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Chapter 6
Introduction to Logic
Copyright © 2002 Delmar Thomson Learning
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
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 (cont’d.)
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.
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Logical Continuity (cont’d.)
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 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.
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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.
On signal stored as a 1, 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 the associated 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
result of 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 a time.
When 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 hard wired 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.
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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 and normally closed 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 programmed to provide the
proper input conditions for your
application.
This is called combination AND-OR
logic.
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Combination AND-OR Logic
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