PLC using logic gates ppt.ppt for education

PLC using logic gates

2. Chapter
7
Programming Logic
Gate Functions in PLCs

3. Permission granted to reproduce for educational use only.© Goodheart-Willcox Co., Inc.
Objectives
•Describe combinational and sequential
logic gate circuits.
•Create PLC ladder logic programs for NOT,
AND, OR, NAND, NOR, XOR, and XNOR
logic gates.
•Create Boolean expressions and logic gate
circuits from truth tables.

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Objectives
•Use the Logic Converter instrument in NI
Multisim to create logic tables and Boolean
expressions from logic gate circuits.
•Convert Boolean expressions to PLC
ladder logic diagrams.
•Convert PLC ladder logic diagrams to logic
gate circuits and Boolean expressions.

5. Permission granted to reproduce for educational use only.© Goodheart-Willcox Co., Inc.
Combinational Logic Gates
•Do not require clock pulses to operate.
•Outputs depend only on their inputs.
•Outputs are generated instantaneously.
•Simply called logic gates.

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Logic Gates
•NOT.
•AND.
•OR.
•NAND.
•NOR.
•XOR (exclusive OR).
•XNOR (exclusive NOR).

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Sequential Logic Devices
•Have outputs that depend on their
inputs as well as time.
•Require clock pulses.
•An inherent delay time is always
present.
•Flip-flop devices.

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Sequential Logic Circuit

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Boolean Expressions
•Every gate logic function has its own
equation called a Boolean expression.
•Boolean algebra:
–Two states are true and false.

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Boolean Expressions (Cont.)
•True state:
–Represented by the number one, called
logic high or logic one in Boolean algebra.
•False state:
–Represented by the number zero, called
logic low or logic zero.

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Boolean Expressions (Cont.)
•Logic high:
–Represented by the presence of a voltage
potential.
–Represented with five volts (+5 V).
•Logic low:
–Represented by the absence of a voltage
potential.
–Represented with zero volts (0 V).

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Truth Tables
•In Boolean algebra,
a table contains the
digital input and
output points.
•This table is called
a truth table.

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Gate Symbols
•For every combinational and sequential
logic device.
•Used to create logic gate circuits.

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NOT Gate
•Output is the
inverse of the input.
•Sometimes called
an inverter.
•Function is
simulated by the
electric circuit
displayed.

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AND Gate
•Two-input AND
logic gate symbol,
its Boolean
expression, and its
truth table.

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OR Gate
•Two-input OR logic
gate symbol, its
Boolean
expression, and its
truth table.

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NAND Gate
•Two-input NAND
logic gate symbol,
Boolean
expression, and its
truth table.

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NOR Gate
•A two-input NOR
logic gate symbol,
its Boolean
expression, and its
truth table.

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XOR (exclusive OR) Gate
•XOR logic gate
symbol, its Boolean
expression, and its
truth table.

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XNOR (exclusive NOR) Gate
•XNOR logic gate symbol, its Boolean
expression, and its truth table.

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Simplifying Boolean
Expressions
•To convert a truth table to a PLC
ladder logic diagram:
–Find its simplified Boolean expression.
–Use the gate logic to PLC ladder diagram
conversion routine to create the PLC
ladder logic diagram.

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Simplifying Boolean
Expressions (Cont.)
•Three methods used to simplify
Boolean expressions:
–Karnaugh maps.
–Quine-McCluskey routine.
–Electronic simulation software.

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Karnaugh Maps (K-Map)
•Graphical representations of truth
tables.
–Use columns and rows to represent each
term in a truth table.
–For an n-variable input truth table, there
are 2
n
boxes in a Karnaugh map.
–A box for every line in the truth table.

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Karnaugh Maps (K-Map)
(Cont.)

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Using K-Maps
•Use the following steps to simplify the
Boolean expressions using K-Maps:
1.Select an appropriate K-Map that has the
correct number of input boxes, such as
two-input and three-input. As stated, for an
n-variable input truth table, there will be 2
n

boxes. Therefore, for a two-variable (A and
B) input table, there will be 2
2
boxes, or 4
boxes.
2.Plot only the terms in which Y = 1.

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Using K-Maps (Cont.)
3. Follow the rules below for grouping the 1s in the
K-Map that lead to simplifying the expression.

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Using K-Maps (Cont.)
•Each group must contain an even number of
binary 1s.

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Using K-Maps (Cont.)
•Every 1 in adjacent cells must be included in a
group.
•The same 1 can be used in two or more
overlapping groups. Each group should be as large
as possible.

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Using K-Maps (Cont.)
•Map can be considered closed, so that end boxes
are grouped adjacently (top and bottom, or left and
right).

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Using K-Maps (Cont.)
•How groups wrap around the K-Map.

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Using K-Maps (Cont.)
4. Write the Boolean expressions for each group, and
then simplify the expression.

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Using K-Maps (Cont.)
5. Sum common variables from each group to create
simplified sum of product (SOP) Boolean
expression.

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Quine-McCluskey Routine
•For more than five input variables, a
better method for simplifying Boolean
expressions.
•Complicated method that uses the
Boolean algebraic simplification rules
to find the simplified Boolean
expression.
•Might be used in an advanced course.

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Electronic Simulation
Software
•Easiest method to find simplified
Boolean expression:
–Enter input and output data.
–Solves and simplifies the expression.
–NI Multisim is an example of this software.

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NI Multisim Software
•Open NI Multisim program.
•From the Instruments toolbar, click
the Logic Converter icon.
•Click a space in the work area to place
the converter.
•Double-click the Logic Converter
image to open the Logic Converter
dialog box.

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Creating PLC Ladder Logic
Diagrams from Logic Gate Circuits
•Convert each gate to its equivalent
ladder logic diagram.

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Creating PLC Ladder Logic Diagrams
from Boolean Expressions
•Some manufacturers use Boolean
expressions to program PLCs.
Example
•Create the PLC ladder logic diagram
for the following Boolean expression.
Y = A′ + B + CD + EB

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Creating PLC Ladder Logic Diagrams
from Boolean Expressions (Cont.)
•To create the diagram, each rung or
each portion of a rung is created by
replacing the Boolean letter with the
inputs that match.

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PLC Ladder Logic Diagrams
from Boolean Expressions

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Creating Logic Gate Circuits from
PLC Ladder Logic Diagrams
•Converting to logic gate circuits:
–Find Boolean expression that represents
ladder logic diagram.
–Draw the logic gate circuit using the
Boolean expression similar.
–Use logic converter instrument in NI
Multisim program to find truth tables and
Boolean expressions.

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Creating Logic Gate Circuits from
PLC Ladder Logic Diagrams (Cont.)
•Create the logic gate circuit for the PLC
ladder logic diagram displayed.

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Creating Logic Gate Circuits from
PLC Ladder Logic Diagrams (Cont.)
•Turn the PLC ladder logic diagram into a
Boolean expression as shown in the ladder
diagram.