Chapter 1: Logic gates

Question 1

What is an analogue signal?

Answer 1

-It gives an an analogy-copies the behaviour of a physical quantity. A temperature sensor outputs a voltage which copies the behaviour.
-It can have any value between the minimum and maximum of the power supply

Question 2

What is a digital signal?

Answer 2

-It carries information in the form of a number. Electronic systems usually employ the binary number system
-It has only 2 possible values: 1 or 0.

Question 3

When a digital signal is at minimum voltage it is referred to as what logic signal?

Answer 3

Logic 0

Question 4

When a digital signal is at maximum voltage it is referred to as what logic signal?

Answer 4

Logic 1

Question 5

What are logic gates?

Answer 5

Logic gates are the basic building block of all digital electronic systems. They are the decision making units in electronic systems and there are different types for different
situations.

Question 6

State the truth table for a NOT gate

Answer 6

A Q
0 1
1 0

Question 7

State the Boolean expression for the NOT gate

Answer 7

Q=A

Question 8

State the truth table for an AND gate

Answer 8

B A Q
0 0 0
0 1 0
1 0 0
1 1 1

Question 9

State the Boolean expression for the AND gate

Answer 9

Q=A.B

Question 10

State the truth table for an OR gate

Answer 10

B A Q
0 0 0
0 1 1
1 0 1
1 1 1

Question 11

State the Boolean expression for the OR gate

Answer 11

Q=A + B

Question 12

State the truth table for NAND gate

Answer 12

B A Q
0 0 1
0 1 1
1 0 1
1 1 0

Question 13

State the Boolean expression for the NAND gate

Answer 13

Q=Ā.

Question 14

State the truth table for a NOR gate

Answer 14

B A Q
0 0 1
0 1 0
1 0 0
1 1 0

Question 15

State the Boolean expression for the NOR gate

Question 16

State the truth table for a two input XOR gate

Answer 16

B A Q
0 0 0
0 1 1
1 0 1
1 1 0

Question 17

State the truth table for a 3 input XOR gate

Answer 17

C B A Q
0 0 0 0
0 0 1 1
0 1 0 1
0 1 1 0
1 0 0 1
1 0 1 0
1 1 0 0
1 1 1 1
The output Q is logic 1 when either A OR B OR C is logic 1, or A AND B AND C are all logic 1 but NOT
when any two inputs are logic 1.

Question 18

State the Boolean expression for the XOR gate

Answer 18

Q=A⊕B

Question 19

State the truth table for a 2 input XNOR gate

Answer 19

Inverted form of the XOR gate
B A Q
0 0 1
0 1 0
1 0 0
1 1 1

Question 20

State the truth table for a 3 input XNOR gate

Answer 20

Exact opposite of the 3 input XOR gate
C B A Q
0 0 0 1
0 0 1 0
0 1 0 0
0 1 1 1
1 0 0 0
1 0 1 1
1 1 0 1
1 1 1 0

Question 21

State the Boolean expression for the XNOR gate

Question 22

What counts as logic 1? What counts as logic 0?

Answer 22

70% of the power supply. 30% of the power supply. Anything in between is a faulty chip

Question 23

What is a sourcing current?

Answer 23

When a load is connected to a logic gate in such a way that the logic gate supplies current to the load, the logic gate is said to be sourcing current.

Question 24

What is a sinking current?

Answer 24

When a load is connected so that current flows from the power supply through the load to the logic gate, then the logic gate is said to be sinking current.

Question 26

A.1 =

Answer 26

A.1=A

Question 27

A.0=

Answer 27

A . 0 = 0

Question 28

A.A=

Answer 28

A . A = A

Question 29

A. Ā =

Answer 29

A . Ā= 0

Question 30

A+1=1

Answer 30

A + 1 = 1

Question 31

A+0=

Answer 31

A + 0 = A

Question 32

A+A=

Answer 32

A + A = A

Question 33

A + Ā=

Answer 33

A + Ā = 1

Question 34

What is de Morgan’s Theorem?

Answer 34

Break the line, change the sign or vise versa

Question 35

Each square in a Karnaugh map corresponds to what?

Answer 35

A cell in a truth table

Question 36

How would you complete a Karnaugh map using a truth table?

Answer 36

First find all the ‘cells’ where the output is high. Place ones into the corresponding Karnaugh map squares. Then fill the remaining squares with 0.

Question 37

How would you complete a simplified Boolean expression using a Karnaugh map?

Answer 37

Identify groups of 2,4,8 which contain logic 1. Then finds the terms to each group and then combine them. This gives a simplified expression of Boolean.

Question 38

How would you create an AND using NAND

Answer 38

NAND gate followed by a NAND inverter with its two inputs connected together

Question 39

How would you create a NOT gate using NAND gates?

Answer 39

A NAND gate with its two inputs connected together

Question 40

How would you create an OR gate using NAND gates?

Answer 40

Two NAND gates, connected in parallel, with their outputs feeding into another NAND gate

Question 41

How would you create a NOR gate using NAND gates?

Answer 41

Two NAND gates, connected in parallel, with their outputs feeding into another NAND gate, which then gets inverted

Question 42

How can some systems of just NAND gates been simplified?

Answer 42

By eliminating redundant gates?

Question 43

What is a Multiplexer?

Answer 43

A device which is able to transfer a signal from any one of a number of inputs to an output, using a combination of digital signals applied to its input select pins

Question 44

What are the S inputs connected to in a Multiplexer?

Answer 44

C,B,A

Question 45

One disadvantage of a multiplexer?

Answer 45

Much more expensive than logic gates

Question 46

One advantage of a multiplexer?

Answer 46

They can be reprogrammed to produce the output of any logic gate.

Question 47

On a multiplexer, what does input 0000 select?

Answer 47

D0

Question 48

On a multiplexer, what does input 1111 select?

Answer 48

D7

Question 49

On a multiplexer, D inputs are connected to what?

Answer 49

Logic 1 or logic 0

Question 50

What is the region between Logic 1 and logic 0?

Answer 50

Open band

Question 51

If a circuit is in the region between logic 1 and logic 0, how does this effect the overall circuit?

Answer 51

It results in highly unpredictable behaviour.