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This an informative practical manual on digital electronics...

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ELECTRICAL/ELECTRONICS DEPARTMENT

PRACTICAL MANUAL COURSE TITLE: DIGITAL ELECTRONICS COURSE CODE: EEE 408 NAME MATRIC NO LEVEL SEMESTER SESSION CONTENT: LOGIC GATES 7 SEGMENT DISPLAY

1

PRACTICAL 1 Title: Logic Gates Objectives: At the end of the practical; student should be able to:

  

Write out the truth table of the gates AND, OR, NOT, NAND, NOR and XOR. The behavior/characteristics of the various gates. How to use gates as blocking gates.

Equipment Required:  

Digital trainer Jumper wires

Discussion: In this experiment we will use with the common gates AND, OR, NOT, NAND, NOR and XOR. We will identify the truth table of each gate. We will use two gates with two inputs to create one gate with three inputs. We will also examine how to block digital signals with gates.

Logic components The logic components are the building blocks of the digital system and constitute their basis. Digital systems are systems required to perform an operation or sequence of operations according to signals delivered at their inputs. A nearly unlimited number of digital systems are in existence, starting with a switch lighting a light up to a sophisticated computer executing an infinite number of operations. These components are called logic components because their operations resemble a kind of thinking and decision making. The logic component is looked upon as a "Black Box" having a number of inputs (one, two, or more) and a single output.

A B

Y

C

Figure 1-1 Logic Component Representation 2

Logic components with several outputs are also found, but they may be regarded as a number of separate components constructed with the same inputs and different outputs.

A B

X

C

A B

X

A

Y

B

C

Z

C

A B

Y

Z

C

Figure 1-2 3-Outputs Component In every logic component, the signal at the output is a function of the signals at the inputs. We are dealing with logic systems; hence, the inputs and the outputs can acquire one of two values ('0' or '1'). A digital system is not necessarily only a computer system where the digits represent different voltage levels. The digital system may be a mechanical system, an electro-mechanical system, or an electrical one. If the inputs are switches, then a closed switch may be defined as logic '1', and an open switch may be defined as logic '0'. In the realm of systems in which we deal, logic '1' state shall be defined as one voltage level (usually +5V), and the logic '0' state may be defined as another voltage level (usually 0V). There are several possibilities of implementing this type of device "electronically". We shall not dwell on the internal (circuit wise) structure of these systems. A computer system is made of thousands of logic devices. There are scores of different types of logic components. The basis for all of them is a small group of devices known as logic gates. A logic gate is the primary basic logic component from which all other logic components are assembled. They are the building blocks of all the digital systems. When explaining the diverse logic gates, it is customary to introduce simple electric circuits demonstrating the operation of a logic gate, which by themselves form a kind of a similar gate. In these circuits, the inputs are switches and the outputs are light bulbs. For the latter, the logic '1' state is defined by a turned on light and the logic '0' is defined by an extinguished light. In the following presentation in this chapter, we shall discuss two-input gates, though, as stated earlier, various multi-inputs gates are also in use.

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PRACTICAL 2 "AND" gate A

A B

B

Y

Y

Figure 1-3

Figure 1-4

The gate is presented by the symbol in figure 1-3, and its operation can be understood from the circuit in figure 1-4. The light shall be turned ON (i.e., will be in the logic '1' state) when both switches A "AND" B are closed (be in the logic '1' state). A similar situation holds for the gate in figure 1-3. Y will be at +5V if +5V occurs at both A "AND" B. These switches are also referred to as N.O. (Normally Open) switches. They are OPEN under their regular (normal) condition. Upon activation, they close and constitute a short circuit. In order to describe the relation between Y and the A and B inputs, a "truth table" depicting the possible states is used. A truth table shows the value at the output for every possible combination of input states. Thus, the table of the AND gate possible states are: A

B

0

0

0

1

1

0

1

1

Y

The AND gate can be described by the equation:

Y = A AND B It has been already mentioned that the gates can posses a number of inputs larger than 2. Thus, Y of a 4-inputs AND gate shall be logic '1' only if all four inputs shall be at logic '1' state.

4

PRACTICAL 3 "OR" gate A B A B

Y

Y

Figure 1-5

Figure 1-6

Figure 1-5 presents the symbol of the gate and figure 1-6 demonstrates its operation. The lamp is turned ON (light) when switch A "OR" switch B is closed. It will also be turned ON when both switches are closed. Hence, logic '1' state shall be constructed at Y when A "OR" B is in the logic '1' state. In conclusion, the truth table of an "OR" gate is as shown: A 0 0 1 1 The OR gate can be described by the equation:

Y = A OR B

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B 0 1 0 1

Y

PRACTICAL 4 "NOT" gate – Inverting gate A

A

Y

Y

Figure 1-7

Figure 1-8

Figure 1-7 presents the symbol of the gate and figure 1-8 demonstrates its operation. Another type of switch encountered is the N.C. (Normally closed) switch. Its regular (normal) state is the CLOSED state. Upon activation, it opens and constitutes an open circuit. Switch A is an N.C. switch. When it is not activated (it constitutes a short circuit), that is, the lamp is lit (turned ON) but when A is activated (it is an open circuit) the light is extinguished. The logic state of Y is always the reverse of the logic state of A. Note that a small circle appended to the output of the gate symbolizes an inverting gate. The truth table of the "NOT" gate has two states: A

Y

0 1

This situation can be described by the equation:

Y = NOT A The above three gates are basic gates. We will describe three additional gates, which are used a lot. Although these additional gates can be built based on the first three gates, they are also considered basic gates.

