connect 74LS47 to seven-segment display does not work?

[John Loomis]

I'm trying to connect a 74LS47 to a seven-segment display (which I can do fine in real hardware).

On Multisim the two devices work as advertised until I connect the outputs of the 74LS47 to the

seven-segment display inputs - then nothing.

[John Loomis]

I set digital simulation to "real" - that helped - but now the simulation stops at the number "2",

Maybe the problem now is the Word generator?

I am enclosing the ms10 file.

[lacy]

I have been going throught your circuit very carefully to find out what was giving the problems. This is why it has taken me a while to get back to you on this. I will outline the changes I made to your circuit below.

1. I removed the digital DCD Hex display and all the digital probes and replaced them with the standard version.(it seems that these were causing some of the problem, but I am not sure why).

2. I increased the voltage to 12V because the 74 series IC's are TTL and I believe they require 12 volts in order to function properly (at least it does in the simulator apparently.

3. Adjusted your wiring because you had segment A not tied to A0 on the IC (This may have been intentional, but I weent ahaead and tied it in). I also added a resistor to ground to the H segment of the disply to light the decimal point. I also cleaned up the wiring a little to make it look better (not that it looked bad, but I just like neat schematics)

The first 2 items is what seemed to cure you problems and the last was just some things I added to it that I thought were needed.

The adjusted circuit is posted below:

[John Loomis]

Thanks, Lacey.  I think the key was setting the digital simulation settings.

Here is the original file, with some minor changes, now running - but very slowly.

The students were to use a 555 timer to trigger a counter and then the seven-segment display.

[lacy]

I am glad to see that it is working for you. I believe the DCD Hex Dsiplay was causing a lot of the problems with the original circuit. This is probably why I had to increase the voltage in order for the BCD Counter to work right (I never could get it to work correctly at 5V with that display in the circuit. I don't know why that would be but that's the way it was for me). Also, when I did my tests I didn't change your settings from what you had them and tried to find another way to cure the error. Either way it is working for you now and that is want we wanted.

One thing to try in order to speed up the simulation is to goto Simulate>Interactive Simulation Settings>Set maximum timesteps. Click the circle that says Maximum Timesteps (TMAX) and then enter 1e-003. This will increase the speed of the simulator to make it more easy to see that it is working as intended. I tried this and received no error messages after simulating it.

Message Edited by lacy on 11-12-2007 05:00 PM

[John Loomis]

Thanks again, Lacy.

I tried what you suggested and it actually simulated faster than I wanted, so I set the time step down to 1e-4 and that slowed it down

so I could watch it change at a reasonable speed.

John

[jonnyBGood]

I too have not been able to get the 74LS47 to work with the Seven Segment display.

See my attached test file.

[lacy]

I got it going. Take the Analog Ground on Pin 15 of the 74157 and delete it. Replace this with the DGND. This should get it up and running.

This is just a note for everyone reading this post. The Digital components require that VCC or VDD and DGND be placed somewhere or the schematic. I know the documentation says that you don't have to do this if the Digital Simulation Settings are set to Ideal, but I have found that some IC's and Displays just will not work correctly with VCC, VDD and DGND not being present. Please double check that these are present on your schematic as this will cure a bunch of simulation problems. I thought I would share this information so as to maybe save everyone a bunch of headaches when it comes to digital parts.

[Lodin]

Hello lacy

2. I increased the voltage to 12V because the 74 series IC's are TTL and I believe they require 12 volts in order to function properly (at least it does in the simulator apparently.

74-TTL ICs are only working with 5V (4.5V-5.5V, some low voltage family types 3.3V) in reality. 12V will destroy them. Only CMOS family (40xx + 45xx) can be used up to 15V.  If the simulation only works with 12V, there is something wrong with the simulation or the models. 

regards

Lutz

[lacy]

You are correct. I failed to see that. I am going to re-examine this circuit with 5V and see if it will indeed work and if it doesn't then we may have a problem with the TTL components.