Chapter 06 Intro to Logic
Transcript of Chapter 06 Intro to Logic
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Chapter 6
Introduction to Logic
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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.
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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.
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Hardwired Relay Circuit
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Logical Continuity (1 of 3)
• Hardwire relay circuits have electrical
continuity or current flow.
• PLCs do not operate on electrical continuity,but logical continuity.
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Logical Continuity (2 of 3)
• 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.
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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.
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Signal Flow into and out of a PLC
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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Combining and Solving
Ladder Instructions• Three basic types of logic:
– And
– Or
– And / Or (combinational)
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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.
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Conventional Series Circuit
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Switch 1 AND Switch 2 Need
to Be Closed to Energize L-1
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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.
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Truth Table for AND Logic
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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.
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PLC Representation of a Rung
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Truth Table for
Two-Input AND Logic
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Three-Input Series Circuit
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Three-Input PLC Ladder Rung
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Three-Input AND Logic Truth Table
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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.
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Conventional Parallel Circuit
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Programmable Controller
Ladder Diagram
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Two-input OR Truth Table
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Truth Table for NOT Logic
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Conventional Ladder Diagram
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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.
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Normally Open and Normally
Closed Instruction Identification
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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.