic device that can be programmed to control a process or machine operation. It typically consists of a power supply, processor, memory, inputs and output.

See also: PLC, Soft PLC
PROMsearch for term

Programmable Read Only Memory is a type of non-volatile memory that can only be written to once but read many times. Unlike ROM memory this comes from the factory blank so that the user can burn the contents onto the memory chip. Once the memory is set it cannot be reprogrammed.

PRTsearch for term
See also: Platinum Resistance Thermometers
Racksearch for term

A housing that holds the CPU and modules. A rack typically refers to a backplane with siding that has slots used to guide the modules into plugs on the backplane.

See also: Chassis
RAMsearch for term

Random Access Memory is a type of memory where information can be read and stored (written) at any time. Most RAMs memory is volatile and requires a battery to maintain its information.

See also: EEPROM, EPROM, PROM, ROM, Volatile
Real Numbersearch for term
See also: Floating Point
Registersearch for term

An area of memory used to store information typically a word.

See also: Word
Resistance Temperature Detectorsearch for term

A temperature measuring device made of wire wound around a core and isolated in a seath. It works on the basis that resistance changes with temperature. That is, the hotter they become, the larger or higher the value of their electrical resistance. They are very precise, respond quickly and are nearly linear over a wide range of temperatures. They are most commonly made from platinum.

See also: Platinum Resistance Thermometers, RTD, Thermocouple
Retentive Memorysearch for term

Memory that is configured by the user to maintain values through a power cycle or a "program" to "run" transition.

See also: Battery Backup
RIUPsearch for term

Remove and Insert Under Power is a feature that allows the modules to be removed and inserted without powering down the PLC.  Allen Bradley ControlLogix has this feature.

RoHSsearch for term

Restriction of Hazardous Substances - EU Directive 2002/95/EC: restriction on the use of certain hazardous substances in electrical or electronic equipment sold or used in the European Union after July 1, 2006. These substances are lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyls, and polybrominated diphenyl ethers.

ROMsearch for term

Read Only Memory is a type of non-volatile memory that can only be written to once but read many times. The contents of the memory are burned onto the chip when they are manufactured and cannot be reprogrammed.

See also: EEPROM, EPROM, PROM, RAM, Volatile
RS-232search for term

An EIA standard that specifies electrical, mechanical and functional characteristics for serial communications. It is used for point to point communications less then 15 meters long.

See also: RS-485
RS-485search for term

An EIA standard that specifies electrical, mechanical and functional characteristics for serial communications. It is used for multi-point (networked) communications.

See also: RS-232
RTBsearch for term

Removable Terminal Block, or the harness that plugs into most modules that interface with field devices.

RTDsearch for term
See also: Resistance Temperature Detector
RTSsearch for term

Request To Send – A serial communication signal sent from a transmitting station to a receiving station requesting permission to transmit.

Rungsearch for term

A ladder logic term that refers to a series or branch of instructions connected together that drive an output. The term is derived the look of ladder logic which resembles a ladder with rungs.

SCADAsearch for term

Supervisory Control And Data Acquisition. Monitoring and control of processes using a computer.

Signed Integersearch for term

A numbering system using a word of memory (16 bits) that can display from -32,768 to +32,767.

See also: Double Integer, Integer, Long Integer, Unsigned Integer
SINTsearch for term

Short Integer uses only 8 bits of memory and therefore is a value of 0 to 255 or if signed from -128 to +127.

See also: Integer
Soft PLCsearch for term

Software used on a personal computer or embedded system that emulates the functions of a PLC.

Related Links: SoftPLC

See also: Programmable Controller
Symbolsearch for term

A name given symbollically to an absolute address in memory. Siemens SIMATIC Step 7 uses symbols for naming addresses.

See also: Tag
Tagsearch for term

A named area of the controller's memory where data is stored. Allen Bradley ControlLogix uses tags.

See also: Symbol
TCsearch for term
See also: Thermocouple
Thermistersearch for term

A resistor that changes with temperature. This is based on the fact that as temperature increases, resistance decreases. Thermistors typically work accurately over a relatively small temperature range.

Thermocouplesearch for term

A temperature measuring device made of two metals. When the two metals are put together it creates a voltage differential which is measured to determine the temperature.

See also: Resistance Temperature Detector, TC
Totalizersearch for term

A device which keeps track of the total amount of fluid, gas or other material used in a process. With a flow rate of 100 m3/hr a totalizer would read 100 m3 in one hour time.

Transducersearch for term

A device used to convert physical parameters like pressure, temperature or weith into electrical signals.

Unsigned Integersearch for term

A number that occupies one word in memory (16 bits). It varies from 0 to 65,535 (in hexadecimal notation it goes from 0 to FFFF).

See also: Double Integer, Integer, Long Integer, Signed Integer
Uploadsearch for term

Transfer of the program in the PLC to the computer. Most of the time depending on the PLC there will be no associated documentation with an uploaded program.

See also: Download
Volatilesearch for term

If the memory loses its information when power is removed, this type of memory is called volatile memory.

See also: RAM, ROM
Wordsearch for term

A memory location consisting of a number of bits to form a piece of information (normally a number or letter). Typically in PLCs the length of a word is 16 bits.

See also: Binary Coded Decimal, Bit, Block, Byte, Register