Half-duplex line. A communication line that can transmit data in two directions, but in only one direction at a time. Hamming code. An error-detecting code that combines parity and data bits to generate a byte containing a value that identifies the erroneous bit. hard copy. A printed document. Hardware. All the physical components of a programmable controller, including peripherals, as opposed to the software components that control its operation. hardwired logic. Logic control functions that are determined by the way a system’s devices are physically interconnected. Hexadecimal number system (hex). A base 16 number system that uses the numbers 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9 and the letters A, B, C, D, E, and F to represent numbers and codes. Host. A central computer in a network system. I: IEC 1131 programming standard. A standardized set of PLC programming guidelines, set forth by the International Electrotechnical Commission, that includes general PLC information, equipment and test requirements, programminglanguages, user guidelines, and communication standards. IEEE 802. A family of standards specified by the Institute of Electrical and Electronic Engineers for data communication over local and metropolitan area networks. IL. See instruction list. Image table. An area in a PLC’s memory dedicated to I/O data where 1s and 0s represent ON and OFF conditions, respectively. Individual control. A PLC control system organization in which a PLC controls a single machine or process. Inference engine. The section of an AI system where all decisions are made using the knowledge stored in the knowledge database. Input. Information sent to the processor from connected devices. Input device. Any connected equipment, such as control devices (e.g., switches, buttons, and sensors) or peripheral devices (e.g., cathode ray tubes and manual programmers), that supply information to the central processing unit. Each type of input device has a unique interface to the processor. Input/output system. A collection of plug-in modules that transmit control data between a PLC and field devices. Input table. The area of a PLC’s memory where information about the status of input devices is stored. Instruction list (IL). A low-level, text-based PLC programming language that uses assembly language–like mnemonics to represent the control program. Integer variable. A nondiscrete variable whose value is transmitted in the form of a whole number. Integral controller. A continuous-mode controller whose output to the control field device changes according to how the error signal changes over time. Integral of time and absolute error open-loop tuning method (ITAE). A method used to determine the proper tuning constants for a controller based on the minimization of the integral of time and the absolute error of the response. Integral windup. The situation in which the control variable in a system remains at its maximum level even though the amount of error in the system starts to decrease. Intelligent I/O interface. A microprocessor-based module that can perform sophisticated processing functions independently of the central processing unit. Interface. A circuit that permits communication between a central processing unit and a field input or output device. Different devices require different interfaces. Interlock. A device actuated by the operation of another device to which it is linkedto govern the succeeding operation of the same or allied devices. Internal output. A program output that does not drive a field device and is used forinternal purposes only. It provides interlocking functions like a hardwired control relay. An internal output may also be referred to as an internal storage bit or an internal coil. Internal storage address assignment document. A document that identifies the address, type, and function of every internal used in a control program. International Standards Organization (ISO). An organization established to promote the development of international standards. Interrupt. The act of redirecting a program’s execution to perform a more urgent task. I/O address. A unique number, assigned to each input/output device, that corresponds to the device’s location in the rack enclosure. The address number is used when programming, monitoring, or modifying a specific input or output. I/O address assignment document. A document that identifies every field device by address, type of input/output module, type of field device, and the function the field device performs. I/O bus network. A network that lets input and output devices communicate directly to a PLC through digital communication. I/O bus network scanner. A device connected to a PLC that reads and writes to field devices connected to an I/O bus network, as well as decodes the data in the network information packet. I/O module. A plug-in assembly, containing two or more identical input or output circuits, that provides the connection between a processor and connected devices. Normal I/O module capacities are 2, 4, 8, and 16 circuits. I/O scan time. The time required to update all local and remote I/O. I/O update scan. The process of revising the bits in a PLC’s I/O tables based on the latest results from reading the inputs and processing the outputs according to the control program. I/O wiring connection diagram. A drawing that shows the actual connections of the field I/O devices to a PLC, including power supplies and subsystem connections. ISO. See International Standards Organization. Isolated I/O interface. An input module in which each input has a separate return line. Isolated I/O interfaces can connect field devices powered from different sources to one module. Isolation transformer. A transformer that protects its connected devices from surrounding electromagnetic interference. ITAE. See integral of time and absolute error open-loop tuning method. K: K. 210. Used to denote memory size in either bits, bytes, or words. knowledge AI system. A mid-level AI system that detects faults based on resident knowledge and also makes decisions about the cause of the fault and ensuing process actions. knowledge database. The section of an AI system that stores information extracted from the expert. knowledge inference. A decision-making methodology used to gather and analyze process data in order to draw conclusions. knowledge representation. The way an artificial intelligence system strategy is organized. |
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