3.6.4.1 Logic Gates.

Question 1

Define a logic gate.

Answer 1

Devices which apply logical operations to one or more Boolean inputs in order to produce a single output.

Question 2

What name is given to a collection of connected logic gates?

Answer 2

Logic circuit.

Question 3

What is the purpose of a logic circuit?

Answer 3

To perform more complex operations like binary addition.

Question 4

How many outputs do logic gates have?

Answer 4

One.

Question 5

Define the purpose of a truth table.

Answer 5

To show every possible combination of inputs and the corresponding output for a logic gate or logical circuit.

Question 6

How are the inputs on a truth table labelled?

Answer 6

Alphabetically, starting with A with the output usually labelled as Q.

Question 7

Outline a NOT gate.

Answer 7

One input and one output.
Output is the opposite of the input.
Truth table has two columns, labelled A and Q.

Question 8

Outline the Boolean algebra equation for NOT.

Answer 8

_

A = A

Question 9

Outline an AND gate.

Answer 9

Two inputs, out output.
Output is the product of the inputs.
Only outputs true (1) when or one, or both, inputs are true, otherwise it outputs false.

Question 10

Outline the Boolean equation for AND.

Answer 10

Q = A x B

Question 11

Outline an OR gate.

Answer 11

Two inputs, out output.
Input + input = output.
Only outputs false (0) when both inputs are false, otherwise it outputs true.

Question 12

Outline the Boolean equation for OR.

Answer 12

Q = A + B

Question 13

Outline a XOR gate.

Answer 13

Aka exclusively or.
Outputs true when strictly one of its inputs are true.
Outputs false when both inputs are true.
Used in the vernam cipher.

Question 14

Outline the Boolean equation for XOR.

Answer 14

Q = A ⊕ B

Question 15

Outline a NAND gate.

Answer 15

Aka not and.
Combination of the NOT and AND gates.
Only outputs false (1) when or one, or both, inputs are false, otherwise it outputs
true.

Question 16

Outline the Boolean equation for NAND.

Answer 16

____

Q = A × B

Question 17

Outline a NOR gate.

Answer 17

Aka not or.
Combination of NOT and OR.
Both inputs must be false to elicit a positive output.

Question 18

outline the Boolean equation for NOR.

Answer 18

____

Q = A + B

Question 19

Outline a full adder.

Answer 19

Three inputs and two outputs, enabling it to input two Boolean values and a carry bit from the previous, less significant operation.

Question 20

In a full adder, outline what A, B and Cin stand for.

Answer 20

Carry in.

Question 21

In a full adder, outline what S stands for.

Answer 21

Sum.

Question 22

In a full adder, outline what Cout stands for.

Answer 22

Carry out.

Question 23

Define an Edge-triggered D-type flip-flop.

Answer 23

A logic circuit which can be used as a memory unit for storing the value of a single bit.

Question 24

How many inputs does an Edge-triggered D-type flip-flop have?

Answer 24

Two, one for data and one for clock signal.

Question 25

How many outputs does an Edge-triggered D-type flip-flop have?

Answer 25

One, which always holds the value of a stored bit.

Question 26

How is clock signal generated in an How many inputs does an Edge-triggered D-type flip-flop?

Answer 26

Generated by the computer and alternates between 0 and 1 at a set frequency.
The value of the stored bit is set to the value of the data input with each change
of the clock signal

Question 27

What other purpose does clock signal have?

Answer 27

The clock signal can also be used to synchronise numerous flip-flops when they form part
of a larger system such as a shift register.