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Glossary Glossary of Terms used in Programmable Controller-based Systems From Industrial Text and Video Co. the leader in Electrical, Motor Control and PLCs Video Training Programs www.industrialtext.com 1-800-752-8398 1 Glossary GLOSSARY A AC/DC I/O interface. A discrete interface that converts alternating current (AC) voltages from field devices into direct current (DC) signals that the processor can use. It can also convert DC signals into proportional AC voltages. action. A set of control instructions prompting a PLC to perform a certain control function during the execution of a sequential function chart step. acyclic message. An unscheduled message transmission. A/D. See analog-to-digital converter. address. (1) The location in a computer’s memory where particular information is stored. (2) The alphanumeric value used to identify a specific I/O rack, module group, and terminal location. addressability. The total number of devices that can be connected to a network. address field. The sequence of eight (or any multiple of eight) bits immediately following the opening flag sequence of a frame, which identifies the secondary station that is sending (or is designated to receive) the frame. AI. See artificial intelligence. algorithm. A set of procedures used to solve a problem. alphanumeric code. A character string consisting of a combination of letters, numbers, and/or special characters used to represent text, commands, numbers, and/or code groups. ambient temperature. The temperature of the air surrounding a device. American National Standards Institute (ANSI). A clearinghouse and coordinating agency for voluntary standards in the United States. American Wire Gauge (AWG). A standard system used to designate the size of electrical conductors. Gauge numbers have an inverse relationship to size; larger gauges have a smaller diameter. analog device. An apparatus that measures continuous information signals (i.e., signals that have an infinite number of values). The only limitation on resolution is the accuracy of the measuring device. analog input interface. An input circuit that uses an analog-to-digital converter to translate a continuous analog signal, measured by an analog device, into a digital value that can be used by the processor. analog output interface. An output circuit that uses a digital-to-analog converter to translate a digital value, sent from the processor, into an analog signal that can control a connected analog device. analog signal. A continuous signal that changes smoothly over a given range, rather than switching suddenly between certain levels as discrete signals do. analog-to-digital converter (A/D). A device that translates analog signals from field devices into binary numbers that can be read by the processor. AND. A logical operator that requires all input conditions to be logic 1 for the output to be logic 1. If any input is logic 0, then the output will be logic 0. ANSI. See American National Standards Institute. www.industrialtext.com 1-800-752-8398 2 Glossary application. (1) A machine or process monitored and controlled by a PLC. (2) The use of computer or processor-based routines for specific purposes. application memory. The part of the total system memory devoted to storing the application program and its associated data. application program. The set of instructions that provides control, data acquisition, and report generation capabilities for a specific process. arithmetic instructions. Computer programming codes that give a PLC the ability to perform mathematical functions, such as addition, subtraction, multiplication, division, and square root, on data. artificial intelligence (AI). A subfield of computer science dealing with the development of computer programs that solve tasks requiring extensive knowledge. ASCII. For American Standard Code for Information Interchange. A seven-bit code with an optional parity bit used to represent alphanumeric, punctuation, and control characters. ASCII I/O interface. A special function interface that transmits alphanumeric data between peripheral equipment and a PLC. assembly language. A symbolic programming language that can be directly translated into machine language instructions. asynchronous. Recurrent or repeated operations that occur in unrelated patterns over time. AWG. See American Wire Gauge. B back plane. A printed circuit board, located in the back of a chassis, that contains a data bus, power bus, and mating connectors for modules that will be inserted into the chassis. backup. A device or system that is kept on hand to replace a device or system that fails. backward chaining. A method of finding the causes of an outcome by analyzing its consequents to obtain its antecedents. bandwidth. The range of frequencies expressed in Hertz over which a system is designed to operate. base. The maximum number of digits used to represent values in a number system. baseband coaxial cable. A communication medium that can send one transmission signal at a time at its original frequency. BASIC module. An intelligent I/O interface capable of performing computational tasks without affecting the PLC processor’s computing time. battery backup. A battery or set of batteries that will provide power to the processor’s memory in the event of a power outage. baud. (1) The reciprocal of the shortest pulse width in a data communication stream. (2) The number of binary bits transmitted per second during a serial data transmission. Baye’s theorem. An equation that defines the probability of one event occurring based on the fact that another event has already occurred. BCC. See block check character. www.industrialtext.com 1-800-752-8398 3 Glossary BCD. See binary coded decimal. binary coded decimal (BCD). A binary number system in which each decimal digit from 0 to 9 is represented by four binary digits (bits). The four positions have a weighted value of 1, 2, 4, and 8, respectively, starting from the least significant (right-most) bit. binary number system. A base 2 number system that uses only the numbers 0 and 1 to express all values. Each digit position of a binary number has a weighted value of 1, 2, 4, 8, 16, 32, 64, and so on, starting with the least significant (right-most) digit. bit. For binary digit. The smallest unit of binary information. A bit can have a value of 1 or 