Page 416 - Programmable Logic Controllers, Fifth Edition

Computer interface A device designed for data communication
between a programmable logic controller and a computer. D
Data Information encoded in a digital form, which is stored in an
Consumed tag References data that come from another controller. assigned address of data memory for later use by the processor.
Contact The current-carrying part of an electric relay or switch. Data address A location in memory where data can be stored.
The contact engages to permit power flow and disengages to interrupt Data file A group of data memory words acted on as a group rather
power flow to a load device.
than singly.
Contact bounce The uncontrollable making and breaking of a contact Data highway A communications network that allows devices such
during the initial engaging or disengaging of the contact.
as PLCs to communicate. They are normally proprietary, which means
Contact histogram An instruction sequence that monitors a designated that only like devices of the same brand can communicate over the
memory bit or a designated input or output point for a change of state. A highway.
listing is generated by the instruction sequence that displays how quickly Data latching A technique used to read the value of the input data
the monitored point is changing state. that will be operated on by the instructions with a function block.
Contactor A special-purpose relay designed to establish and interrupt Data link The equipment that makes up a data communications
the power flow of high-current electric circuits. network.
Contact symbology A set of symbols used to express the control Data manipulation The process of exchanging, altering, or moving
program with conventional relay symbols. data within a programmable logic controller or between programmable
Continuous current per module The maximum current for each logic controllers.
module. The sum of the output current for each point should not exceed Data manipulation instructions A classification of processor
this value. instructions that alter, exchange, move, or otherwise modify data
Continuous current per point The maximum current each output is memory words.
designed to supply continuously to a load. Data table The part of processor memory that contains input and
Controlled variable The output variable that the automatic control output values as well as files where data are monitored, manipulated,
adjusts to keep the process value at a set-point. and changed for control purposes.
Control logic The control plan for a given system; the program. Data transfer The process of moving information from one location
to another, in other words, from register to register, from device to
Control loop The method of adjusting the control variable in a device, and so forth.
process control system by analyzing the process variable data and
then comparing it to the set-point to determine the amount of error Data transmission line A medium for transferring signals over a
in the system. distance.
ControlNet An open, high-speed, deterministic network that Debouncing The act of removing intermediate noise states from a
transfers on the same network time-critical I/O updates, controller-to- mechanical switch.
controller interlocking data, and non-time-critical data such as data Debug The process of locating and removing mistakes from a software
monitoring and program uploads and downloads. program or from hardware interconnections.
Control relay A relay used to control the operation of an event or a Decimal number system A number system that uses ten numeral
sequence of events. digits (decimal digits): 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. Each digit position
has a place value of 1, 10, 100, 1000, and so on, beginning with the
Counter An electromechanical device in relay-based control least significant (rightmost) digit; base 10.
systems that counts numbers of events for the purpose of controlling
other devices based on the current number of counts recorded; a Decrement The act of reducing the contents of a storage location or
programmable logic controller instruction that performs the functions value in varying increments.
of its electromechanical counterpart. Determinism The ability to reliably predict when data will be
Cross reference In ladder diagrams, letters or numbers to the right delivered.
of coils or functions. The letters or numbers indicate where on other DeviceNet An open communication network designed to connect
ladder lines contacts of the coil or function are located. factory-floor devices together without interfacing through an I/O system.
Up to 64 intelligent nodes can be connected to one DeviceNet network.
Crosstalk Undesired energy appearing in one signal path as a result
of coupling from other signal paths or use of a common return line. Diagnostic program A user program designed to help isolate
Current The rate of electrical electron movement, measured in hardware malfunctions in the programmable logic controller and the
amperes. application equipment.
Diagnostics The detection and isolation of an error or malfunction.
Current-carrying capacity The maximum amount of current a
conductor can carry without heating beyond a predetermined safe limit. Digital device One that processes discrete electric signals.
Current sinking Refers to an output device (typically an NPN Digital gate A device that analyzes the digital states of its inputs and
transistor) that allows current flow from the load through the output outputs and an appropriate output state.
to ground. Digital signals A system of discrete states: high or low, on or off,
Current sourcing Output device (typically a PNP transistor) that 1 or 0.
allows current flow from the output through the load and then to ground. Digital-to-analog converter An electrical circuit that converts
Cycle A sequence of operations repeated regularly; the time it takes binary bits to a representative, continuous, analog signal.
for one such sequence to occur. DINT A data type that stores a 32-bit (4-byte) signed integer value.

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