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# Chapter 6 - PowerPoint PPT Presentation

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

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 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)

• Hardwire relay circuits have electrical continuity or current flow.

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

Logical Continuity (2 of 3)

• A PLC ladder program resembles an electric schematic.

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

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.

• 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.

• 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.

• 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.

• 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

• 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 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.

• 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.

• Three basic types of logic:

• And

• Or

• And / Or (combinational)

• 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

• 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 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

• 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

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

• Different manufacturers use different names for the instructions.

• 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.