0. bit rate. See baud. bit-wide bus network. An I/O bus network that interfaces with discrete devices that transmit less than 8 bits of data at a time. blackboard architecture. The distribution of knowledge inferencing, as well as global and knowledge databases, in a control system through the use of several subsystems containing local, global, and knowledge databases that work independently of each other. block. A group of words transmitted as a unit. block check character (BCC). A character, placed at the end of a data block, that corresponds to the characteristics of the block. block diagram. A schematic drawing. block length. The total number of words transmitted at one time. block transfer. A programming technique used to transfer up to 64 words of data to or from an intelligent I/O module. Boolean action. A set of control instructions that assigns a discrete value to a variable during a sequential function chart step. Boolean language. A PLC programming language, based primarily on the Boolean logic operators, that implements all of the functions of the basic ladder diagram instruction set. Boolean operators. Logical operators, such as AND, OR, NAND, NOR, NOT, and exclusive-OR, that can be used singly or in combination to form logical statements that have output responses of TRUE or FALSE. Boolean variable. A single-bit variable whose value is transmitted in the form of 1s and 0s. Bourdon tube. A pressure transducer available in spiral, helical, twisted, and C-tube configurations that converts pressure measurements into displacement. branch. A parallel logic path within a rung. breadth-first search. A method of rule evaluation that evaluates each rule in the same level of a decision tree before proceeding downward. bridge circuit. A mechanism found in transducer circuits that uses resistors to change the parameters (e.g., voltage and current) of an incoming signal. broadband coaxial cable. A communication medium that can transmit two or more transmission signals at one time via frequency division multiplexing. burn-in procedure. The process of operating a device at an elevated temperature to identify early-failing parts. www.industrialtext.com 1-800-752-8398 4 Glossary bus. (1) A group of lines used for data transmission or control. (2) Power distribution conductors. bus topology. A network configuration in which all stations are connected in parallel with the communication medium and all stations can receive information from any other station on the network. bypass/control station. A device that allows a process to be switched to either PLC or manual control. byte. A group of eight adjacent bits that are operated on as a unit, such as when moving data to or from memory. byte-wide bus network. An I/O bus network, which interfaces with discrete and small analog devices, that can transmit between 1 and 50 or more bytes of data at a time. C cascade control. The use of two controllers to regulate a process so that the feedback loop of one controller is the set point of the other controller. center of gravity method. A method of calculating the final output value of a fuzzy logic controller by finding the value that corresponds to the center of the mass under the control output curve. centralized control. A PLC control system organization in which a central PLC controls several machines or processes. central processing unit (CPU). The part of a programmable controller responsible for reading inputs, executing the control program, and updating outputs. Sometimes referred to as the processor, the CPU consists of the arithmetic logic unit, timing/control circuitry, accumulator, scratch pad memory, program counter, address stack, and instruction register. centroid. The point in a geometrical figure whose coordinates equal the average of all the other points comprising the figure. channel. A designated path for a signal. channel capacity. The amount of information that can be transmitted per second on a given communication channel depending on the medium, line length, and modulation rate. character. One symbol of a set of elementary symbols, such as a letter of the alphabet or a number. chassis. A hardware assembly that houses PLC devices, such as I/O modules, adapter modules, processor modules, power supplies, and processors. checksum. A transmission verification algorithm that adds the binary values of all the characters in a data block and places the sum in the block check character position. chip. A very small piece of semiconductor material that holds electronic components. Chips are normally made of silicon and are typically less than 1/4 inch square and 1/100 inch thick. closed loop. A control system that uses feedback from the process to maintain outputs at a desired level. coaxial cable. A transmission medium, consisting of a central conductor surrounded by dielectric materials and an external conductor, that possesses a predictable characteristic impedance. www.industrialtext.com 1-800-752-8398 5 Glossary code. (1) A binary representation of numbers, letters, or symbols that have some meaning. (2) A set of programmed instructions. coil. A ladder diagram symbol that represents an output instruction. cold junction compensation. A compensation factor that allows a thermocouple to operate as though it has an ice-point reference. collision detection (CSMA/CD). A network access method in which each node waits until there is no traffic on the network then transmits its message. If the node detects another transmission on the network, it will disable its transmitter and wait until the network clears before retransmitting the message. combined error. See propagation error. common bus topology. A network configuration in which individual PLCs connect to a main trunkline in a multidrop fashion. compatibility. (1) The ability of various specified units to replace one another with little or no reduction in capability. (2) The ability of units to be interconnected and used without modification. complement. A logical operation that inverts a signal or bit. conditional probability inferencing. The conditional probability of an event happening in an artificial intelligence system. constant voltage transformer. A transformer that maintains a steady output voltage (secondary) regardless of input voltage (primary) fluctuations. contact. A ladder diagram symbol that represents an input condition. contact output