DESCRIPTION OF ALTERNATIVE LOGIC GATE

WEEK: 6

CLASS: SS 2

SUBJECT: COMPUTER SCIENCE

TOPIC: LOGIC CIRCUIT (II)

CONTENT: Alternative logic gates:

• Description of alternative logic gate
• Types of alternative logic gates NAND, NOR
• Symbols of each logic gate
• Input/output signals for: NAND, NOR

Sub-Topic 1: DESCRIPTION OF ALTERNATIVE LOGIC GATE

Although OR, AND, and NOT gate combinations are good enough for the construction of any logic operation, there are alternative methods that could be used. The alternative logic gates are NAND and the NOR logic gates. Alternative methods using NAND and NOR gates are more economical and therefore more widely used.

All logic operations can be performed with combinations of just NANDs or NORs, but a combination of NORs with NANDs is usually preferable to one provided just by NANDs.

TYPES OF ALTERNATIVE LOGIC GATES

Basic digital logic gates perform logical operations using the following standard single logic gates:

1. Logic “NAND” gate
2. Logic “NOR” gate

THE LOGIC “NAND” GATE

The logic NAND gate is a combination of the digital logic AND gate with that of an inverter, or NOT gate connected together in series. The NAND (Not AND) gate has an output that is normally at logic level “1” and only goes “LOW” to logic level “0” when ALL of its inputs are at logic level “1”. It is the reverse of the ‘complementary’ form of AND gate.

The Boolean expression for a logic NAND gate, is denoted by a single dot or full stop symbol (.) with a line or overline (-) over the expression to signify the NOT or logical negation of the NAND gate, giving us the Boolean expression of A.B=Q.

Sub-Topic 2 : SYMBOLS OF NAND LOGIC GATE

Inputs                                               Output

Shows invasion

Inputs                                                                                                      Output

THE LOGIC “NOR” GATE

The logic NOR or inclusive NOR gate is a combination of the digital logic OR gate with that of an inverter or NOT gate connected together in series. The NOT (Not OR) gate has an output that is normally at logic “1” and only goes “LOW” to level of “0” when ANY of its inputs are at logic level “1”. The logic NOR gate is the reverse or “complementary” from of the OR gate.

The Boolean expression for a logic NOR gate is denoted by a plus sign (+) with a line or overline (-) over the expression to signify the NOT or logical negation of NOT gate giving us the Boolean expression of A+B=Q

Symbol of NOR Logic gate

A.

INPUTS                                                                                                              OUTPUT

Shows inversion

B.

INPUTS                                                                                              OUTPUT

EVALUATION

1. Mention the two main types of alternative logic
2. Show the arithmetic operations of NAND and NOR

READING ASSIGNMENT

Read construction of truth table for NAND and NOR gate, also uses of logic gates and summarize in not more than one page.

WEEKEND ASSIGNMENT

Objectives

1. The output of gate negates the output of OR gate.
2.  gate has a ‘D’ shape.
3. The combination of and                          gates is more economical for constructing comparators.
4. The shape of gate is triangle.
5. The symbols that negate the output of a gate are and                    

ESSAY QUESTIONS

1. Construct a truth table of NAND for two
2. List three uses of logic

QUIZ QUESTIONS

 

Results

#1. When both inputs of an XOR gate are the same, what is the output?

It cannot be determined

It depends on previous inputs

0

1

#2. Which logic gate is also known as an “Inverse OR” gate?

XOR

NOR

XNOR

NAND

#3. How many inputs does a XNOR gate have?

2

4

3

1

#4. Which logic gate is equivalent to the logical statement “Either A or B, but not both”?

XOR

OR

NOR

AND

#5. What is the symbol for an XOR gate?

&

!

^

||

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