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page 240

A = B*C*D + B*C*E + B*C*F + B*C -----

Canonical/Disjunctive normal form

20.2 DISCRETE LOGIC

• The basic types of discrete logic problems are,

1. Conditional - if a set of conditions can directly cause an action.

e.g. if the temperature is too high and there is an ingot in only one bay, turn on fan 1.

B1 = bay 1 ingot detect

B2 = bay 2 ingot detect

F1 = fan 1

T1 = temperature overheat sensor

in Boolean F1 = T1 * (B1 EOR B2)

or F1 = T1 * (( B1 * B2 ) + ( B2 * B1 )) or F1 = T1 * B1 * B2 + T1 * B2 * B1

 

In ladder logic,

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Note: recall the rules of Boolean algebra

 

 

 

 

 

 

 

 

 

 

 

B1

 

B2

 

 

T1

 

 

 

 

 

 

 

 

 

 

 

F1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

B1

 

B2

 

 

 

 

* the result for conditional logic is a single

 

 

 

 

 

 

 

 

 

step in the ladder

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2. Sequential - when the system is in a certain state, the controls will do certain things. e.g., when an oven is on, the PLC adjusts temperature.When it is off doors

can be opened/closed.

page 241

ladder logic for temperature control only used when oven on

ladder logic for doors only used when oven off

• Try the example shown below,

page 242

A

B

C

X B

A

C

Develop the Boolean equation for the circuit above. Simplify the expression, and then convert this to ladder logic, a digital circuit, and a computer program.

X = ( ABC) + B B( A + C)

X = ( A + B + C + B) B( AC)

X = ( ABAC + BBAC + CBAC + BBAC)

X = ( BAC + BAC + 0 + BAC)

X = BAC

• Consider the more complicated design that follows,

page 243

A = B * ( C * ( D + E + C ) + F * C ) ---- Original equation

* remember operations in brackets are done first, like normal algebra

D

E

C

A

F

B

 

 

 

 

 

 

 

The gates can be purchased for

 

 

 

 

 

 

 

about $0.25 each in bulk.

 

 

 

 

 

 

 

Inputs and outputs are

D

 

C

 

B

A

typically 5V

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

E

An inexpensive PLC is worth at least a few hundred dollars

C

F C

Consider the cost trade-off!

page 244

A = B * C * ( D + E + F ) ---- Simplified Form

D

E

F A C

B

D C B A

E

F

A = (B * C * D ) + (B * C * E ) + (B * C * F ) ---- Disjunctive Normal Form

B

 

 

 

C

 

 

 

D

 

 

 

 

 

 

A

E

 

 

 

F

 

 

 

B

C

D

A

 

 

 

B

C

E

 

B

C

F

 

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