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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.
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.
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.