Introduction

About the Experiment

This experiment enables a student to learn

• How to realize the functionality of IC 7485 which is used to compare magnitudes of two binary numbers.
• How to realize an eight bit magnitude comparator by cascading two comparator (one is lower nibble comparator,another is higher nibble comparator)

Theory

IC 7485 is used to compare magnitude of two binary numbers .The two 4-bit numbers are applied at the inputs A3,A2,A1,A0 and B3,B2,B1,B0 Where A3 and B3 are the most significant bits of the two 4-bit operands to be Compared.The Three cascading inputs (A = B)_IN,(A < B)_IN and (A > B)_IN should be connected to logic '1'(HIGH),logic '0'(LOW) and logic '0'(LOW) respectively.The Three outputs (A > B)_OUT,(A = B)_OUT and (A < B)_OUT will reflect the results of comparing the two 4-Bit numbers A=A3A2A1A0 and B=B3B2B1B0.

Figure 1

The students of virtual DEC Laboratory can verify the functionality of the 7485 chip fully rather than testing for a total of 2⁸=256 inputs,the students can observe the outputs listed in the compressed function table given below.

Figure 2

Part II

Implement an 8-bit magnitude comparator using two 7485 chips.Let A= A7...A0 and B= B7...B0 be the two 8-bit numbers.Note that no external gates are required. The three outputs of the 7485 comparing the lower nibbles , viz (A > B)_OUT,(A = B)_OUT and (A < B)_OUT are connected to the corresponding cascading inputs of the 7485 used to compare the higher nibbles.Final results of comparison are obtained at the(A > B)_OUT,(A = B)_OUT and (A < B)_OUT terminals the higher nibble comparator.