# Logic gates and short circuits

Discussion in 'Digital Circuit Design' started by microchips-n-dip, Nov 8, 2016.

1. ### microchips-n-dip Thread Starter New Member

Nov 8, 2016
5
0
Hey AAC!

I'm making a spiking neural network using hardware as a bit of a challenge for myself, but I've run into a problem with my logic gates. For my synapse update circuitry, I need an IMPLIES gate, and I know how to make one but my version involves the inevitable shorting of my power supply which prevents me from having any other components. Really, I get this problem whenever I make some kind of digital inverter, be it a NOT, a NAND, or my IMPLIES gate. If anybody has designs for inverters that don't involve a direct connection to ground, or manage to avoid shorting the power supply, I'd love to hear it.

Cheers,
microchipsndip

2. ### WBahn Moderator

Mar 31, 2012
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7,695
Perhaps if you would show how you are trying to implement your gates we might be able to make some useful observations. Otherwise you are relying on the quality of our crystal balls.

3. ### JUNELER Member

Jul 13, 2015
181
25
Hi,
can you show a little bit drawing of that so we can see where to start.

4. ### microchips-n-dip Thread Starter New Member

Nov 8, 2016
5
0
Hey!

Thanks for the quick reply. I attached a picture of my circuits for NOT and IMPLIES.

Cheers,
microchipsndip

5. ### kubeek Expert

Sep 20, 2005
5,584
1,093
A->B is equivalent to NOT A OR B. Use a NOT gate and an OR gate.

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6. ### microchips-n-dip Thread Starter New Member

Nov 8, 2016
5
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kubeek,

I know how logic gates work, what I'm trying to avoid is the ground that comes with the inverter.

Cheers,
microchipsndip

7. ### kubeek Expert

Sep 20, 2005
5,584
1,093
I don´t get it. what you have is an RTL gate, and the output is either at 0V or at 2.5V. What do you mean by "the ground that comes with the inverter"?
Can you show what you mean? So far I don´t see any way the cricuits you show could short the Vcc to ground.

8. ### microchips-n-dip Thread Starter New Member

Nov 8, 2016
5
0
kubeek,

The short happens whenever the transistor is turned on. When closed, the transistor creates a route direct from Vcc to ground.

Cheers,
microchipsndip

9. ### kubeek Expert

Sep 20, 2005
5,584
1,093
You mean, direct through the resistor? That resistor would be somewhere in the 10k range, which I really wouldn´t call a short. Yes it loads the supply a bit, but really is not that big of a problem.

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10. ### WBahn Moderator

Mar 31, 2012
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How does it do that? You have a resistor between the collector of the transistor and the positive supply rail.

What sizes are the resistors you are using?

Why aren't you just using standard TTL/CMOS logic gates?

11. ### microchips-n-dip Thread Starter New Member

Nov 8, 2016
5
0
kubeek & Wbahn,

I'll use CMOS gates, I've got my IMPLIES working. Thanks for the help. I'm still getting a serious drop in voltage when I turn on my transistor though.

Cheers,
microchipsndip

12. ### kubeek Expert

Sep 20, 2005
5,584
1,093
Then post a complete circuit with component values, much better than just a blank sketch.

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13. ### WBahn Moderator

Mar 31, 2012
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A serious drop in voltage WHERE???!!!

Post a COMPLETE schematic, with component values.

14. ### AnalogKid AAC Fanatic!

Aug 1, 2013
8,080
2,285
That's because your circuit is not correct. Your schematic has no resistor values, so it cannot be analyzed. But given what we know from the last *90 years* of digital logic circuit design , it is fair to say that your design is flawed. If the circuit has a problem with bipolar transistors, it will have the same problem with MOS and CMOS parts. There is no magic here. Your circuit is incorrect, and as long as you keep critical details secret it will stay that way.

ak

15. ### PhilTilson Well-Known Member

Nov 29, 2009
75
13
Once again someone seeks help with a problem but either refuses or is unable to give any reasonable sort of information to those from whom he seeks help!

I am not quite sure why people take this attitude. If you REALLY have something super-secret that you don't want the world to know about, then this is absolutely not the right forum to be looking for help in!

If that doesn't apply, then for heaven's sake give the people who are trying to help you something to go on - and by that I mean EVERYTHING! And if you really don't understand what it is you are trying to do, don't make nonsense statements like: "When closed, the transistor creates a route direct from Vcc to ground", which it clearly doesn't, and only tends to confuse.

I am not having a go at the OP (well, maybe a bit!) just trying to make it easier for people to get answers.

State EXACTLY what it is you are trying to achieve, state CLEARLY how you are trying to achieve it and describe FULLY the problems you are experiencing or the undesirable results you are getting. Then you might get some sensible answers!

Phil

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16. ### WBahn Moderator

Mar 31, 2012
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What "diagrams below"?

Look at the first several posts. For whatever reason, the TS is trying to implement the implication function using RTL (resistor-transistor logic). They haven't provided enough info to determine if this route is a reasonable one to pursue and it has been suggested that they at least consider using off-the-shelf gates. Beyond that we can only accept that their decision to use RTL is reasonable and proceed on that basis.

Sep 20, 2005
5,584
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Mar 31, 2012
24,559
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19. ### AnalogKid AAC Fanatic!

Aug 1, 2013
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I have no idea what this means, so I guess everyone old is noob again.

ak

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20. ### WBahn Moderator

Mar 31, 2012
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In many forums (including this one) there are privileges that members don't have until they get at least so many posts under their belt. For instance, here you can't start a Conversation until you have a certain number of posts. In some forums you can't post any links unless you reach a threshold. Spammers, of course, want to post links. Since most forums don't advertise these settings and since spammers are about volume, many just automatically post garbage in order to get a post count up quickly on the assumption that this will get them link-posting privileges. Keep in mind that most of the time these are brain-dead bots using brute force techniques (though the sophistication of some of these bots is pretty amazing).