Page 83 - Programmable Logic Controllers, Fifth Edition

NOT truth table
When the input pushbutton is not actuated, the output
A NOT A normally closed pushbutton in series with a lamp.
A A (NOT A) lamp is ON. When the input pushbutton is actuated,
Input Output 0 1
1 0 the output lamp switches OFF.
The NOT function is most often used in conjunction
Figure 4-9 NOT function.
with the AND or the OR gate. Figure 4-11 shows the NOT
function connected to one input of an AND gate for a low-
The NOT Function pressure indicator circuit. If the power is on (1) and the
The symbol drawn in Figure 4-9 is that of a NOT function. pressure switch is not closed (0), the warning light will
Unlike the AND and OR functions, the NOT function can be on (1).
have only one input. The NOT output is 1 if the input is 0. The NOT symbol placed at the output of an AND gate
The output is 0 if the input is 1. The result of the NOT opera- would invert the normal output result. An AND gate with
tion is always the inverse of the input, and the NOT function an inverted output is called a NAND gate. The NAND
is, therefore, called an inverter. The NOT function is often gate symbol and truth table are shown in Figure 4-12. The
depicted by using a bar across the top of the letter, indicat- NAND function is often used in integrated circuit logic
ing an inverted output. The small circle at the output of the arrays and can be used in programmable controllers to
inverter is referred to as a bubble and indicates that an inver- solve complex logic.
sion of the logical function has taken place. The same rule about inverting the normal output result
The logical NOT function can be performed on applies if a NOT symbol is placed at the output of the OR
a contact input simply by using a normally closed gate. The normal output is inverted, and the function is re-
instead of a normally open contact. Figure 4-10 shows ferred to as a NOR gate. The NOR gate symbol and truth
an example of the NOT function constructed using a table are shown in Figure 4-13.

Hardwired circuit

Truth table Logic representation
N.C. pushbutton
+ Pushbutton Light Light
Light Not pressed (0) On (1)
– Pressed (1) O (0)

Single input
Figure 4-10 NOT function constructed using a normally closed pushbutton.

(Power on) NAND truth table
A = 1 Low-pressure Inputs Output
indicator on (1) A A B Y
Inputs Y
B Output 0 0 1
B = 0 (1) 0 1 1
(Pressure Two-input 1 0 1
switch open) NAND gate 1 1 0
Truth table
Figure 4-12 NAND gate symbol and truth table.
Pressure Pressure
switch Power indicator
0 1 1
1 1 0
(Power on)
A = 1 Low-pressure NOR truth table
indicator o (0) Inputs Output
A A B Y
B = 1 (0) Inputs Y 0 0 1
B Output
(Pressure 0 1 0
switch closed) Two-input 1 0 0
NOR gate 1 1 0
Figure 4-11 NOT function is most often used in
conjunction with an AND gate. Figure 4-13 NOR gate symbol and truth table.

64 Chapter 4 Fundamentals of Logic

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