help with an electrical engineering topic BINARY SUBTRACTOR?

Question

Design a single cell for an iterative network to do subtractions directly. This will NOT
use complement techniques. The inputs to the cell are the digits Xi , Yi and Bi. The
outputs are Si and Bi+1. The operations must be X-Y and Bi is the borrow from the
previous bit operation and Bi+1 is the borrow passed to the next bit. Show the Cell as a
box with inputs and outputs and the truth table.

Answer 1

So what's the problem? Write out the truth table (it will have 8 lines to it corresponding to the 8 possible combinations of Xi, Yi, and Bi) and show the states of the Si and Bi+1 outputs. I'll do the first two for you:
X1 = 0, Y1 = 0, B1 = 0 => S1 = 0, B2 = 0
X1 = 1, Y1 = 0, B1 = 0 => S1 = 1, B2 = 0
After you get that done, realize the circuit with simple and, or, and not functions. (Note: Modern design would use a very fast, 8 address by 2 bit PROM for this function. But doing it the 'old fashioned' way will give you a deeper understanding of binary arithmetic and it will develop 'character' ☺)


Doug