Why do the 7 segment LED's not light up in this TTL circuit simulation?

[mode656]

In the following circuit, none of the 7 segment LED's light up when i start simulation. I have been told that I do not need to connect anything to the LT, RBI and BI/RBO ports of the 74247 TTL. Does that affect anything maybe?[PLAIN]http://img708.imageshack.us/img708/390/7seg.png

 

[berkeman]

In the following circuit, none of the 7 segment LED's light up when i start simulation. I have been told that I do not need to connect anything to the LT, RBI and BI/RBO ports of the 74247 TTL. Does that affect anything maybe?[PLAIN]http://img708.imageshack.us/img708/390/7seg.png[/QUOTE]

What does the datasheet say?

 

[vk6kro]

You could take the wire that goes to pin 13 of U2 off U2 and ground it.

This will tell you if you have a LED problem or a driver problem because one of the LED segments should light up if you do this.

 

[Jack Chappy]

I gave my 13 year old son a simular lab which seem to work fine. Take a look, this may work for you as well.

 

[Feldoh]

I'd check the spec sheet for what they do. Also you probably did but I'll ask anyways. Have you tried a different combination? The logic might be 0000000 for your particular initial condition.

http://pdf1.alldatasheet.com/datasheet-pdf/view/27400/TI/SN74247N.html

 

[vk6kro]

That circuit won't work for the 74247 which is TTL.

TTL has to be pulled low to go low and that circuit gives the inputs 5 volts or leaves them open.

So they would be high all the time.


Did you try this suggestion:
You could take the wire that goes to pin 13 of U2 off U2 and ground it.

This will tell you if you have a LED problem or a driver problem because one of the LED segments should light up if you do this.

 

[Jack Chappy]

The circuit has four dipswitches that feed the 7-segment decoder inputs.
This enables us to generate a complete truth table as illustrated in the data sheet.

I am currently creating EWB labs for my son where he will be simulating RC transistor oscillators driving JK flip-flops and NAND gates. He just loves it.

 

[vk6kro]

There are different families of logic and they have to be driven correctly.

This is an input for a TTL device:

Can you see that it makes no difference whether you connect 5 volts to A or B or just leave them open circuit?
It still counts as a "1".
To get a "0" input you have to pull A or B to ground.