Page 423 - Programmable Logic Controllers, Fifth Edition - Mobile version

Register A memory word or area for the temporary storage of data
used within mathematical, logical, or transfer functions. Sensor A device used to gather information by the conversion of a
physical occurrence to an electric signal.
Relay An electrically operated device that mechanically switches Sequencer A mechanical, electric, or electronic device that can be
electric circuits. programmed so that a predetermined set of events occurs repeatedly.
Relay contacts The contacts of a relay that are either opened or Sequence table A table or chart indicating the sequence of opera-
closed according to the condition of the relay coil. Relay contacts are tion of output devices.
designated as either normally open or normally closed in design. Sequential control A process that dictates the correct order of events
Relay logic A representation of the program or other logic in a form and allows one event to occur only after the completion of another.
normally used for relays. Sequential Function Chart (SFC) Graphical language whose basic
Remote input/output (I/O) system Any input/output system that language elements are steps or states with associated actions and
permits communication between the processor and input/output hardware transitions with associated conditions used to move from the current
over a coaxial or twin axial cable. Remote input/output systems permit state to the next.
the placement of input/output hardware at any distance from the Serial communication A type of information transfer in which the
processor. bits are handled sequentially; contrasted with parallel communication.
Resolution The smallest distinguishable increment into which a Series circuit A circuit in which the components or contact symbols
quantity is divided. are connected end to end, and all must be closed to permit current flow.
Response time The amount of time required for a device to react to Servo module The device whose feedback is used to accomplish
a change in its input signal or to a request. closed-loop control. Though programmed through a PLC, once
Retentive instruction Any programmable logic controller instruction programmed it can control a device independently without interfering
that does not need to be continuously controlled for operation. Loss with the PLC’s normal operation.
of power to the instruction does not halt execution or operation of the Set-point The value that the process value is to be held to by the
instruction. automatic control function.
Retentive timer An electromechanical relay that accumulates time Shield A barrier, usually conductive, that substantially reduces the
whenever the device receives power and maintains the current time effect of electric and/or magnetic fields.
should power be removed from the device. Loss of power to the device
after reaching its preset value does not affect the state of the contacts. Shift To move binary data within a shift register or other storage
device.
Retentive timer instruction A programmable logic controller
instruction that emulates the timing operation of the electromechanical Shift register A PLC function capable of storing and shifting
retentive timer. binary data.
Short circuit An undesirable path of very low resistance in a circuit
Retentive timer reset instruction A programmable logic controller
instruction that emulates the reset operation of the electromechanical between two points.
retentive timer. Short-circuit protection Any fuse, circuit breaker, or electronic
hardware used to protect a circuit or device from severe overcurrent
Ring topology A network topology that that forms a data path in
a ring. conditions or short circuits.
Signal The event or electrical quantity that conveys information
Routine A series of instructions that perform a specific function or task.
from one point to another.
RS-232 An Electronic Industries Association (EIA) standard for data
transfer and communication for serial binary communication circuits. Significant digit A digit that contributes to the precision of a number.
The number of significant digits is counted beginning with the digit
Run The single, continuous execution of a program by a programmable contributing the most value, called the most significant digit (leftmost),
logic controller. and ending with the digit contributing the least value, called the least
Rung A group of programmable logic controller instructions that significant digit (rightmost).
controls an output or storage bit, or performs other control functions Silicon-controlled rectifier (SCR) A semiconductor device that
such as file moves, arithmetic, and/or sequencer instructions. A rung is functions as an electronic switch.
represented as one section of a ladder logic diagram.
Single-scan function A supervisory instruction that causes the
S control program to be executed for one scan, including input/output
update. This troubleshooting function allows step-by-step inspection
SCADA An acronym for supervisory control and data acquisition. of occurrences while the machine is stopped.
Scan time The time required to read all inputs, execute the control Sink mode output A mode of operation of solid-state devices in
program, and update local and remote input and output statuses. Scan which the device controls the current from the load. For example, when
time is, in effect, the time required to activate an output controlled by the output is energized, it connects the load to the negative polarity of
programmed logic. the supply.
Schematic A diagram of graphic symbols representing the electrical SINT A data type that stores an 8-bit (1-byte) signed integer value.
scheme of a circuit.
Snubber A circuit generally used to suppress inductive loads; it
Search function Allows the user to display quickly any instruction consists of a resistor in series with a capacitor (RC snubber) and/or a
in the programmable logic controller program. MOV placed across the alternating current load.
Self-diagnostic The hardware and firmware within a controller that Software Programs that control the processing of data in a system,
monitors its own operation and indicates any fault it can detect. as contrasted to the physical equipment itself (hardware).

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