Experiment 4

Digital Electronics-II

Experiment No. 4

Experiment No. 4 1.

Title: Design and verify the logic circuit for full substractor using logic gates.

2.

Prior Concepts: Rules for binary subtraction, K-map, knowledge of half subtractor.

3.

New Concept : Proposition 1 : A half subtractor is a combinational circuit that subtracts two bits and produces their differences. The second output designed Cn for borrow. Proposition 2 : The full subtractor circuit performs subtraction of two one bit numbers with borrow from the previous stage. An (Minuend)

Cn (Borrow)

B (subtrahend)

Full subtractor

Cn-1 (Borrow from previous stage)

Dn (Difference)

4.

Learning Objectives : 4.1

Intellectual Skill :4.1.1 The truth table preparation for half subtractor. 4.1.2 The truth table preparation for full subtractor.

4.2

Motor Skills :4.2.1 To build the full subtractor circuit on breadboard. 4.2.2 To verify the practical results (truth table entries) with the help of LEDs, Multimeter, or on CRO.

5.

Apparatus : D.C. power supply (5V), connecting wires , breadboard , LED's, current limiting resistors ( e.g.270W ), write IC's required -……………., …………… , ………….., ………………, ………………, multimeter, CRO, etc

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Experiment No. 4

6.

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Digital Electronics-II

Diagram : 6.1

Draw required IC configuration.

6.2

Draw circuit diagram of half subtractor.

6.3

Draw block diagram of full subtractor using half subtractor.

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Digital Electronics-II

7.

Experiment No. 4

Procedure : 7.1

Prepare the truth- table for half subtractor.

A full subtractor circuit for performing multi bit subtraction where in a borrow from previous bit position may also be there. A full subtractor will have three inputs, An(minuend), B (subtrahend), and Cn-1 (borrow from previous stage) and two outputs Dn (Difference) and Cn (borrow). 7.2

Prepare truth table for full subtractor. Inputs

Outputs Borrow from

Minuend

Subtrahends

previous

Difference Borrow

Borrow Difference

Dn Cn

Cn Dn

stage

7.4

An

Bn

Cn-1

0

0

0

1

1

1

Prepare a K-map for Difference (Dn)

Expression for Difference

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Dn =

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Experiment No. 4

7.5

Digital Electronics-II

Prepare K-map for Borrow ( Cn )

Expression for Borrow

8.

Cn =

7.6

Draw the desired circuit diagram.

7.7

Build the designed full subtractor circuit on bread board.

7.8

Connect LEDs, multimeter or CRO at output terminals of full subtractor.

7.9

Switch ON power supply.

7.10

Verify truth table by observing status of LED or on Multimeter or on CRO for different inputs.

Conclusion : Comment on the output state for different combinations of inputs for LED to glow.

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Digital Electronics-II

9.

Experiment No. 4

Questions : Attempt three questions as directed by teacher. 1) State the expressions for difference and borrow of full subtractor and implement full subtractor using NAND gate. 2) Show how a full adder can be converted to a full subtractor with the addition of one inverter circuit. 3) Draw the circuit diagram to implement full subtractor with two half subtractors. 4) Refer the circuit diagram given in fig .1 and answer the following questions.

Cn

is in dark Fig. 1 a) b) c) d) 5)

State the status of Red and green LEDs when both photo transistors Q1 and Q2 are under radiation. State the status of Red and green LEDs when both photo transistors Q1 and Q2 are in dark. State the status of Red and green LEDs when photo transistors Q1 is in dark and Q2 is under radiation. State the status of Red and green LEDs when both photo transistors Q1 is under radiation and Q2 is in dark.

Redraw the circuit as given in fig. 1 with transistor Q3 is under illumination. Also write the answer of a), b), c), and d) as in question no.4. (Space for answer)

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Experiment No. 4

Digital Electronics-II

(Space for answer)

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Digital Electronics-II

Experiment No. 4

(Space for answer)

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