Chapter 06 Intro to Logic

7/29/2019 Chapter 06 Intro to Logic

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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

• Hardwired circuit has actual current flow.

• This is called electrical continuity.• Current flows through switching device to

directly control the load.



Hardwired Relay Circuit



Logical Continuity (1 of 3)

• Hardwire relay circuits have electrical

continuity or current flow.

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




• A PLC ladder program resembles an electric

schematic.

• A PLC program is a set of instructions storedin memory.

• PLC ladder symbols represent ladder

program instructions.




• 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 Flow into and out of a PLC



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

• Discrete input module determines if input is a

valid ON signal.

• The discrete input module converts theincoming 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

• 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

• 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 theassociated memory location for a 1 or 0.



Ladder Program Solved

• Ladder instructions have rules regarding

when they are true or false.

– Instructions are made true or false as a resultof the examined bit.

– If there is a path of true input instructions to

the output instruction, the rung will be true.



Output Update

• When all ladder rungs have been solved, the

output status table data is sent to the

respective output modules one word at atime.

• When a rung is solved, the output status

table is updated.



Combining and Solving

Ladder Instructions• Three basic types of logic:

– And

– Or

– And / Or (combinational)



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.



Conventional Series Circuit



Switch 1 AND Switch 2 Need

to Be Closed to Energize L-1



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.



Truth Table for AND Logic



PLC Instructions in Series

• PLC instructions work the same basic way as

hardwired field devices.

• Instructions in series are considered ANDedtogether, or AND logic.

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

come on.



PLC Representation of a Rung



Truth Table for

Two-Input AND Logic



Three-Input Series Circuit



Three-Input PLC Ladder Rung



Three-Input AND Logic Truth Table



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.



Conventional Parallel Circuit



Programmable Controller

Ladder Diagram



Two-input OR Truth Table



Truth Table for NOT Logic



Conventional Ladder Diagram



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.



Normally Open and Normally

Closed Instruction Identification



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.



Combination AND-OR Logic

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Chapter 06 Intro to Logic