Safety control relay (SCR). A hardwired or softwired relay instruction that will deenergize its associated I/O devices when de-energized. Scaling. Changing analog output data to reflect engineering units. Scan. The process of reading all inputs, executing the control program, and updating all outputs. Scan time. The time required to complete the scan. Effectively, this is the time required to activate an output that is controlled by programmed logic. SCR. See safety control relay. Scratch pad memory. A temporary storage area used by the CPU to store a relatively small amount of data used for interim calculations or control. Data that is needed quickly is stored in this area to avoid the extra access time involved in retrieving data from the main memory. Second-order response. A process response to a rapid change in the control variable characterized by two lag time and a process response curve that either oscillates around the set point or overshoots the set point before settling to it. Sequential function charts (SFC). An object-oriented programming framework that organizes actions written in IEC 1131-3 programming languages (ladder diagram, instruction list, function block diagram, and structured text) into a unified sequential control program. Series circuit. A circuit in which the components or contact symbols are connected end to end. All components must be closed to permit current flow. Servo motor interface. An intelligent I/O module used in applications requiring position control via a servo drive controller, which translates the rotational movement of a servo motor into linear displacement. Set point. The target process variable value in a process control system. SFC. See sequential function charts. SFC action. A set of IEC 1131-3 instructions, organized as an SFC program, that is activated when a certain step in the main SFC program becomes active. Single-ended input/output. An analog I/O connection in which the commons are electrically tied together resulting in only one return line. Single-precision arithmetic. Arithmetic instructions that use one register each to hold the operands and one or two registers to hold the result of the operation. Sinking configuration. An electrical configuration that causes a device to receive current when the device is ON. Slave. A remote system or terminal whose function is controlled by a master device. Software. The programs that control the processing of data in a system. Solenoid. A transducer that converts a current into linear motion through the use of one or more electromagnets that move a metal plunger. Solid-state. Circuitry designed using only integrated circuits, transistors, diodes, etc., without any electromechanical devices, such as relays. Sourcing configuration. An electrical configuration that causes a device to provide current when the device is ON. Special function instructions. Computer codes that allow a PLC to perform special operations, such as sequencing, diagnostics, and PID control. ST. See structured text. Stand-alone action. A set of IEC 1131-3 programming instructions, not attached to the SFC program itself, that directs the program to jump to a particular step when the action’s logical conditions are satisfied. standard deviation. A measure of the dispersion of a set of data readings about the mean. Star-shaped ring topology. A network architecture in which signals from one node are relayed through all the other nodes in the network, yet a node can be bypassed in the event of its failure to avoid a break in the ring. Star topology. A network architecture in which all network nodes are connected to a central device that routes the nodes’ messages. Static input wiring check. A procedure performed with power applied to the PLC and input devices that verifies that each input device is connected to the proper input terminal and is operating properly. Static output wiring check. A procedure performed with power applied to the PLC and output devices that verifies that each output device is connected to the proper output terminal and is operating properly. Steady state. The situation in which the error in a process control system is at zero or within the error dead band. Step. A stage in a control process as defined by the process’s sequential function chart. Stepper motor interface. A positioning interface that controls a stepper motor, which translates incoming pulses into mechanical motion, by generating a pulse train indicating distance, rate, and direction commands to the motor. Step response. The process variable’s response to a sudden change in the process input (i.e., the control variable). Step test. A forced, sudden change in the control variable used to elicit a response from the process. Storage area. The area of a PLC’s memory that stores blocks of input/output data, as well as data about the status of internal bits. Storage register assignment document. A document that lists the storage registers used in a control program, including their contents and a description of their function. Strain gauge. A mechanical transducer that measures body deformation (or strain) due to the force applied to a rigid body. Structured text (ST). A high-level, text-based PLC programming language, resembling the BASIC and PASCAL computer languages, that allows a control program or any other complex task to be broken down into smaller tasks. Subprogram. A semi-independent program, embedded in a larger, main control program, that executes a specialized control sequence when activated by the main program. Subroutine. A program segment in a ladder diagram that performs a separate task. Subsystem. A part of a larger system having the properties of a system in its own right. Sum-of-the-weights method. A method of changing values from other