Draw the logic diagram of a full subtractor using half subtractors and explain its working with the help of a truth table

Ans:

Full Subtractor: It has to take care of repeated borrow by the next higher bit. At some stage along with the two bits (one of that is to be subtracted from another) is the other input Bin, that is, borrow bit from the D_i and borrow B_o. Table illustrates the truth table.

Fig.(a) Block Diagram of Full Subtractor

Fig.(b) Block Diagram of Full Subtractor as Combination of two Half Subtractors and OR Gate

Fig.(c) Logic Diagram of Full Subtractor

Fig.(a) demonstrates a block diagram for a full subtractor. This can be constructed from two Half Subtractors and an OR gate as demonstrated in Fig.(b). The logic diagram is demonstrated in Fig.(c). This logic diagram is according the truth table.

Truth Table for Full Subtractor