Version 1.0, August 31, 2001, Copyright, Hugh Jack 1993-2001

20.2 DISCRETE LOGIC

· The basic types of discrete logic problems are,

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,

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.

· Try the example shown below,

· Consider the more complicated design that follows,

20.2.1 Boolean Algebra for Circuit and Ladder Logic Design