Combinational Logic Circuits Lab

Combinational Logic Circuits Lab - Computer Systems Lab 2...

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Computer Systems Lab 2 – 1 – Combinational Logic Circuits Object To investigate the properties of combinational logic circuits, as illustrated by the half adder, full adder, and data selec- tor/multiplexer. Parts (1) 7400 Quad 2 input NAND gate (1) 7408 Quad 2 input AND gate (2) 7486 Quad 2 input XOR gate (1) 74151 Eight-line to one-line data selector (1) 7404 Hex inverter Study section Computer Systems , Fourth Edition, Jones and Bartlett Publishers: Section 10.4, Combinational Devices. General information Combinational circuits are interconnections of logic elements in which the output at any time re fl ects the condition of the inputs at that time. Combinational circuits do not retain information as do sequential circuits, and their outputs change with any change in input. One example of the combinational circuit is the binary adder. Procedure 1. The half adder The half adder is the simpler of the two types of adders, as it does not use a carry input from a previous stage. The input consists only of the two binary bits to be added. The outputs are the sum of the two bits and the carry, if any, resulting from the addition. As there is no carry input, the half adder may be used only as the least signi fi cant adder of an array of adders. The sum and carry outputs are base on the addition table of Figure 1(a). If the addition table is converted to a truth table, the truth table would appear as in Figure 1(b). Co is the carry out, and S is the sum.
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Computer Systems Lab 2 – 2 – Note that the sum outputs of the truth table correspond to those of the XOR gate, and that the carry outputs corre- spond to those of the AND gate. Thus the half adder may be implemented with only two gates, as shown in Figure 2. In the fi gure, G1 stands for a gate in IC number 1 (a 7486), and G2 stands for a gate in IC number 2 (a 7408). Construct the circuit as shown in Figure 2. Set SW2 and SW3 to the positions indicated in the truth table and record the light indications. DO NOT tear down the circuit after you have fi nished. You will need it for the circuit of Part 3.
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  • Spring '14
  • StanWarford
  • Addition, Half adder, Logic gate

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