# Logic gates

Explanations about logic gates and what they can be used for

1. ### Anon_LG

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Jun 24, 2014
Lavaguava submitted a new resource:

Logic gates - Explanations about logic gates and what they can be used for

2. ### Supercap2F

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Mar 22, 2014
Some random thoughts

It depends on what kind of logic you are talking about. CMOS uses MOSFETs, TTL uses Transistors, DTL uses diodes and transistors, and RTL uses Resistors and transistors (although DTL and RTL are pretty much gone).

You better say what they mean. i.e.

• 4xxx = CMOS
• 74HCxxx = CMOS
• 74LSxxx = TTL
• etc.

You and me know what high and low voltages would mean, but a complete beginner at this would not. just say something like: the ones = positive voltage, the zeros = ground. Or something like this:

 High Low 1 0 + Volts Ground
You will need to have the truth tables for: XOR, XNOR, NAND, OR and NOR.

Edit: You might want to say something about not leaving logic gates inputs connected to nothing.

Regards

Dan

Last edited: Jul 17, 2014
Arouse1973 likes this.
3. ### Anon_LG

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Jun 24, 2014
Yes I was aware of these things, and was about to add them to the resource however I had limited time last night. The resource may be in development for a while yet. I was planning on putting NOR gates first but thought that AND was a good one to start with as it is easiest to understand.

Last edited: Jul 17, 2014
4. ### Supercap2F

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Mar 22, 2014
Cool! Were did you learn about RTL and DTL logic? Most people our age would not know about them!

All the best!

Dan

5. ### Anon_LG

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Jun 24, 2014
Wikipedia. I know lots of people think Wikipedia is dodgy but I like it, I think they do an all right job of keeping it tidy and I think their objective of world-wide available knowledge is a good one. I am not going to put in the resource about RTL and DTL as I am making resources for absolute beginners, I do not want to confuse them, however I have put "the 2 most popular are called TTL..." leaving it open what the other 2 are, they could research what these types are when they have learnt the basics of logic.

6. ### Supercap2F

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Mar 22, 2014
I totally agree, wikipedia is amazing.

I wouldn't put them in the resource even if you were writing it for experts,Because RTL and DTL logic was out of date 35+ years ago!

Would you like some more ideas for you resource?

Regards

Dan

7. ### Anon_LG

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Jun 24, 2014
Yes please if you are happy to give them, I am currently working on the resource now. I will use the suggestions tomorrow.

What type+series of logic gate do you generally use in your electronics?

Last edited: Jul 17, 2014
8. ### Supercap2F

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Mar 22, 2014
CMOS, always CMOS.

You might want to say something about driving TTL inputs from CMOS outputs. Like how you can only drive one 74LS series input from a 4000 series output without a pull up resistor.

I thought I had more suggestions then that..

Anyway's, it's look'n good

If you want me to I can get a photo of a CMOS IC thats a bit easier to see.

Regards

Dan

9. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Here's a couple of pointers for you:

That's a good start. But perhaps you should describe what the reader should already understand and the key things they'll get out of it.

Maybe something like:

"I will assume that you have some understanding of electronics and are familiar with what bipolar transistors and mosfets are, but don't know anything about digital electronics. In this resource I will aim to introduce you to some of the elementary logic gates."​

Perhaps the next thing you should talk about is what is digital electronics and how it differs from analog electronics. Something like:

"Electronics can be broken down into analog and digital. Analog electronics involves circuits which deal with continuously variable voltages, an example would be an audio amplifier. Digital electronics involves circuits which deal with discrete states. A light switch is an elementary digital device".

After you have laid some groundwork, you can talk about these discrete states:

"Most digital electronics involves binary states, that is any given part of a circuit is in one of two distinct states. These may be called on and off, o and 1, true and false, low and high, etc."
Then we need to know how these are represented.