interface. A discrete interface, which does not require an external power source, that is triggered by the change in state of a normally open or normally closed contact. contact symbology. A set of symbols used to express a control program through conventional relay symbols (e.g., normally open contacts, normally closed contacts, etc.). continuous-mode controller. A process controller that sends an analog signal to a process control field device. control element. The output field device that regulates the actual control variable level in a process control system. control logic. The control plan for a given system. control loop. The method of adjusting the control variable in a process control system by analyzing process variable data and then comparing it to the set point to determine the amount of error in the system. control panel. A panel that contains instruments used to control devices. control program checkout. A final review of a PLC’s control program prior to starting up the system. control program printout. A hard copy of the control logic program stored in a PLC’s memory. control strategy. The sequence of steps that must occur during a process or PLC program to produce the desired output control. control task. The desired results of a control program. control variable. The independent variable in a process control system that is used to adjust the dependent variable, the process variable. www.industrialtext.com 1-800-752-8398 6 Glossary convergence. A point in a sequential function chart where many elements flow into one element. counter. An electromechanical device that counts the number of times an event occurs. counter instructions. Computer programming codes that allow a PLC to perform the counting functions (count up, count down, counter reset) of a hardware counter. CPU. See central processing unit. CRC. See cyclic redundancy check. critically damped response. A second-order control system response in which the damping coefficient equals 1, causing the response to overshoot the set point and then quickly settle back to it. CSMA/CD. See collision detection. current loop. A two-wire communication link in which the presence of a 20 milliamp current level indicates a binary 1 (mark) and its absence indicates no data, a binary 0 (space). CX-ORC. See cyclic exclusive-OR checksum. cyclic exclusive-OR checksum (CX-ORC). An error detection method in which the words in the data block are exclusive-ORed with the checksum word and then rotated to the left. This action is repeated until all of the words in the block have been operated on. cyclic message. A scheduled message transmission. cyclic redundancy check (CRC). An error detection method in which all the bits in a block are divided by a predetermined binary number. The remainder becomes the block check character. D D/A. See digital-to-analog converter. data. A general term for any type of information. data link layer. Layer 2 of the OSI network protocol. This layer provides functional and procedural means for establishing, maintaining, and releasing data link connections among network entities. data manipulation instructions. Computer codes that provide a PLC with the ability to compare, convert, shift, examine, and operate on data in multiple registers. data table. The part of a processor’s memory, containing I/O values and files, where data is monitored, manipulated, and changed for control purposes. data transfer instructions. Computer codes that allow a PLC to move numerical data within a controller, either in single register units or in blocks of registers. DC I/O interface. A discrete module that links a processor with direct current field devices. dead time. The delay between the time a control system’s control variable changes and the time the process variable begins to respond to the change. debouncing. The act of removing intermediate noise from a mechanical switch. decimal number system. A base 10 number system that uses ten numbers—0, 1, 2, 3, 4, 5, 6, 7, 8, and 9—to represent all values. Each digit position has a weighted value of 1, 10, 100, 1000, and so on, beginning with the least significant (rightmost) digit. www.industrialtext.com 1-800-752-8398 7 Glossary defuzzification. The process of converting a fuzzy logic controller’s output conclusions into real output data and sending the data to the field device. depth-first search. A rule evaluation method that evaluates all the rules in a downward branch of a decision tree before proceeding to the next branch. derivative controller. A continuous-mode controller whose output to the control field device is proportional to the rate of change of error in the system. device bus network. A network that allows low-level input/output devices that transmit relatively small amounts of information to communicate directly with a PLC. diagnostic AI system. The lowest level of artificial intelligence system. This type of system primarily detects faults within an application but does not provide information about possible solutions. diagnostics. The detection and isolation of an error or malfunction. differential input/output. A signal transmission system where inputs and outputs have individual return lines for each channel, as opposed to all data running through one line. digital device. A device that processes and sends discrete (two-state) electrical signals. digital signal. A noncontinuous signal that has a finite number of values. digital-to-analog converter (D/A). A device that translates binary numbers from a processor into analog signals that field devices can understand. direct-acting controller. A closed-loop controller whose control variable output increases in response to an increase in the process variable. direct action I/O interface. A special I/O interface that detects, preprocesses, and transmits low-level and fast-speed signals. discrete input interface. An input circuit that allows a PLC to receive data from digital field devices. discrete-mode controller. A controller that sends a noncontinuous signal to the field device controlling a process. discrete output interface. An output circuit that allows a PLC to send data to digital field equipment. displacement transducer. A device that measures the movement of an object. distributed control. A PLC control system organization in which factory or machine control is divided into several subsystems, each managed by a separate