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Chapter 6. Introduction to Logic. Objectives. Explain AND logic. Describe OR logic. Explain NOT logic. Explore a truth table for specified logic. Determine if a PLC ladder rung is true or false under specified conditions. Hardwired Electrical Circuit Has Electrical Continuity.

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Chapter 6

Chapter 6

Introduction to Logic


  • Explain AND logic.

  • Describe OR logic.

  • Explain NOT logic.

  • Explore a truth table for specified logic.

  • Determine if a PLC ladder rung is true or false under specified conditions.

Hardwired electrical circuit has electrical continuity
Hardwired Electrical Circuit Has Electrical Continuity

  • Hardwired circuit has actual current flow.

  • This is called electrical continuity.

  • Current flows through switching device to directly control the load.

Logical continuity 1 of 3
Logical Continuity (1 of 3)

  • Hardwire relay circuits have electrical continuity or current flow.

  • PLCs do not operate on electrical continuity, but logical continuity.

Logical continuity 2 of 3
Logical Continuity (2 of 3)

  • A PLC ladder program resembles an electric schematic.

  • A PLC program is a set of instructions stored in memory.

  • PLC ladder symbols represent ladder program instructions.

Logical continuity 3 of 3
Logical Continuity (3 of 3)

  • PLC instructions examine the status of a bit at the associated address.

  • Bits are either a 1 or 0.

  • Instructions are either true or false.

  • PLCs have logical continuity.

Input signal seen by input module
Input Signal Seen by Input Module

  • Signal from field device is wired to input module screw terminals.

  • These signals are called input signals or simply inputs.

  • Each input point has a unique identifier called an address.

Input module isolates and converts
Input Module Isolates and Converts

  • Discrete input module determines if input is a valid ON signal.

  • The discrete input module converts the incoming electrical signal to +5V DC, or 0V DC.

  • An ON state will be converted to +5V DC.

  • An OFF state will be converted to 0V DC.

Input section to back plane
Input Section to Back Plane

  • The high voltage input signal is isolated from the lower voltage microprocessor and supporting circuitry in the processor.

  • Signal is then transferred to the chassis back plane and sent to processor memory for storage.

Processor and input status file
Processor and Input Status File

  • Field input signals are transferred from the input module by way of the back plane for storage in the processor’s input status file.

  • There is one storage location in the status table for the ON or OFF status of each input device.

  • The ON signal is stored as a 1; the OFF as a 0.

Processor examines status table bit
Processor Examines Status Table Bit

  • When running, the processor solves ladder instructions by examining the ON or OFF status of the input device by examining the associated memory location for a 1 or 0.

Ladder program solved
Ladder Program Solved

  • Ladder instructions have rules regarding when they are true or false.

    • Instructions are made true or false as a result of the examined bit.

    • If there is a path of true input instructions to the output instruction, the rung will be true.

Output update
Output Update

  • When all ladder rungs have been solved, the output status table data is sent to the respective output modules one word at a time.

  • When a rung is solved, the output status table is updated.

Combining and solving ladder instructions
Combining and Solving Ladder Instructions

  • Three basic types of logic:

    • And

    • Or

    • And / Or (combinational)

Series logic
Series Logic

  • When two switches are wired in series, both switches must be on before power will flow.

  • Switch 1 AND switch 2 must all pass power before light 1 will turn on.

Switch 1 and switch 2 need to be closed to energize l 1
Switch 1 AND Switch 2 Need to Be Closed to Energize L-1

Truth table
Truth Table

  • A truth table shows all possible input conditions and the expected output conditions.

  • The following slide shows a truth table for AND logic.

Plc instructions in series
PLC Instructions in Series

  • PLC instructions work the same basic way as hardwired field devices.

  • Instructions in series are considered ANDed together, or AND logic.

  • Switch 1 AND switch 2 must be on for L-1 to come on.

Truth table for two input and logic
Truth Table for Two-Input AND Logic

Parallel is also or logic
Parallel Is also OR Logic

  • Inputs in parallel are said to be ORed.

  • Switch 1 is ORed with switch 2.

  • If switch 1 is closed OR switch 2 is closed, light 1 will turn on.

  • If both SW1 and SW2 are closed, light 1 will turn on.

Plc representation of n o and n c instructions
PLC Representation of N.O. and N.C. Instructions

  • All PLCs use the standard normally open (N.O.) and normally closed (N.C.) instructions.

  • Different manufacturers use different names for the instructions.

Combination and or logic
Combination AND-OR Logic

  • Input devices can be combined in series (ANDed) and also in parallel (ORed) on a PLC ladder rung.

  • Instructions are programmed to provide the proper input conditions for your application.

  • This is called combination AND-OR logic.