What is the definition of "examine off"?
A basic PLC logic instruction symbol. Also referred to as Examine If Open.

Learn more about examine off in the class Intro to PLCs 200 below.

PLCs Training

Class Information
PLCs Training Tooling U-SME classes are offered at the beginner, intermediate, and advanced levels. The typical class consists of 12 to 25 lessons and will take approximately one hour to complete.
Class Name:Intro to PLCs 200
Description:This class introduces the parts and operations of programmable logic controllers (PLCs) and describes the functions and different programming languages you will find on most PLCs. Includes an Interactive Lab.
Number of Lessons:17
Language:English, Spanish
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Class Outline
  • Objectives
  • What Is a PLC?
  • History of PLCs
  • PLC Components
  • How a PLC Works
  • The Structure of PLC Applications
  • Principles of Logic
  • Logic Symbols
  • CPU
  • Input Devices
  • Output Devices
  • Internal Relays
  • Counters
  • Timers
  • Sequencers
  • PLC Standardization
  • Summary
Class Objectives
  • Define PLCs.
  • Describe the history of PLCs.
  • Identify the main components of a PLC system.
  • Describe how a PLC works.
  • Describe how a PLC is structured.
  • Identify basic PLC logic functions.
  • Identify the most common PLC logic symbols.
  • Describe the role of PLC central processing units.
  • Identify common input devices.
  • Identify common output devices.
  • Identify common internal relays.
  • Describe how counters work in a PLC.
  • Describe how timers work in a PLC.
  • Describe how sequencers work in a PLC.
  • Describe PLC standardization.

Class Vocabulary

Vocabulary TermDefinition
accumulator A register in PLC memory that stores the results of counting functions.
address The unique location assigned to each I/O module so that it may be correctly referenced and signaled by the PLC program.
AND A logic function where both A and B must be true to trigger C. AND works similar to a series circuit.
automation Self-regulated machines that meet predetermined requirements in a continuous process without the need for direct human intervention.
binary A number system used for PLCs that has either 0 or 1 as a value. Using binary, a function is either on or off.
block I/O A smaller type of PLC that is a self-contained box. A block I/O has fewer input and output modules.
Boolean algebra A branch of mathematics that deals with the use of logical operators OR, AND, and NOT.
count down counter A type of PLC software counter that counts down from the reset value. Count down counters are less popular because they may use negative numbers.
count up counter A type of PLC software counter that counts up from the reset value, usually 0.
counter An internal relay program that counts items to a preset number before starting the process over again.
CPU The main processor of information in a PLC that performs all logic and math operations.
Ethernet A common method used to link computers or PLCs together through a local area network.
event-driven trigger A sequencer trigger actuated by some type of mechanical motion that is sensed by an input.
examine off A basic PLC logic instruction symbol. Also referred to as Examine If Open.
examine on A basic PLC logic instruction symbol. Also referred to as Examine If Closed.
I/O Abbreviation for input/output device.
IEC 61131-3 The IEC standard that seeks to limit the number of PLC protocols in use.
input device A type of device, usually a sensor, that sends information into a PLC. Inputs are connected to the PLC by input modules.
input module The jack where an input device is physically connected to a PLC. The input modules transfer signals to the internal relays.
internal relay The part of a PLC that simulates the physical connections of relay circuits by using processor driven software.
International Electrotechnical Commission An organization that provides conformity assessment for government, business, and society for all electrical, electronic, and related technologies.
ladder logic A method for drawing electrical logic schematics. Ladder logic is now most often used to program PLCs.
limit switch A protective device used to open or close electrical circuits when temperature or pressure limits are reached.
logic The sequence of instructions performed by a PLC program.
logic gate A circuit with several inputs but only one output that can be activated by particular combinations of inputs. Logic gates are used to calculate operations in Boolean algebra.
module The physical connection on a PLC where an input or output is plugged in.
motor starter A type of PLC output device that starts a motor when triggered. PLCs do not have the current capacity to start a motor on their own.
NAND A logic function where both A and B must be false to trigger C. If one input is present, the output will not be turned on.
NOR A logic function where C will be triggered if neither A nor B are present.
normally closed contact An electrical contact that regularly allows electricity to flow.
normally open contact An electrical contact that rarely allows electricity to flow.
NOT A logic function where the output will not be triggered if a specified input is present.
OR A logic function where C is triggered if either A or B are true. OR works similar to a parallel circuit.
output device A device that performs a mechanical action after receiving the electrical signal to do so from the PLC output modules.
output energize The most common PLC output symbol. Using this means to energize, or turn on, the output.
output module The jack where an output device is physically connected to a PLC. The output modules transfer signals from the internal relays.
PC A personal computer. PCs are processor driven and are now used to do most PLC programming.
photo eye A type of PLC input that senses motion. Once the motion is detected, the photo eye sends a signal to begin an output.
pilot light A light that indicates whether power is on or a motor is in operation.
power supply The device that provides power to a PLC CPU and I/O modules. The power supply must convert the line voltage to around 5VDC for the PLC to use.
preset A value that is programmed into a PLC counter. The preset is the target number you want the system to reach.
programmable logic controller A processor driven device that uses logic-based software to provide electrical control to machines.
programming device The physical device used to enter a program into PLC memory. PCs are the most common programming devices.
protocol The language spoken between computers to help them exchange information.
pushbutton A type of PLC input that when manually depressed lets the system know the circuit has been closed and a process should begin. Pushbuttons are commonly used as machine start buttons.
rack I/O A larger type of PLC that is a collection of I/O cards that are linked together and stored in a rack. A rack I/O can handle thousands of inputs and outputs.
relay circuit Hard-wired physical devices that transfer electrical signals from input devices to output devices. PLCs use software to digitally simulate these connections.
reset A value that is programmed into a PLC counter. The reset is the number at which the counter begins counting, usually zero.
scan One complete cycle of a PLC checking inputs, executing its programs, and updating the status of its outputs.
sensor A device that detects a change in a physical stimulus and turns it into a signal that can be measured or recorded.
sequencer A logic device that takes a set of inputs coming into the PLC and orders the steps that lead to a series of outputs.
software Written operating instructions for a processor to carry out.
solenoid A type of output device coil that utilizes voltage to convert electrical energy to mechanical energy via magnetic fields.
solid-state device A type of circuit used in digital equipment that replaced physical connections.
time-driven trigger A sequencer trigger actuated by an internal clock.
timer An internal relay program that delays sending input data to the outputs for a preset amount of time.
timer off delay A timer that waits to turn off the output after receiving an off signal from the input.
timer on delay A timer that waits to turn on the output after receiving an on signal from the input.