Page 424 - Programmable Logic Controllers, Fifth Edition - Mobile version

Solid-state switch Any electronic device incorporating a transistor,
silicon-controlled rectifier, or triac semiconductor switch to control the Timed contact A normally open and/or normally closed contact that
is actuated at the end of a timer’s time-delay period.
on/off flow of electric power. Timer In relay-panel hardware, an electromechanical device that can
Source mode output A mode of operation of solid-state output devices be wired and preset to control the operating interval of other devices. In
in which the device controls the current to the load. For example, a programmable logic controller, a timer is internal to the processor;
when the output is energized, it connects the load to the positive polarity that is, it is controlled by a user-programmed instruction.
of the supply. Toggle switch A panel-mounted switch with an extended lever;
Star topology A network architecture in which all network nodes normally used for on/off switching.
are connected to a central device that routes the nodes’ messages. Token The logical right to initiate communications in a communication
State The logic 0 or 1 condition in programmable logic controller network.
memory or at a circuit input or output. Token passing A technique in which tokens are circulated among
Station Any programmable logic controller, computer, or data terminal nodes in a communication network.
connected to, and communicating by means of, a data highway.
Topology The structure of a communications network; examples are
Status indicators LEDs that indicate the on-off status of an input or bus, ring, and star.
output point and are visible on the outside of the PLC.
Transducer A device used to convert physical parameters such as
Stepper-motor module Provides pulse trains to a stepper-motor temperature, pressure, and weight into electric signals.
translator that enables control of a stepper motor. Transformer An electric device that converts a circuit’s electrical
STI An acronym for selectable time interrupt, a subroutine that energy into a circuit or circuits with different voltages and current ratings.
executes on a time basis rather than an event basis.
Transistor A three-terminal active semiconductor device composed
Storage bit A bit in a data table word that can be set or reset but that of silicon or germanium that is capable of switching or amplifying an
is not associated with a physical input or output terminal point. electric current.
Structure Text (ST) High-level, text-based language with commands Transistor-transistor logic (TTL) A semiconductor logic family in
that support a highly structured program development and the ability which the basic logic element is a multiple-emitter transistor. This family
to evaluate complex mathematical expressions. of devices is characterized by high speed and medium power dissipation.
Subroutines Program files that are scanned only when called on by Transitional contact A contact that, depending on how it is
logic and can be used to break the program into smaller segments. programmed, will be on for one program scan every 0 to 1 transition,
Subtract A programmable logic controller instruction that performs or every 1 to 0 transition, of the referenced coil.
the mathematical subtraction of one number from another. Transmission line A system of one or more electric conductors used
Suppression device A unit that attenuates the magnitude of to transmit electric signals or power from one place to another.
electrical noise. Triac A solid-state component capable of switching alternating current.
Surge A transient wave of current or power. True As related to programmable logic controller instructions, an
Synchronous shift register A shift register in which only one on, enabled, or 1 state.
change of state occurs per control pulse.
Truth table A table listing that shows the state of a given output as a
Synchronous transmission A type of serial transmission that function of all possible input combinations.
maintains a constant time interval between successive events.
TTL-input module Enables devices that produce TTL-level signals
T to communicate with a PLC’s processor.
TTL-output module Enables a PLC to operate devices requiring
Tag A text-based name for an area of the controller’s memory where TTL-level signals to operate.
data are stored.
Twisted pair cable A pair of wires that can transmit data; the wires
Tap A device that provides mechanical and electrical connections to a are twisted to provide protection against crosstalk.
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. U
Task It holds the information necessary to schedule the program’s Unlatch instruction One-half of a programmable logic controller
execution and sets the execution priority for one or more programs.
instruction pair that emulates the unlatching action of a latching
Terminal address The alphanumeric address assigned to a particular relay. The unlatch instruction de-energizes a specified output point or
input or output point. It is also related directly to a specific image table internal coil until re-energized by a latch instruction. The output point
bit address. or internal coil remains de-energized regardless of whether or not the
Thermocouple A temperature-measuring device that utilizes two unlatch instruction is energized.
dissimilar metals for temperature measurement. As the junction of Up-counter An event that starts from 0 and increments up to the
the two dissimilar metals is heated, a proportional voltage difference, preset value.
which can be measured, is generated.
Thumbwheel switch A rotating switch used to input numeric V
information into a controller. Variable A factor that can be altered, measured, or controlled.
Time base A unit of time generated by a microprocessor’s clock circuit Variable data Numerical information that can be changed during
and used by PLC timer instructions. Typical time bases are 0.01, 0.1, application operation. It includes timer and counter accumulated values,
and 1.0 second. thumbwheel settings, and arithmetic results.

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