Elenco Understanding Logic Gates and Circuits User Manual download pdf (Page 16)

U19

Project 9: S-R NOR Latch

This circuit demonstrates how the S-R NOR Latch works.

Turn the slide switch (S1) on. Connect the loose end of

the red wire (S input) to 0 and the loose end of the black

wire (R input) to 1. The red LED will be off (S=0, R=1

resets output Q to 0). Now disconnect the black wire

from 1 and connect it to 0. The red LED remains off (S=0,

R=0 holds the last output state). Now disconnect the red

wire from 0 and connect it to 1. The red LED will be on

(S=1, R=0 sets the output to 1). Now disconnect the red

wire from 1 and connect it to 0. The red LED remains on

(S=0, R=0 holds the last output state). S=R=1 is the

forbidden state, can you explain why? The green LED

should always be the opposite of the red LED (green LED

off when red LED on and green LED on when red LED off)

since the green LED represents Q which is the opposite

of Q.

S-R NOR Latches can be used to eliminate bouncing in switches.

U19

 Stays the same when S=R=0

 Reset to 0 when S=0 & R=1

 Set to 1 when S=1 & R=0

 S=R=1 is forbidden

Input (S) Input (R) Output (Q) Output (Q)

0 0 Hold State Hold State

1 0 1 0

0 1 0 1

1 1 Not Allowed Not Allowed

1 2 ... 11 12 13 14 15 16 17 18 19 20 21 ... 41 42

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Elenco Understanding Logic Gates and Circuits User Manual Page 16