Half & Full Subtractor Circuit

Detailed Discussion

1)What is a Half Subtractor Circuit?
  A half-subtractor is a combinational logic circuit that have two inputs and two outputs (i.e. difference and borrow). The half subtractor produces the difference between the two binary bits at the input and also produces a borrow output (if any).

Half Subtractor Block Diagram
 
  Half Subtractor Truth Table
 
  Half Subtractor Circuit/ Logical Diagram
 
 

1)What is a Full Subtractor Circuit?
  The Half Subtractor is used to subtract only two numbers. To overcome this problem, a full subtractor was designed. A full-subtractor is a combinational circuit that has three inputs A, B, C (B-in) and two outputs Diff and Borrow. Where, A is the minuend, B is subtrahend, C (B-in) is borrow produced by the previous stage, Diff is the difference output and Borrow is the borrow output. As we know that the half-subtractor can only be used for subtraction of LSB (least significant bit) of binary numbers. If there is any borrow during the subtraction of the LSBs of two binary numbers, then it will affect the subtraction of next stages. Therefore, the subtraction with borrow are performed by a full subtractor.

Full Subtractor Block Diagram
 
  Full Subtractor Truth Table
 
  Full Subtractor Circuit/ Logical Diagram
 

  Full Subtractor using two Half Subtractor Block Diagram
 

Full Subtractor using two Half Subtractor Circuit Diagram
 

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