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. 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 usedto adjust the dependent variable, the process variable. 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.
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. 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. Dynamic system checkout. The process of verifying the correct operation of a control program by actually implementing it. |
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