BE Lab Manual Experiment 8

 Experiment 8 

Aim: - To study IC 7485 as a four-bit comparator truth table.


Apparatus: - Four-bit comparator kit, patch chords.


Theory:- Comparators can be designed for comparing multi-bit numbers. The block diagram of an N-bit comparator is shown below. It receives two N-bit numbers A & B as inputs and the outputs are A>B, A=B, and A<B. depending upon the relative magnitude of the two numbers, one of the outputs will be HIGH. The IC7485 is a comparator IC in the following figures.


These ICs can be cascaded to compare words of greater length without external gates. The A>B, A=B, and A<B output of a stage handling less – significant bits are connected to the corresponding A>B, A=B, and A<B cascading inputs of the next stage handling more – significant bits. The stage handling the least – significant bits must have A =B input connected to logic level A>B, A<B input connected to logic 0 or 1 level. The function of IC 7485 is given by truth tables.



D:\bhupesh shukla 29-01-10\D LAB\comparator.jpg

Truth table of a 2-bit Comparator


Function table 7485 4 – bit comparator

For a 4 bit comparator


Procedure:-

  1. Study the circuit provided on the panel of the kit.

  2. Switch ‘ON’ the power supply. Check +5 V at Vcc.

  3. Apply the logic inputs to A3-- A0 (A - input) & B3--B0 (B- input) from logic input section.

  4. Also apply the inputs to cascading inputs from the logic section.

  5. For a 2-bit comparator apply the inputs A1--A0 & B1--B0. Change the logic inputs as per the truth table & observe the corresponding outputs.   

      Repeat the step for the different combinations.

  1. For a 4-bit comparator apply the inputs to A3-- A0 & B3--B0. Change the logic inputs as per the truth table & observe the corresponding outputs. Repeat the step for the different combinations.

Observation table:

 For 2-bit comparator

For 4-bit comparator

Result:-

The 2 – bit and 4 – bit comparator using IC 7485 is studied.

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