Basics of Ladder Logic 220

This class describes the basic principles of ladder logic, identifies the symbols used to program a PLC and explains the primary logic functions those symbols create. Includes an Interactive Lab.

Class Details

Class Name:
Basics of Ladder Logic 220
Number of Lessons:
Additional Language:

Class Outline

  • Objectives
  • What Is Ladder Logic?
  • Line Diagrams
  • PLC Symbols
  • Inputs and Outputs
  • Locating Ladder Elements
  • Logic Functions
  • AND Logic
  • OR Logic
  • NOT Logic
  • Other Logic Functions
  • Logic Order
  • Limitations of Ladder Logic
  • Function Block Diagram
  • Summary


  • Define ladder logic.
  • Define line diagrams.
  • Identify common PLC logic symbols.
  • Describe PLC inputs and outputs.
  • Explain rules for locating ladder elements.
  • Describe logic functions.
  • Identify AND logic.
  • Identify OR logic.
  • Explain the function of NOT logic.
  • Identify other types of PLC logic.
  • Describe PLC logic order.
  • Describe the limitations of ladder logic.
  • Describe function block diagram.


  • CMfgT


Vocabulary Term Definition
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 in which both A and B must be true to trigger C. AND works similar to a series circuit.
binary A number system that has either 0 or 1 as a value. PLCs use binary, in which a function is either on or off.
Boolean algebra A branch of mathematics that deals with the use of logical operators OR, AND, and NOT.
coil The main PLC output symbol. A coil is a device that needs to be energized.
concurrent parallel processing The conceptually simultaneous execution of more than one sequential program on a computer or network of computers.
CPU The main processor of information in a PLC that performs all logic and math operations.
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.
false A PLC input contact condition that does not trigger an output--an OFF condition. A false condition exists when the presense or absence of an input does not match the logic of the contact.
function block 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.
graphical language A programming language that uses pictures and symbols to represent executable code. While graphical languages still use letters and numbers, textual languages rely on them exclusively.
inverter Another name for a NOT gate because when used it reverses the function of other gates.
ladder diagram Another name for a line diagram, due to its resemblance to a ladder. In this sense, each circuit line is a "rung" of the ladder.
ladder logic A method for drawing electrical logic schematics. Ladder logic is now most often used to program PLCs.
latching contact A contact that mechanically locks itself in an open or closed position so that it remains even after the signal is removed.
line diagram A diagram that shows the logic of an electrical circuit or system using standard symbols. A line diagram is also known as a ladder diagram because the diagram appears as individual lines or "rungs" connected between two vertical lines.
logic The sequence of instructions performed by a PLC program.
NAND A logic function in which both A and B must be false to trigger the output. If one input is present, the output will not be turned on.
NOR A logic function which the output 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 which the output will not be triggered if a specified input is present.
OR A logic function in which the output is triggered if either A or B is true. OR works similar to a parallel circuit.
order of operations A set of mathematical rules that dictates the sequence in which operations must be performed.
output energize The most common PLC output symbol. Using this means to energize, or turn on, the output.
programmable logic controller A processor-driven device that uses logic-based software to provide electrical control to machines.
programming language A set of symbols and rules used to represent information to a processor so that it can understand and apply instructions.
relay circuit Hard-wired physical devices that transfer electrical signals from input devices to output devices. PLCs use software to digitally simulate these connections.
rung The horizontal programming lines in ladder logic. Each rung controls one output.
sequential Performing only one step at a time, followed by the next step in a logical order.
sheet In function block diagrams, a distinct set of PLC instructions. Each sheet is comparable to a single line on a ladder diagram.
troubleshooting The systematic examination of the various parts of a system, circuit, or process to identify a malfunctioning part using process of elimination reasoning.
true A PLC input contact condition that triggers an output--an ON condition. A true condition exists when the presense or absence of an input matches the logic of the contact.
truth table A table that shows all the possible ON and OFF combinations for input devices and the corresponding state of the output. Whether the output is ON or OFF is based on the logic of the program line.
wire In function block diagrams, a line that represents whole or bit data and links inputs to outputs.
XOR A logic function in which the output is triggered when only A or B is true, but not if both are true.