Design an 8 bit comparator using a two numbers of IC 7485 or using a NAND gates.
The Correct Answer and Explanation is :
To design an 8-bit comparator using the IC 7485 or NAND gates, we need to understand the working of the comparator circuit.
IC 7485 (4-bit Magnitude Comparator):
The IC 7485 is a 4-bit magnitude comparator that compares two 4-bit binary numbers and gives three output signals:
- A>B: Output is high if input A is greater than input B.
- A: Output is high if input A is smaller than input B.
- A=B: Output is high if both inputs A and B are equal.
Since the IC 7485 is a 4-bit comparator, for an 8-bit comparator, we can use two 7485 ICs in a cascaded configuration. Here’s how to do that:
Step-by-Step Design:
- Divide the 8-bit inputs into two 4-bit groups. Let the 8-bit number A be A7A6A5A4A3A2A1A0 and the 8-bit number B be B7B6B5B4B3B2B1B0. We split these into two groups: A7A6A5A4 and A3A2A1A0 for number A, and B7B6B5B4 and B3B2B1B0 for number B.
- Use two 7485 comparators:
- The first 7485 compares the most significant 4 bits (A7A6A5A4 with B7B6B5B4).
- The second 7485 compares the least significant 4 bits (A3A2A1A0 with B3B2B1B0).
- Cascade the outputs: The outputs of the two ICs are then fed into logic gates that combine their results:
- The “A>B” output from both ICs are connected with an AND gate to give the overall “A>B” output.
- Similarly, the “A<B” outputs from both ICs are connected with an AND gate for the “A<B” output.
- The “A=B” outputs are connected with OR gates, combining the results of both 7485 ICs to determine if the entire 8-bit number A equals B.
Alternative Design Using NAND Gates:
If you prefer to design the comparator using NAND gates, you’ll need to compare each pair of corresponding bits from the two numbers using XOR gates (since XOR will produce a “1” if the bits are different). Then, the results of XOR gates can be fed into a series of AND and OR gates to determine the overall comparison. Using only NAND gates requires constructing AND, OR, and NOT gates from NAND gates, and combining them appropriately.
This design ensures that the 8-bit comparator will provide accurate comparison results for both equality, greater than, and less than scenarios, using either the IC 7485 or logic gates like NAND gates.