Octal number system. A base 8 number system that uses eight numbers—0, 1, 2, 3, 4, 5, 6, and 7—to represent all values. Off-line. The state of not being in continuous direct communication with the processor. One’s complement. An operation that represents the negative value of a binary word by assigning the most significant bit of the word with a value equal to its normal value minus one. One shot. A programming technique that sets a storage bit or output to a certain state for only one scan. On-line. The state of being in continuous communication with the processor. open loop. A control system that does not receive process feedback in order to perform self-correcting actions. Optical coupler. A device that couples signals from one circuit to another by means of electromagnetic radiation. OR. A logical operator that yields a logic 1 output if any input is logic 1 and a logic 0 output if all inputs are logic 0. Orifice plate. A transducer that measures fluid flow by measuring the pressure differential between two points. OSI model. A description of network communications functions organized in seven layers to promote open system interconnections. Output. Information sent from the processor to connected field devices. output device. Any connected equipment, such as control devices (e.g., motors, solenoids, and alarms) or peripheral devices (e.g., line printers, disk drives, and color displays), that receives information or instructions from the central processing unit. Each type of output device has a unique interface to the processor. Output table. The area of a PLC’s memory where information about the status of output devices is stored. Overdamped response. A second-order control system response in which the damping coefficient is greater than 1, causing the response to overshoot the set point and then slowly settle back to it. P: Packet. Data and sequences of control bits arranged in a specified format and transferred as an entity during data transmission. Panel enclosure. The physical enclosure that houses a PLC’s hardware and components. Parallel circuit. A circuit in which two or more of the connected components or contact symbols in a ladder program are connected to the same pair of terminals so that current may flow through all the branches. Parity. The even or odd characteristic of the number of 1s in a byte or word of memory. Parity bit. A bit added to a memory word as a means of error detection. parity check. A check for a certain number of 1s and 0s in a memory word to ensure data integrity. Peripherals. External devices, such as line printers, disk drives, recorders, etc., that are connected to a PLC. PID interface. See proportional-integral-derivative interface. PLC. See programmable logic controller. Polling. A network access method where a master controller manages the communication process by interrogating each slave controller under it to determine whether the slave has any information to send. Positive logic. The conventional use of binary logic in which logic 1 represents a positive logic level (e.g., logic 1 = +5 V, logic 0 = 0 V). Potentiometer. A simple transducer that measures displacement based on resistance changes due to the movement of a wiper arm. power supply. The unit that supplies the necessary voltage and current to a system’s circuitry. Presentation layer. Layer 6 of the OSI protocol. This layer communicates data while resolving syntax differences between network devices. Pressure transducer. A transducer that measures pressure by transforming exerted force into an electrical signal. Process. (1) Continuous and regular production executed in a definite, uninterrupted manner. (2) One or more entities threaded together to perform a requested service. process bus network. A network that allows high-level analog input/output devices that transmit large amounts of information to communicate directly with a PLC. Process control. The regulation of process parameters to within specified target parameters through the manipulation of the control variable. Process gain. The ratio between a process’s output and its input. In an ideal process control situation, the process gain equals one. Process variable. A process control system’s dependent variable, which is controlled by its independent variable, the control variable. Program. A planned set of instructions stored in memory and executed in an orderly fashion by the central processing unit. Program coding. The process of translating a logic or relay diagram into PLC ladder program form. Program/flow control instructions. Computer codes that give a PLC the ability to direct the flow of operation and alter the order of execution of a control program. programmable logic controller (PLC). A solid-state control device that can be programmed to control process or machine operations. It consists of five basic components: the processor, memory, input/output modules, the power supply, and the programming device. Programmable read-only memory (PROM). A read-only memory that can be programmed once and never altered again. Programming device. A device that is used to enter the control program into memory and make changes to the stored program. Program scan. The time required by the processor to evaluate and execute the control logic. This time does not include the I/O update time. The program scan repeats continuously while the processor is in the run mode. PROM. See programmable read-only memory. Propagation error. A combined error caused by the interaction of two or more independent variables, each causing a different error. Proportional controller. A continuous-mode controller whose output to the control field device in proportional to the change in error. Proportional-derivative controller. A continuous-mode controller that uses both proportional and derivative actions to determine the control variable output based on both the amount of error and its rate of change. Proportional-integral controller. A continuous-mode controller that uses both proportional and integral actions to determine the control variable output based on the amount of error and its change over time. Proportional-integral-derivative controller. A continuous-mode controller that uses proportional, integral, and derivative actions to determine the control variable output based on the amount of error, its change over time, and its rate of change. This type of controller provides the optimum type of control in most process applications. Proportional-integral-derivative (PID) interface. An intelligent I/O module that provides automatic, closed-loop control of multiple, continuous-process control loops. protocol. A formal definition of how communication will occur in a network. pulse action. A set of IEC 1131-3 instructions that is executed only once after a step becomes active. Q: quarter-amplitude response. A process variable response whose amplitude diminishes by one-fourth during each cycle.
Rack enclosure. The location in a PLC that physically houses plug-in devices, such as I/O modules and supplementary power supplies. RAM. See random-access memory. Random-access memory (RAM). A volatile, alterable memory that provides storage for the application program and data. Random error. An error resulting from an unexpected action in a process line. read. (1) To acquire data from a storage device. (2) The transfer of data between devices, such as a peripheral device and a computer. Read-only memory (ROM). A type of memory that permanently stores an unalterable program or set of instructions. Real variable. A nondiscrete variable whose value is transmitted in the form of fractional and floating-point data. Register. A temporary storage device for data and information (e.g., timer/counter preset values). A PLC register is normally 16 bits wide. Register/BCD I/O interface. A multibit module that uses thumbwheel switches to interface between discrete devices and a programmable controller. Relay. An electrically operated device that mechanically switches electrical circuits. Relay logic. The representation of a control program or other logic in relay form (i. e., using electrically operated devices to mechanically switch electrical circuits). Remote I/O subsystem. A system where some or all of the I/O racks are mounted away from the PLC. Remote rack. An enclosure, containing I/O modules and a remote I/O processor, located away from the CPU. Resistance temperature detector (RTD). A temperature transducer composed of conductive wire elements typically made of platinum, nickel, copper, or nickeliron. resistance temperature detector (RTD) interface. An intelligent I/O module that interprets temperature information from RTD devices. Resolution. The smallest detectable increment of measurement. Response time. The time, including terminal delay, network delay, and service node delay, between the transmission of the last character of a network node’s message and its receipt of the first character of the reply. Reverse-acting controller. A closed-loop controller whose control variable output decreases in response to an increase in the process variable. Ring topology. A network architecture where signals from one node are relayed through all the other nodes in the network. ROM. See read-only memory. RTD. See resistance temperature detector. RTD interface. See resistance temperature detector interface. Rule. An algorithm consisting of IF conditions and THEN actions that a fuzzy logic module uses to interpret input data and respond with a corresponding output value. Rule-based knowledge representation. A method of expressing an expert’s knowledge in an AI system using IF…THEN rules that determine the actions and decisions to be made. Rung. A ladder program term that refers to the programmed instructions that driveone output. A complete control program may have several rungs. |
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