Elenco Understanding Logic Gates and Circuits User Manual download pdf (Page 26)

U15

U16

U20

Project 16: Half Subtractor

This circuit demonstrates how a half

subtractor works. Turn the slide switch (S1)

on. The green LED represents the Difference

(D) of input A minus input B and the red LED

represents the Borrow (B). When the loose

ends of the red and black wires are both

connected to 0 (0-0) or to 1 (1-1), both LEDs

are off (D=B=0). When the loose end of the

red wire is connected to 1 and the loose end

of the black wire is connected to 0 (1-0), the

green LED is on and the red LED is off (D=1,

B=0). When the loose end of the red is

connected to 0 and the loose end of the black

wire is connected to 1 (0-1), the green and red

LEDs are both on (D=1, B=1), indicating a

borrow condition.

Subtractors are often implemented with adders since the incremental cost is low (one

additional NOT gate).

Represents the

Difference

Represents the

Borrow

Input (A) Input (B) Output (D) Output (B)

0 0 0 0

0 1 1 1

1 0 1 0

1 1 0 0

1 2 ... 21 22 23 24 25 26 27 28 29 30 31 ... 41 42

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Elenco Understanding Logic Gates and Circuits User Manual Page 26