PLC, yet all interconnected to form a single entity. distributed I/O processing. The allocation of various control tasks to several intelligent I/O interfaces. divergence. A point in a sequential function chart where one element flows into many elements. documentation. An orderly collection of recorded hardware and software information about a control system. These records provide valuable reference data for installing, debugging, and maintaining the PLC. double-precision arithmetic. Arithmetic instructions that use double the number of registers than single-precision arithmetic to hold the operands and result (i.e., two registers each for the operands and two or four registers for the result). downtime. The time when a system is not available for use. www.industrialtext.com 1-800-752-8398 8 Glossary dynamic system checkout. The process of verifying the correct operation of a control program by actually implementing it. E EAROM. See electrically alterable read-only memory. EEPROM. See electrically erasable programmable read-only memory. EIA. See Electronic Industries Association. electrically alterable read-only memory (EAROM). A type of nonvolatile, programmable, read-only memory that can be erased completely by applying the proper voltage to the memory chip. electrically erasable programmable read-only memory (EEPROM). A type of nonvolatile, programmable, read-only memory that can be erased by electrical pulses. Electronic Industries Association (EIA). An agency that sets electrical/electronic standards. encoder/counter module. An interface, which is used in positioning applications, that links encoders and high-speed counter devices with programmable logic controllers. enhanced ladder language. A PLC language that implements basic ladder language instructions, as well as more sophisticated functional block instructions, which can perform multiple operations in a single instruction. EPROM. See erasable programmable read-only memory. erasable programmable read-only memory (EPROM). A type of nonvolatile, programmable, read-only memory that can be erased with ultraviolet light. error. The difference between the set point and the process variable in a control system. error-correcting code. A code in which each acceptable expression conforms to specific rules of construction that also define one or more nonacceptable expressions, so that if certain errors occur, they can be detected and corrected. error deadband. The amount that the process variable can fluctuate from the set point before the control system provides corrective action. error-detecting code. A code in which each expression conforms to specific rules of construction, so that if an error occurs in an expression, it can be detected. exclusive-OR (XOR). A logical operation, which has only two inputs, that yields a logic 1 output if only one of the two inputs is logic 1 and a logic 0 output if both inputs are the same, either logic 1 or logic 0. execute. To perform a specific operation by processing either one instruction, a series of instructions, or a complete program. execution time. The time required to perform one specific instruction, a series of instructions, or a complete program. executive memory. The part of the system memory that permanently stores a system’s supervisory programs, as well as instruction software. This area of memory is not accessible to the user. expert AI system. The highest level of AI systems. This type of system detects process faults, provides information about possible causes of the faults, and makes complex decisions about resulting actions based on statistical analysis. www.industrialtext.com 1-800-752-8398 9 Glossary F FALSE. As related to PLC instructions, a reset logic state associated with a binary 0. fast-input interface. An intelligent I/O module that functions as a pulse stretcher, detecting very fast input pulses that regular I/O modules cannot read. fast-response interface. A special I/O module designed to detect fast inputs and respond with an output. FBD. See function block diagram. feedback. The signal or data transmitted to a PLC from a controlled machine or process to denote its response to the command signal. fiber-optic cable. A communication medium composed of thin fibers of glass or plastic enclosed in a material with low refraction. first-order response. A process response to a rapid change in the control variable characterized by one lag time and a process response curve that slowly approaches the set point. floating-point math. A data manipulation format, which is used to express a number by expressing the power of the base, that usually involves the use of two sets of digits. For example, in a floating decimal notation where the base is 10, the number 8,700,000 would be expressed as 8.7(10)6 or 8.7E6. flowchart. A graphical representation of the definition or solution of a task or problem. flowcharting. A method of pictorially representing the operation of a process in a sequential manner. flow transducer. A device that measures the amount of solid, liquid, or gaseous materials flowing through a process by measuring either weight, differential pressure, or fluid motion. forward chaining. A method for determining all possible outcomes for a given set of inputs. frequency shift keying (FSK). A signal modulation technique that offers a high amount of noise immunity in which a carrier frequency is shifted to high or low to represent a binary 1 or 0, respectively. FSK. See frequency shift keying. full-duplex line. A communication line used to simultaneously transmit data in two directions. function block diagram (FBD). A graphical PLC programming language in which instructions are programmed as blocks that are then used as needed to control process elements. fuzzification. The translation of input data into fuzzy logic membership sets. fuzzy logic. The branch of artificial intelligence that deals with reasoning algorithms used to simulate human judgment. fuzzy logic interface. A special I/O interface that provides intelligent, closed-loop process control by analyzing input data according to specified mathematical algorithms and then providing a correlating output response. fuzzy processing. The interpretation of fuzzy input data to determine an appropriate outcome based on user-programmed IF…THEN rules. fuzzy set. A group of membership functions. www.industrialtext.com 1-800-752-8398 10