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page 247

This logic can also be implemented in an embedded controller

w = 1; s = 2; m = 3; a = 4 input m; input w; input s output a

loop:

if (in2 = 1) and (in1 = 0 or in3 = 1) then on low a; goto loop ‘alarm off

on:

high a; goto loop ‘alarm on

• This example was quite simple. To do more complicated problems we will need to review some basic theory first.

20.2.2 Boolean Forms

• Canonical, truth tables

20.3 SIMPLIFYING BOOLEAN EQUATIONS

20.3.1 Karnaugh Maps for Combinatorial Design

Karnaugh maps allow us to convert a truth table to a simplified Boolean expression without having to use Boolean Algebra.

Consider the example below,

page 248

We will add an alarm quit switch to the last example

therefore,

A, W, M, S as before

Q = Alarm Quiet ( 0 = quiet )

Step1 : Draw the truth table

S

M

W

Q

 

A

 

 

 

 

 

 

0

0

0

0

 

0

0

0

0

1

 

0

0

0

1

0

 

0

0

0

1

1

 

0

0

1

0

0

 

0

0

1

0

1

 

0

0

1

1

0

 

0

0

1

1

1

 

0

1

0

0

0

 

0

1

0

0

1

 

1

1

0

1

0

 

0

1

0

1

1

 

0

1

1

0

0

 

0

1

1

0

1

 

1

1

1

1

0

 

0

1

1

1

1

 

1

Step 2 : Divide the input variables up. I choose SQ and MW

Step 3 : Draw a Karnaugh map based on the input variables

 

MW (=00)

MW (=01)

MW (=11)

MW (=10)

SQ (=00)

 

 

 

 

SQ (=01)

 

 

 

 

SQ (=11)

1

 

1

1

SQ (=10)

 

 

 

 

Added for clarity

page 249

Step 4 : Look for patterns in the map

M is the common term

MW MW MW MW

SQ

SQ

SQ 1

1

1

SQ

W is the common term

In addition to the common terms, These both occur in row SQ, therefore, A = S*Q*(W) + S*Q*(M)

A = S*Q*(W + M)

M

 

S

 

Q

A

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

W

*This technique works well for complicated systems.

• Design ladder logic for a car that considers,

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