6

PRACTICAL 5 "NAND" gate This gate can be constructed as an AND gate with an appended inverter at its output.

A

C

Y

B

Figure 1-9 A NAND gate The truth table of this system is: Inputs

Outputs

A

B

C

0 1

0 0

0 0

1

0

0

C = A AND B

1

1

1

Y = NOT C

Y

The symbol of the "NAND" gate is as follows:

A

Y

B

Figure 1-10 The truth table of the "NAND" gate is as follows: A 0 0 1 1 Its equation is:

Y = NOT (A AND B) 7

B Y 0 1 0 1

PRACTICAL 6 "NOR" gate This gate, too, can be constructed by two gates – an "OR" gate and an appended inverter at its output.

C

A

Y

B

Figure 1-11 The truth table of this system is: Inputs

Outputs

A

B

C

0

0

0

0

1

1

1

0

1

C = A OR B

1

1

1

Y = NOT C

Y

The symbol of the "NOR" gate is as follows:

A

Y

B Figure 1-12 Its truth table is as presented below:

Its equations is:

Y = NOT (A OR B) 8

A

B

0

0

0

1

1

0

1

1

Y

PRACTICAL 7 "XOR" (eXclusive OR) gate This gate is symbolized as follows:

A B

Y

Figure 1-13 We can understand its operation by studying its truth table, which is: A

B

0

0

0

1

1

0

1

1

Y equals logic '1' only when A "OR" B equals '1', but not when they are both equal '1' (as in the "OR" gate). This is the reason for its name, the "eXclusive OR". Its equations is:

Y = A XOR B

Questions and exercises: 1)

How would a 2-inputs NAND gate behave when its two inputs are short circuited (to one another)?

2)

How would a 2-inputs NOR gate behave when its two inputs are short circuited (to one another)?

3)

How would a 2-input XOR gate behave when its two inputs are short circuited (to one another)?

4)

How do you think the following gates (a, b, and c) would behave? Write down their tables of state.

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Procedure: Identifying tables of the states of the gates: Step 1:

Connect the TPS-3351 to the power supply and connect the power supply to the Mains.

Step 2:

Turn ON the TPS-3351.

Step 3:

Connect both inputs of the AND gate to two switches S0, S1 on TPS-3351, and connect the output of the gate to the LED L0. AND

A B C

S0 S1

Y L0

Step 4:

Write the truth table for the circuit in your notebook.

Step 5:

Change the state of the switches according to the truth table and record the result of every state in the table. S1 A 0 0 1 1

Step 6:

AND S0 B 0 1 0 1

L0 Y

Record your conclusion. Does the truth table that you have obtained conform to the theory presented in the theoretical background material?

Step 7: Repeat steps 3 to 6 for the following gates and complete the truth table for each gate. OR A B

NOT A

Y

Y

A B

NAND A B

Y

Y

A 0 0 1 1

B 0 1 0 1

Y

A 0 1

Y

10

A 0 0 1 1

B 0 1 0 1

Y

NOR A B

XOR A B

Y

A 0 0 1 1

B 0 1 0 1

Y

Y

A 0 0 1 1

B 0 1 0 1

Y

Expansion of gates: Step 8:

Connect inputs A, B, and C of the following circuit to three switches on TPS-3351 trainer. A 0 0 0 0 1 1 1 1

B 0 0 1 1 0 0 1 1

C Y 0 1 0 1 0 1 0 1

A B

C

Y

= A B Y

C

Step 9:

Connect the L0 LED to the output of the circuit (Y).

Step 10:

Complete the truth table for the circuit in your notebook.

Step 11:

Change the state of the switches according to the truth table. Record the result for each state in the table.

Step 12:

As a conclusion, write down whether the truth table that you have obtained conforms with the theory presented in the theoretical background material.

Step 13: tables.

Repeat steps 8 to 12 for the following gate combinations and complete their truth

A B

C

Y

= A B C

A B Y

C

11

Y

A 0 0 0 0 1 1 1 1

B 0 0 1 1 0 0 1 1

C Y 0 1 0 1 0 1 0 1

A 0 0 0 0 1 1 1 1

B 0 0 1 1 0 0 1 1

C Y 0 1 0 1 0 1 0 1

Blocking gates: Step 14:

Connect the A and B inputs of an AND gate to two switches on the TPS-3351. AND S0 S1

A B C

Y L0

Step 15:

Connect the output of the gate to the LED input.

Step 16:

Assume that the B input serves as a control input. Write two truth tables in your notebook – one for B=0 and one for B=1. B=1 A Y 0 1

B=0 A Y 0 1

Step 17:

Change the state of the switches according to the truth tables, and record the results in the tables.

Step 18:

Write your conclusions regarding the function of the gate in each of the states under the control input conditions.

Step 19:

Repeat steps 14 to 18 for the following gates and complete their truth tables. OR B=1 A 0 1

Y

NAND B=1

NOR B=1

XOR B=1

A

A

A

0 1

Y

0 1

12

Y

0 1

Y

B=0 A

B=0 Y

A

Y

B=0 A

Y

B=0 A

0

0

0

0

1

1

1

1

Y

Note the special behavior of the XOR gate in this case.

Summary questions: 1)

Collect all your experiments and tables results. Write in the table above each experiment result, the experiment name and its drawing.

2) Compare the experiment results with the theory. ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… Precaution: ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… Conclusion: ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ………………………………………………………………………………………………………

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