Ladder diagram. An industry standard for representing relay logic control systems. Ladder diagram language (LD). A graphical set of instructions that implements basic relay ladder functions in a PLC. Ladder relay instructions. Computer codes that implement relay coils and contactsand their corresponding functions in a PLC. Ladder rung matrix. A rectangular array that defines the maximum number of contacts that can be programmed in a ladder rung, along with the maximum number of parallel branches allowed in the rung. Lag time. The delay between the initial response of the process variable to a change in the control variable and the process variable’s optimal response to it. LAN. See local area network. Language. A set of symbols and rules for representing and communicating information between people and machines. Laplace transform. A mathematical function used to convert differential equations from the time domain into the frequency domain so that they become easy-to manage algebraic equations. LCD. See liquid crystal display. LD. See ladder diagram language. Lead resistance compensation. A factor that compensates for signal loss due to resistance present in electrical wires. Least significant bit (LSB). The bit representing the smallest value in a nibble, byte, or word. Least significant digit (LSD). The digit representing the smallest value in a byte or word. LED. See light-emitting diode. Light-emitting diode (LED). A semiconductor diode whose junction emits light when current passes through it in a forward direction. Limit switch. An electrical switch actuated by the motion of a machine or equipment. Linear variable differential transformer (LVDT). An electromechanical mechanism that provides a voltage reference that is proportional to the movement or displacement of a core inside a coil. Liquid crystal display (LCD). A display device consisting of a liquid crystal hermetically sealed between two glass plates. Load. The power used by a machine or apparatus. Load cell. A force or weight transducer that is based on a direct application of a bonded strain gauge. Local area network (LAN). An ensemble of interconnected processing elements (nodes), which are typically located within a few miles of each other. local rack. An enclosure, placed in the same area as the master rack, that contains a local I/O processor, which sends data to and from the central processing unit. Location. A storage position or register in memory identified by a unique address. logic. The process of solving complex problems through the use of simple functions that can be either true or false. Logic diagram. A drawing that uses interconnected AND, OR, and NOT logic symbols to graphically describe a system’s operation or control. Longitudinal redundancy check (LRC). An error-checking technique based on an accumulated exclusive-OR of transmitted characters. LRC characters are accumulated at both the sending and receiving stations. Loop tuning. The process of determining the proportional, integral, and derivative constants that will allow a PID controller to perform optimally. LRC. See longitudinal redundancy check. LSB. See least significant bit. LSD. See least significant digit. LVDT. See linear variable differential transformer. M: MAC. See medium access control. Macrostep. A small sequential function chart program embedded as an action within a larger sequential function chart. Mask. A logical function used to set certain bits in a word to an established state. Master. A device used to control other devices. Master control relay (MCR). A hardwired or soft wired relay instruction that will de-energize its associated I/O devices when the instruction is de-energized. Master rack. The enclosure containing the CPU or processor module. Master/slave bus topology. A network configuration in which one master controller manages several slave controllers. Maximum value method. A method of calculating the final output value of a fuzzy logic controller by finding the rule output value with the highest membership function grade. MCR. See master control relay. Mean. The average value of a set of data readings. Mean-time-between-failures study. A study, which contains data about the average time between equipment failures, that provides information about the reliability of a product. Median. The middle value of a set of data readings organized in ascending order. Medium access control (MAC). A technique that ensures that only one device is transmitting on a network at any given time. Membership function. A group of fuzzy logic rules used to divide input data into sets, which are then analyzed to provide reasoned control of a field device. Memory. The part of a programmable controller that stores data, instructions, and the control program either temporarily or semipermanently. Memory map. A diagram showing a system’s memory addresses, as well as which programs and data are assigned to each section of memory. Message. A group of data and control bits transferred as an entity from a data source. Microprocessor. A digital, electronic logic package (usually on a single chip) capable of performing the program execution, control, and data-processing functions of a central processing unit. A microprocessor usually contains an arithmetic logic unit, temporary storage registers, instruction decoder circuitry, a program counter, and bus interface circuitry. Miniprogrammer. A portable device used for programming, changing, and monitoring a PLC’s control logic. Mode. The most frequently occurring value in a set of data readings. Module. An interchangeable, plug-in item containing electronic components. Most significant bit (MSB). The bit representing the greatest value of a nibble, byte, or word. Most significant digit (MSD). The digit representing the greatest value of a byte or word. MSB. See most significant bit. MSD. See most significant digit. Multidrop link. A cable that terminates at more than one point. Multiplexing. The act of channeling two or more signals to one source using the same channel. Multiprocessing. Concurrent execution of two or more tasks residing in memory. N: NAND. A logical operator that yields a logic 1 output if any input is logic 0 and a logic 0 output if all inputs are logic 1. This operator is a negated AND function, the result of negating the output of an AND gate by following it with a NOT symbol. Negative logic. The use of binary logic so that logic 0 represents the voltage level normally associated with logic 1 (i.e., logic 0 = +5 V, logic 1 = 0 V). Network. A series of points (or devices) connected by some type of communication medium. Network communications instructions. Computer codes that allow a PLC to share data with other PLCs connected to a local area network. Network interface module. A special function interface that allows PLCs and other intelligent devices to communicate and transfer data over a high-speed local area communication network. Network layer. Layer 3 of the OSI protocol. This layer routes information in the network. Nibble. A group of four bits. Node. A station, such as a personal computer or a PLC, that is connected to a network and can thereby send and receive messages through the network. Nonreturn to zero invert on ones (NRZI). A self-clocking pulse code used to establish reliable synchronous transmission. Nonvolatile memory. A type of memory whose contents are not lost or disturbed if operating power is lost. NOR. A logical operator that yields a logic 1 output if all inputs are logic 0 and a logic 0 output if any input is logic 1. This operator is a negated OR function, the result of negating the output of an OR gate by following it with a NOT symbol. Normal action. A set of IEC 1131-3 instructions that is executed continuously for the duration of an SFC step’s activity. Normally closed contact. (1) A relay contact pair that is closed when the coil of the relay is not activated and open when the coil is activated. (2) A ladder program symbol that allows logic continuity (flow) if the referenced input is logic 0 when evaluated. Normally open contact. (1) A relay contact pair that is open when the coil of the relay is not activated and closed when the coil is activated. (2) A ladder program symbol that allows logic continuity (flow) if the referenced input is logic 1 when evaluated. NOT. A logical operator that yields a logic 1 output if a logic 0 is entered at the input and a logic 0 output if a logic 1 is entered at the input. The NOT function, also called an inverter, is normally used in conjunction with AND and OR functions. NRZI. See nonreturn to zero invert on ones. |
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