number systems into their decimal equivalents by multiplying each digit by the weighted value of its position and then summing the results. Synchronous. A type of serial transmission that maintains a constant time interval between successive events. Syntax. Rules governing the structure of a language. System. A set of one or more PLCs, I/O devices and modules, computers, associated Software, peripherals, terminals, and communication networks that together provide a means of performing information processing to control a machine or process. System abstract. A definition of the process to be controlled including a clear statement of the control problem, a description of the design strategy, and a statement of objectives. System configuration diagram. A drawing of the PLC control system that shows the location, simplified connections, and minimum details of the system’s major hardware components. System error. An error resulting from an instrument or from the environment. System layout. The planned approach to placing and connecting PLC components to satisfy the control strategy and to provide system reliability and ease of maintenance. T: Tap. A device that provides mechanical and electrical connections to a trunk cable. A tap allows the signals on the trunk to be passed to a station and the signals transmitted by the stations to be passed to the trunk. Task. A set of instructions, data, and control information capable of being executed by a CPU to accomplish a specific purpose. TCP/IP. See transmission control protocol/internet protocol. Termination. (1) The load connected to the output end of a transmission line. (2) A provision for ending a transmission line and connecting to a bus bar or other terminating device. Thermal transducer. A device that measures changes in temperature. Thermistor. A temperature transducer made of semiconductor material, such as oxides of cobalt, nickel, manganese, iron, and titanium, that exhibits changes in internal resistance proportional to changes in temperature. Thermocouple. A bimetallic temperature transducer that provides a temperature value by measuring the voltage differential caused by joining together two different metals at different temperatures. Thermocouple input module. A module that amplifies, digitizes, and converts the input signal from a thermocouple into a digital signal equivalent to the temperature reading. Thermopile. The connection of several thermocouples in series to enhance their resolution. Three-position controller. A discrete-mode controller that provides three output levels—ON, 50% ON, and OFF. Throughput. The speed at which an application or part of an application is performed. Throughput depends on the transmission speed, medium, protocol, packet size, and amount of data handled by a network. Thumbwheel switch. A rotating switch used to input numeric information into a controller. Time base. A unit of time generated by the system clock and used by software timer instructions. Typical time bases are 0.01, 0.1, and 1.0 seconds. timer instructions. Computer codes that allow a PLC to perform the timing functions (ON-delay energize/de-energize, OFF-delay energize/de-energize, reset) of a hardware timer. Token. (1) A signal that grants bus transmission rights to a node on a network. (2) A signal that enables a transition or action in a sequential function chart. token passing. A network transmission technique in which a token is passed along the bus and each node has a set amount of time to receive it and respond to it. Topology. The way in which a network or system is physically structured. Transducer. A device used to convert physical parameters, such as temperature, pressure, and weight, into electrical signals. Transfer function. The unique characteristics of a process that determine its output due to changes over time. Transient response. The behavioral response of a process. Transistor-transistor logic (TTL). A semiconductor logic family characterized by high speed and medium power dissipation in which the basic logic element is a multiple-emitter transistor. Transition. A variable input, action result, conditional statement, or other program element that signals a sequential function chart to progress from one step to another. Transmission control protocol/internet protocol (TCP/IP). A network protocol developed by the U.S. Department of Defense. Transmission medium. The physical device used to transfer data in a transmission system (e.g., coaxial cable, fiber-optic cable, etc.). Transmitter. A device that amplifies a voltage signal. Tree topology. A network architecture in which the network has many nodes located in many branches of the network. Triac. A semiconductor device that functions as an electrically controlled switch for AC loads. TRUE. As related to PLC instructions, a set logic state associated with a binary 1. truth table. A table that shows the state of a given output as a function of all possible input combinations. TTL. See transistor-transistor logic. TTL I/O interface. A discrete interface that allows a controller to accept signals from TTL field devices, which are 5 VDC–level semiconductor devices. Turbine flow meter. A flow transducer that measures fluid flow by measuring the fluid’s motion through the meter’s multibladed rotor. Twisted-pair conductor. A communication medium used mainly for point-to-point applications that can transmit data up to 4000 feet at transmission rates as high as 250 kbaud. Two-position controller. A discrete-mode controller that provides two output levels—ON and OFF. Two’s complement. A numbering system, used to express negative binary numbers, in which all numbers from right to left are inverted after the first 1 is detected. |
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