Programming Logic Gate Functions in PLCs

Chapter

7Programming Logic

Gate Functions in PLCs

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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.

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.

Combinational Logic Gates

• Do not require clock pulses to operate.• Outputs depend only on their inputs.• Outputs are generated instantaneously.• Simply called logic gates.

Logic Gates

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

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.

Sequential Logic Circuit

Boolean Expressions

• Every gate logic function has its own equation called a Boolean expression.

• Boolean algebra:– Two states are true and false.

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.

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).

Truth Tables

• In Boolean algebra, a table contains the digital input and output points.

• This table is called a truth table.

Gate Symbols

• For every combinational and sequential logic device.

• Used to create logic gate circuits.

NOT Gate

• Output is the inverse of the input.

• Sometimes called an inverter.

• Function is simulated by the electric circuit displayed.

AND Gate

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

OR Gate

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

NAND Gate

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

NOR Gate

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

XOR (exclusive OR) Gate

• XOR logic gate symbol, its Boolean expression, and its truth table.

XNOR (exclusive NOR) Gate

• XNOR logic gate symbol, its Boolean expression, and its truth table.

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.

Simplifying Boolean Expressions (Cont.)

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

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 2n boxes in a Karnaugh map. – A box for every line in the truth table.

Karnaugh Maps (K-Map) (Cont.)

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 2n boxes. Therefore, for a two-variable (A and B) input table, there will be 22 boxes, or 4 boxes.

2. Plot only the terms in which Y = 1.

Using K-Maps (Cont.)

3. Follow the rules below for grouping the 1s in the K-Map that lead to simplifying the expression.

• Each group must contain an even number of binary 1s.

• 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.

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

• How groups wrap around the K-Map.

4. Write the Boolean expressions for each group, and then simplify the expression.

5. Sum common variables from each group to create simplified sum of product (SOP) Boolean expression.

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.

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.

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.

Creating PLC Ladder Logic Diagrams from Logic Gate Circuits

• Convert each gate to its equivalent ladder logic diagram.

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

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.

PLC Ladder Logic Diagrams from Boolean Expressions

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.

Creating Logic Gate Circuits from PLC Ladder Logic Diagrams (Cont.)

• Create the logic gate circuit for the PLC ladder logic diagram displayed.

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.

Programming Logic Gate Functions in PLCs

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