Elenco Understanding Logic Gates and Circuits - Manual (Page 17)

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U18 U18
Project 10: S-R NAND Latch
This circuit demonstrates how the S-R NAND Latch
works.
Turn the slide switch (S1) on. Connect the loose
end of the red wire (S input) to 1 and the loose end of
the black wire (R input) to 0. The red LED will be off
(S=1, R=0 resets output Q to 0). Now disconnect the
black wire from 0 and connect it to 1.
The red LED
remains off (S=1, R=1 holds the last output state). Now
disconnect the red wire from 1 and connect it to 0. The
red LED will be on (S=0, R=1 sets the output to 1). Now
disconnect the red wire from 0 and connect it to 1. The
red LED remains on (S=1, R=1 holds the last output
state).
S=R=0 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 NAND Latch is an alternative to S-R NOR Latch where the inputs S and R are now active low.
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Input (S) Input (R) Output (Q) Output (Q)
0 0 Not Allowed Not Allowed
1 0 0 1
0 1 1 0
1 1 Hold State Hold State
Stays the same when S=R=1
Reset to 0 when R=0 & S=1
Set to 1 when R=1 & S=0
S=R=0 is forbidden
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