29-01-2013, 03:00 PM
VERIFICATION OF 4-BIT MAGNITUDE COMPARATOR
VERIFICATION.docx (Size: 11.91 KB / Downloads: 16)
Aim: -
To verify the truth table of one bit and four bit comparators using logic Gates and
IC 7485
Apparatus : -
IC’s -7486, 7404, 7408 and 7485
DESCRIPTION:
The 74F85 is a 4-bit magnitude comparator that can be expanded to almost any length.
It compares two 4-bit binary, BCD, or other monotonic codes and presents the three possible
magnitude results at the outputs. The 4-bit inputs are weighted (A0–A3) and (B0–B3) where A3
and B3 are the most significant bits. The operation of the74F85 is described in the Function
Table, showing all possible logic conditions. The upper part of the table describes the normal
operation under all conditions that will occur in a single device or in a series expansion scheme.
In the upper part of the table the three outputs are mutually exclusive. In the lower part of the
table, the outputs reflect the feed-forward conditions that exist in the parallel expansion scheme.
The expansion inputs IA>B, and IA=B and IA<B are the least significant bit positions.
When used for series expansion, the A>B, A=B and A<B outputs of the lease significant word
are connected to the corresponding IA>B, IA=B and IA<B inputs of the next higher stage. Stages
can be added in this manner to any length, but a propagation delay penalty of about 15ns is
added with each additional stage. For proper operation, the expansion inputs of the least
significant word should be tied as follows: IA>B = Low,
Procedure: -
1. Connect the circuit as shown in fig. Feed the 4-bit binary words A0, A1, A2 , A3 and B0, B1 ,
B2 , B3 from the logic input switches.
2. Pin 3 of IC 7485 should be at logic 1 to enable compare operation.
3. Observe the output A>B, A=B , and A<B on logic indicators. The outputs must be 1 or 0
respectively.
4. Repeat the steps 1 ,2 and 3 for various inputs A0 ,A1 , A2 , A3 and B0 , B1 , B2 , B3 and
observe the outputs at A>B , A=B and A<B .
Precautions:
1. All the connections should be made properly.
2. IC should not be reversed.
Result:
The truth tables of one bit and four bit magnitude comparators are verified.