5.5 Logic circuits

Question 1

Digital computers and Central Processing Units (CPUs) must be able to carry out arithmetic processes and logical combinations

Answer 1

These are done in an Arithmetic Logic Unit (ALU), the heart of each CPU.

Question 2

a system that performs calculations or makes decisions is a logic system

Answer 2

A computer is a logic system that contains memory.

Question 3

Computers and logic systems can handle either

Answer 3

analogue or digital information

Question 4

The ALU needs its inputs in digital form such as:

Answer 4

Logic 1 (also known as logic ’True’)
Logic 0 (also known as logic ‘False’)

Question 5

on/off or true/false in logic is defined as

Answer 5

1 and 0

Question 6

1/0 in a logic system are known as binary digits abbreviated as

Answer 6

bits

Question 7

a combination of bits is called a

Answer 7

word

Question 8

two binary states can be represented in a variety of ways including

Answer 8

most common high and low voltage levels

also two different frequencies and two-phase angles

Question 9

Positive logic is more commonly used where H means 1 and L means 0

Answer 9

.

Question 10

Logic gate operations can be expressed using

Answer 10

Boolean Algebra
X = A or B and C

Question 11

Truth tables specifically describe the

Answer 11

function of a logic gate

Question 12

Logic gates have only three basic functions

Answer 12

• AND Function
• OR Function
• NOT (Inversion) Function

Question 13

By combining the three basic functions
AND
OR
NOT
which others can be made

Answer 13

NAND (not AND)
NOR (not OR)
EXCLUSIVE OR
EXCLUSIVE NOR

Question 14

There are several different ways to define logic functions

Answer 14

Circuit schematic/equivalent circuit
Logic symbol
Truth table
Signal diagram

Question 15

NOT (inverter) Gate

Answer 15

an inverter has only one input, and its output is always the opposite of the input

    - A =  A A - logic 0: produces 1 A - logic 1: produces 0

Question 16

AND Gate

Answer 16

both relays need to be energised (logic 1 inputs) to give a logic 1 output.

Question 17

AND and OR gates are usually what

Answer 17

Transistors

Question 18

AND Gate

Answer 18

X = A . B

the . is read as AND

Question 19

NAND Gate

Answer 19

X = NOT A and B

NOT bubble at the output inverts the output of the AND gate making it a NAND.

Question 20

NAND Gate truth table

Answer 20

The output is a logic 1 when any input is 0.

The output is a logic 0 only when all inputs are 1.

Question 21

OR Gate

Answer 21

either relay needs to be energised (logic 1 inputs) to give a logic 1 output. These relays will be transistors.

Question 22

the + sign means what in logic circuits

Answer 22

OR

Question 23

NOR Gate

Answer 23

the output is inverted (NOT OR)

A NOR gate (Not OR) has two or more inputs and one output

Question 24

NOR Gate truth table

Answer 24

X = NOT A or B

The output is only a logic 1 if all inputs are at logic 0

Question 25

what’s the difference between a NAND and NOR gate

Answer 25

The NOR gate is a gate that produces a logic 1 output only when all of its inputs are logic 0.

The NAND gate is a gate that produces a logic 1 output only when one or more of its inputs are logic 0

Question 26

XOR (exclusive OR)

Answer 26

Some circuits are required to perform only when either input is 1, not both. They will only output a 1 when the inputs are not identical. These are called XOR gates

AKA EXOR

Question 27

⊕ this symbol is called the

Answer 27

XOR operator

Question 28

XNOR (exclusive NOR)

Answer 28

Has two inputs and one output

outputs are the inverse of the XOR gate

Question 29

XNOR truth table

Answer 29

The output is at logic 0 when input A is at logic 1 and input B is at logic 0, or vice versa.

The output is at logic 1 when both inputs A and B are at logic 1, or both inputs are at logic 0.

Question 30

Inverters

Answer 30

Inverters have one input and one output. The output will produce the complement (opposite) of the input

Question 31

when are inverters used

Answer 31

they are used when the complement of a particular signal is required within a logic circuit

Question 32

what does a small circle at the input indicate

Answer 32

indicates that a logic “zero” will now activate the element at that particular input

Question 33

INHIBIT Gate
Occasionally if required to “hold” one input to an “AND” gate at a particular logic level to disable the entire gate

Answer 33

The inhibit is logic 1 to disable the gate. The inhibit at logic 0 enables the gate

Question 34

what is a full adder derived from

Answer 34

two half adders and an OR gate