# Design troubles. This really shouldn't be that hard...

Discussion in 'General Electronics Chat' started by melmac82, Jul 18, 2009.

1. ### melmac82 Thread Starter New Member

Jul 18, 2009
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Ok, trying to design a relatively simple logic circuit with 3 inputs and 3 outputs. (A, B, C and a, b, c) When A is high, a is high and all the rest are low. Then when B goes high a drops out and b goes high even if A is still high. If C goes high, c is high and both a and b are low even if A and B are high. I've tried experimenting with various latches and other combinations. Just when I think I'm getting close I get oscillation issues. Any ideas or help would be great.

2. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Is this a homework assignment?

3. ### millwood Guest

that's easy.

c=C;
b=(!C) & B
a=((!C) & (!B)) & A

where !X = not X.

4. ### melmac82 Thread Starter New Member

Jul 18, 2009
9
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Nope its not a homework problem... just something that got me thinking and now I have to figure it out, heh.

And I should have elaborated more but it can't just be c=C because it has to go backwards too. So if C and c are both high and then A goes high with C still high, c would go low and a high. So no matter what inputs are high, when a new input goes high, all outputs go low except the one associated with the new high input.

5. ### hgmjr Moderator

Jan 28, 2005
9,030
214
A good place to start is to build a truth table relating the inputs to the outputs.

hgmjr

6. ### millwood Guest

that will make it become an oscillator - the circuit will never have a steady state.

7. ### melmac82 Thread Starter New Member

Jul 18, 2009
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I was beginning to think that it was just going to make an oscillator because everything I had tried resulted in that happening. I was just hoping that someone else with a little more experience might be able to tell me otherwise, but oh well, at least it looks like I'm getting good at creating a lot of different circuits that oscillate.

8. ### hobbyist Distinguished Member

Aug 10, 2008
773
62
If there manual inputs,
couldn't you latch the outputs, then use steering diodes connected to the input switchs to reset the other 2 latches.

switch A would connect to S on it's latched output while simultaneously switch A could steer a signal to the R of the other 2 latches. Thereby trigering the opposite outputs.

So when A or B or C switch is inputed the other 2 latches will change to an off state every time a switch is hit.

and do the same for the other 2 switches

However it would have to be the same thing with the input switchs themselves steering diodes and some gating so that 2 switchs on at the same time would feedback a shutoff signal to the other switchs. etc...

Switch A and B and C would be wired to some gating so that A switch blocks B and C switch inputs and vice versa.
I think you know what i mean.

Something like that...If there manual switch inputs.
just an idea, probably not what your looking for..

Last edited: Jul 18, 2009
9. ### melmac82 Thread Starter New Member

Jul 18, 2009
9
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I'm having some problems with latches though because all my inputs and outputs will be low to start. I've found that latches tend to either oscillate or give high outputs with starting low inputs. I can get low inputs to have low outputs when I use XOR gates instead of the usual NOR and NAND gates, but then I run into other oscillation problems later.

I was going to try some stuff with the manual switches, latches, and diodes, but I don't know what software to use. Right now I am using just a basic program that only includes inputs/outputs, gates, plexers, arithmetic, and some memory modules. Any suggestions for some better free software for simulations?

10. ### melmac82 Thread Starter New Member

Jul 18, 2009
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Ok, so now I've been thinking about this way to go. I'm not very knowledgeable about latches, so are we talking NAND latches or NOR latches?

Also, when you are saying to have a steering diode go from one switch to the R input of the other latches, am I correct in assuming then that there would be 2 signals with steering diodes going to each R input of each latch?

And I got a little confused when you were talking about having to do the same thing with the input switches. Does that mean that there would be steering diodes directing the inputs from the two other switches to the S input of the remaining latch?

I've never done anything like this before and it is pretty interesting and its hard to stop thinking about it, lol. And thanks for all the help so far everyone.

11. ### hobbyist Distinguished Member

Aug 10, 2008
773
62
sorry for the delay, I forgot to look at this post.

I'll see if I can come up with a logic diagram.
What I think your asking about.

Something like this.
I never worked with IC's so can't help you there but this logic diagram might be what your asking about.

There RS latchs, when S goes high its output Q goes high
when R goes high the output goes off.

So A switch connects to "S" on "a" output latch and simultaneously connects to R inputs of the other 2 latchs to turn them off.

This only works for inputs that are momentary, if you want to cancel outputs when more than one input is already prisent it will take some elaborate circuitry to cancel input switchs as well.

If you want the switchs to remain on and then they get canceled with another switch input, that's a whole new ballgame, which would take some real heavy thinking and a lot of extra circuitry.

Last edited: Jul 21, 2009
12. ### melmac82 Thread Starter New Member

Jul 18, 2009
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Thanks a lot for posting this. It's pretty much what i was thinking that you were saying and I'll have to look at some more stuff to see if it is more complicated. I did another post that is along the same lines as this. I was trying to explain some stuff better and put a better title to the post to see if I could get some other people to respond. Again, thanks very much for your help