Lab 7: Adder Circuits

 

Introduction

 

            The heart if a computer arithmetic unit is the adder circuit.  Two basic ways for carrying out addition are serial and parallel.  Serial addition is carried out bit by bit using a single full-adder circuit.  Parallel addition provides addition of all bits at the same time, which is very fast, using a full-adder circuit for each bit position.

 

Equipment

 

            Digital Breadboard

            74LS08

            74LS86

            74LS83A

 

  Procedure

 

1.  Construct the circuit shown in Figure 1.  Verify its operation by finding the truth table for this circuit.  Identify the A and B as the Addend and Augend respectively.  Identify the outputs as the Sum and Carry out.

 

 

Figure 1

 

    What kind of adder circuit is this?  What is the purpose of the carry out output?

 

2.  Construct the circuit shown in Figure 2.  Notice this circuit employs an additional input called the carry in.  Verify the proper operation of this circuit by creating a truth table for the circuit.

Figure 2

    What kind of adder circuit is this?  What is the purpose of the carry in input?  What are the differences between the adders in figure 1 and 2? 

 

3.  Add a second full adder (Serial Adder) to the circuit in figure 2 and describe its operation by making a truth table.  Identify which full-adder sum is the LSB and which is the MSB.

 

4.  Use a 4-bit binary full-adder chip (74LS83A) to make a 4-bit parallel full-adder.  Identify which inputs are the low-order bits.

Figure 3

 

     How is a full-adder different from a half-adder?  What is the significance of the carry-out of an adder unit?  What does “look-ahead adder circuitry do?