I don't see any floating inputs.

 

Nathan,

Is that 74HC78_2V a two volt model?

Make sure your initial conditions are proper at energizing. You measure the Highs where there should be highs and lows where you should have lows.

The wiring appears correct, although your drawing could have been cleaner.

Attached is my drawing ... the 74HC21 and 74HC78 are macros vice individual gates. The red dots are highs upon energizing the circuit. I know the reset is HIGH even though it does not have a red dot.

 

I don't see any floating inputs.

My bad. I failed to spot the U7 connection.

 

Nathan,

Is that 74HC78_2V a two volt model? Make sure your initial conditions are proper at energizing. You measure the Highs where there should be highs and lows where you should have lows.The wiring appears correct, although your drawing could have been cleaner. Attached is my drawing ... the 74HC21 and 74HC78 are macros vice individual gates. The red dots are highs upon energizing the circuit. I know the reset is HIGH even though it does not have a red dot.

Thank you joe for the info.
Here is my updated circuit and thank heavens it is working the way it should except for a small glitch.

In my previous circuit Is it absolutely necessary that my nand gates, 4 input and gate and my flip flops belong to the "74 HC" Sub family? Is it bad that i used 7400N for nand gates, 74als for my 4 input and gate ?

can you also let me know what is the difference between a 74HC08N and a 74HC08D (or) what is the difference between a 74HC05N and a 74HC05D ? What is the difference between the "N" and "D"?

Here is the glitch: For example when key "Z" and key "X" are hit simultaneously on my keyboard both the corresponding
LEDs are lighting up.
Let us presume that the "Z" key got closed first .The "Q-bar" of "Z" should ideally make the output of the 4 input and gate LOW thereby not letting "X" light up its corresponding LED, but.... both the LEDs are lighting up.

I want to stress that if i hit Z first and wait a half second or less and then hit "X", only Z's LED is lighting up.

Only when i have my fingers on the Z and X key and push them down at the same time is when both LEDs light up. i would appreciate all feedback as to what might be causing this problem and how to fix it.

p.s. I am using multisim 12.0

Thank You

 

I have also been lucky to get two LEDs to light. I used A, B, C, and D as the letters vice the ones you used.

I say lucky because of the spread, and using two fingers, one on each hand, I was able to do that only once. Build the circuit, get four friends, and put two $5 in the pot. Hold a contest ... the ten bucks is paid out ($5 each) only if two players can get two LEDs to energize. Ask twenty simple two digit math questions and they have to buzz in once they know the answer. Like what's 12+17?

I use TINA version 9

On edit ...

You could add another gate to clock the input, but you still could get two led's to energize using the same method you've done it. Do you expect people to be within nanoseconds of one another?

On second edit ...

You can replace the switches in your simulation with timed switches to close at 10.001 mS and end at 11 mS. Increment the other switches by one microsecond. You will find that the one set the lowest will always energize the LED. Run a transient analysis.