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

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

A software procedure used to solve a particular problem.

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.

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.

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.

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

Related Links: ASCII Chart 

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.

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

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.

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

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).

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

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.

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.

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

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

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.

A parallel logic path on a ladder logic rung.

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

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.

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

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

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.

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.

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

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

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.

A single byte that holds a letter or punctuation.

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.

A term used for an integrated circuit typically memory.

The process of removing values or settings from memory.

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

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

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

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

Cyclic Redundancy Check. Used in communications to ensure that the transmission has no errors. See http://en.wikipedia.org/wiki/Cyclic_redundancy_check for more detailed information.

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

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.

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

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

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

A device that supplies current when activated.

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.

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

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.

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.

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.

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.

Transfer of the program from the computer to the controller.

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

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.

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

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.

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.

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

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

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

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.

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."

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.

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.

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.

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

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

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

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).

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

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

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

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.

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

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 Thermometers. RTDs made of platinum.

Programmable Logic Controller

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.

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.

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.

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.

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.

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

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

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.

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.

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.

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.

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

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.

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

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.

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.

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.

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.

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

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

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

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.

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

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

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

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

Related Links: SoftPLC

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

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

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.

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.

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.

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

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).

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.

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

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.