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A AC See also: Accumulator A dedicated data register in memory that is used for temporary storage when doing calculations. See also: AD See also:, ADC See also:,, Address A code or number used to reference an area of memory in a PLC. See also: Algorithm A software procedure used to solve a particular problem.

Alternating Current 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:, Analog 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:, Synonyms: Analogue Analog to Digital Converter 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:, ASCII American Standard Code for Information Interchange. A code used to represent common numbers, letters, special symbols and control codes. Related Links: See also: Assembly Language 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. B Back Plane The printed circuit board at the back of the PLC rack where the modules connect to a power supply and data bus. See also: BASIC 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 Backup A battery that allows the memory to retain it's values when the PLC is turned off. See also: Baud 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). BCD See also: Binary A base 2 numbering system meaning only the digits 0 and 1 are used. See also: Binary Coded Decimal 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:,, Bit 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:,,, Block A group of bytes or words. A term often used when transmitting or moving areas of memory. See also:, Block Diagram An electrical schematic that represents the exact layout of the components, connections and wiring. See also:, Boolean 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. Branch A parallel logic path on a ladder logic rung. Broadcast A networking term indicating that the message should be sent to all slaves/clients. Buffer Memory 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. Bug Problems in software or hardware design that cause undesired behavior.

See: Burn The process of writing information to PROM memory. The device that does the writing is called a burner. Burn-in 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. Bus 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: Byte A group of bits forming a piece of data.

Usually a subset of a word consisting of eight bits. C CE An approval given to a product that conforms to the standards of the European Common Union. Channel 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: Character A single byte that holds a letter or punctuation. Layby Reminder Template For Payment. See also:, Chassis See also: Checksum 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.

Chip A term used for an integrated circuit typically memory. Clear The process of removing values or settings from memory. Closed Loop A system that measures the output and adjusts operation based on conditions. This is also known as feedback.

See also: Compile The process of translating a program into machine code for the computer to execute. Complement A logical operation that inverts a signal or bit(s). The complement of 0 is 1 and the complement of 1 is 0. CPU The Central Processing Unit also refered to as the Processor. The part of the PLC that runs and interprets the code. CRC Cyclic Redundancy Check. Used in communications to ensure that the transmission has no errors.

See for more detailed information. Crosstalk Signals being induced into other conductors possibly causing communication errors or false transmissions.

CRT 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.

CTS Clear To Send. Used in serial communications to indicate that the device can now receive transmissions. Current Loop Use of a 0-20mA or 4-20mA signal over two wires to transmit a value or state. Current Sink A device which permits current to flow to ground when activated. See also: Current Source A device that supplies current when activated.

D DC See also: DCS See also: Debounce 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. Debug The act of locating and eliminating malfunctioning code in a program. Digital 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:, DINT See also: Direct Current 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:, Distributed Control System 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:, Double Integer 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 - to +. See also:,,,, Download Transfer of the program from the computer to the controller. See also: Driver A small program running in the background that defines the configuration settings for software to communicate with a hardware device. F Fieldbus 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: Floating Point 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:, Full-Duplex A bidirectional mode of communication where data may be transmitted and received simultaneously. L Ladder diagram 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:, LCD See also: LED See also: Light Emitting Diode 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:,, Liquid Crystal Display 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:,, Logic diagram 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:, Long Integer A number that occupies two consecutive words in memory.

It varies from 0 to FFFFFFFFFFFFFFFF in hexadecimal notation. See also:,,, LSB Least significat bit is the rightmost bit or lowest order of a binary number. M mA Abbreviation for milli Amps.

Milli is a standard metric designation for 1/1000th. Therefore 1 mA = 0.001 Amps. Mnemonic 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: Modbus 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: MSB Most significant bit is the leftmost bit or highest order of a binary number. Multitasking The ability of a system to perform two or more actions simultaneously. True multitasking requires multiple processors. O OPC 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 Loop A system that does not monitor the output to adjust control. This is typically used on well behaved systems.

P PAS See also: PID 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 Platinum Resistance Thermometers. RTDs made of platinum.

See also:, PLC Programmable Logic Controller See also:, Points 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:,, Pre-emptive Multitasking 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: Process Automation System 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:, Profibus 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:,,, Profibus-DP 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-FMS 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-PA Profibus Process Automation is a Profibus variant which provides power on the bus, is intrinsically safe and inter-operable with Profibus-DP. See also:, Programmable Controller 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:, PROM 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. See also:,,, PRT See also. S SCADA Supervisory Control And Data Acquisition. Monitoring and control of processes using a computer.

Signed Integer A numbering system using a word of memory (16 bits) that can display from -32,768 to +32,767. See also:,,, SINT 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: Soft PLC Software used on a personal computer or embedded system that emulates the functions of a PLC. Related Links: See also: Symbol A name given symbollically to an absolute address in memory.

Siemens SIMATIC Step 7 uses symbols for naming addresses. T Tag A named area of the controller's memory where data is stored. Allen Bradley ControlLogix uses tags. See also: TC See also: Thermister 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.

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