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concurrent parallel processing
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The conceptually simultaneous execution of more than one sequential program on a computer or network of computers.
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contacts
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Conductive metal parts in an electrical circuit that open or close the circuit by coming in contact with or separating from each other.
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count down counter
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Decreases the current value by an increment of 1 every time an input transitions from FALSE to TRUE. Count down counters should always start counting at the preset value.
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count up counter
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Increases the current value by an increment of 1 every time an input transitions from FALSE to TRUE. Count up counters are started from zero and continue counting until the accumulated value matches the preset value.
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designator
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The label given to each element in ladder diagrams, used to reference that element throughout the program.
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examine if closed
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A PLC input instruction that is true if its addressed bit is ON and false if it is OFF. The same instruction as examine on.
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examine if open
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A PLC input instruction that is true if its addressed bit is OFF and false if it is ON. The same instruction as examine off.
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examine off
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A PLC input instruction that is true if its addressed bit is OFF and false if it is ON. The same instruction as examine if open.
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examine on
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A PLC input instruction that is true if its addressed bit is ON and false if it is OFF. The same instruction as examine if closed.
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function block
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A graphical PLC programming language that allows for both sequential and parallel processing. Function block diagrams are starting to replace ladder logic in higher level PLCs.
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hand-held programmer
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A PLC programming device that combines a display screen and keypad all in one unit. Specially assigned keys allow you to enter program instructions and changes, and navigate and troubleshoot the PLC system.
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internal relay
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Software relays that can be used with ladder logic programs to perform logical operations. Internal relays are represented digitally within a PLC's memory.
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ladder logic
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A method for drawing electrical logic schematics. Ladder logic is now most often used to program PLCs.
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latched contact
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A contact that keeps the output energized even once the input signal is removed.
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line diagram
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A diagram that shows the logic of an electrical circuit or system using standard symbols. Also known as a ladder diagram because the diagram appears as individual lines or "rungs" connected between two vertical lines.
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logical continuity
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The way that PLC programming logic allows power to flow. PLC logical continuity always reads a program from left to right.
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memory
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The ability of a circuit to store a charge and keep the load energized even after the signal is removed.
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mnemonic
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An instruction that is presented in a short, easy-to-remember form.
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momentary contact
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A contact that returns to its original position immediately after sending a signal.
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network
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A group of connected logic elements used to perform a specific function.
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one shot
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A series of ladder instructions that energizes an output for one scan only.
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one shot rising
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A one shot instruction that is triggered when the programmed input transitions from OFF to ON.
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output energize
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The most common PLC output symbol. Using this symbol signals the PLC to energize, or turn on, the output.
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personal computer
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A small, single-user computer. Also known as a PC.
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programmable logic controller
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A processor driven device that uses logic-based software to provide electrical control to machines and devices.
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programming component
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Any component of a PLC software ladder program. Programming components do not physically exist but are representations used by the PLC software.
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programming device
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A device, usually a personal computer, used to enter instructions into the PLC. Some programming devices are small enough to fit in your hand.
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programming language
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A set of symbols and rules used to represent information to a processor so that it can understand and apply instructions.
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proprietary
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Designed by a specific company for use only with its own systems.
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register
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A group of bit data used to store information in a PLC's memory.
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rising edge
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The point at which an AC signal wave transitions from low to high.
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rung
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The horizontal programming lines in ladder logic. Each rung controls one output.
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sequential
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Doing one and only one step at a time, followed by the next step in a logical order.
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shorthand programming
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A programming method that saves time by using mnemonic names for instructions rather than having to search for the graphic symbols.
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timer off delay
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A delay timer that immediately closes contacts when the control coil is energized, then waits for a predetermined amount of time to open them after power is removed from the coil.
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timer on delay
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A timer that waits to turn on the output after receiving an ON signal from the input.
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troubleshooting
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The systematic elimination of the various parts of a system, circuit, or process to locate a malfunctioning part.
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vertical contact
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A contact that bridges a parallel path, shown up and down instead of left and right, on a line diagram. PLC programs cannot use vertical contacts.
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