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Implement 10 bit comparator using IC 7485.

Mumbai University > ELECTRO > Sem 3 > Digital Circuits and Designs

Marks: 10M

Year: May 2015

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• IC 7485 is a four bit comparator IC. It compares two 4-bit words $A (A_3 - A_0) and B (B_3 – B_0)$.

• It is possible to cascade mode than one IC 7485 to compare words of almost any length by making use of the cascade pins of the IC. The logic diagram of IC 7485 is shown below. • In order to compare two 10-bit words, we will require to cascade three IC 7485s. The first IC will compare the 4 LSB bits of the two words. Since it is the LSB IC, the cascade inputs to this IC will be:

$I(A \gt B) = 0,I(A = B) = 1,I(A \lt B) = 0.$

• The IC will give compare outputs which will be connected to cascade inputs of second IC as shown below. Thus, for second IC,

$I(A \gt B)= A \gt B,$

$I(A = B) = A = B,$

$I(A \lt B) = A \lt B.$

• The second IC will compare $A_4-A_7$ with $B_4-B_7$ and give compare outputs accordingly.

• The third IC 7485 will similarly be connected to second IC, where the cascade inputs of last IC will come from compare outputs of second IC. The MSB IC will compare $A_8-A_9$ with $B_8-B_9$. Thus, $A_10-A_11$ and $B_10-B_11$ inputs for third IC will be 0s.

• The output of third IC 7485 will give the final compare answer of the two 10-bit words A and B. 0