"In this resource I will normally be using 0 and 1 as the binary states. Typically (but not universally) 0 is represented by a low voltage and 1 by a high voltage."
Then tell us what digital electronics does at its most basic level:

"Digital electronics involves the manipulation of these states to perform what are essentially mathematical functions."
Then you need to tell us what logic gates are:

"The most basic of these mathematical functions are performed by logic gates".
Having laid the ground rules, you can now talk about the gates themselves. Your discussion starts with what they're made of and the packages they come in. I think this is putting the cart before the horse. I really think you should talk about these basic functions first (the abstract notions) before discussing how they are implemented.

So I've moved that part up here:

Yes, I think that talking about truth tables is a good start. Note that in the introductory part I suggest you already cover the various names for binary states, and say you'll be using 0 and 1. How 0 and 1 relate to on, off, high, low, etc. is an implementation issue (or maybe something that you discuss just prior to the implementation of the gates).

It's usually better to explain things in the simplest possible way. How about:

"Truth tables show the relationship between inputs and outputs.

Again, to simplify:

"Truth tables contain a column for every input and output. They contain as many rows as there are combinations of inputs."​

This may be complexity that can be avoided. And why not start with the most simple logic gate as an example?

"The truth table below is for the inverter (also called a NOT gate). It has only a single input and a single output. The definition of the inverter is that the output is the opposite state of the input. With only a single input, there are 2 rows because there are only two possible combinations of inputs, a 0 or a 1."
Oh, it's also possible to define tables using bbcode. That will be easier than using images.

Yep, I'd go through NOT, AND, OR, then NAND, NOR, and maybe XOR. After that you should point people to boolean logic as a means of symbolically manipulating these logic functions.

Now it's time to talk about implementation issues.

• Relationship between 0 and 1 and high/low, on/off, true/false (careful, it's not as obvious as you may think)
• Voltage levels (and the no-mans land between them (introduce concepts such as the minimum high input voltage and the minimum high output voltage, and why they differ if you're comfortable with that)
• The concept of a "logic family" which is defined as a group of circuits which share the same definitions for logic levels, supply voltages, etc.
• A brief list of logic families, highlighting the ones commonly in use today. (I notice you missed ECL).
In all of these discussions, point to articles on the net that explain the concepts in more detail. Wikipedea is often a good option.

Next you can talk about how you'll find logic circuits.

Remember that they're not always ICs, not always "several transistors", and saying that TTL is popular ignores the fact that LSTTL is far more often used these days. I would tend to list CMOS first.

Be careful of what you're saying because some of it is prehaps misinformed. for example "DO NOT JUST TYPE IN THE LOGIC GATE NAME e.g. AND gate, YOU MUST TYPE IN THE FULL LIST OF LETTERS AND NUMBERS" is misleading. I might want a hex inverter. I don't search for SN74HC04N, I search for 74HC04 because SN is a manufacturer designator and N is packaging information etc. In fact, I might prefer to search for 74*04 because I might be able to use a 74C04, or a 74HC04 (in this case C and HC tell me the logic family)

Also, use terminology that is well accepted and recognised.

I've never heard it called a "clocking mark", and neither has google. The chip has a notch or a circle that is used to identify pin 1.

Also, trim the excessive white space from around your image. Preferably use something clear and simple and appropriately sized. IMHO this image is larger than it needs to be (although perversely they show as very small in these quotes).

Whilst this is well intentioned, it is really needs a bit of work. There is no "typical ESD", and ESD will rarely damage the silicon wafer (it will however rupture a thin oxide coating used to insulate the gate of a mosfet).

I hope I haven't depressed you, I think with a little work and some polishing you could have a useful resource here.

Supercap2F likes this.
10. ### Anon_LG

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Jun 24, 2014
Yes I was about to add many of these things you have suggested, however I just wanted to save the basic outline of the resource so that people/moderators could see roughly what it was about.

I will be working on this resource over the next several days, and will now attempt to take a better image of the CMOS IC (thanks for the offer Supercap2f).

11. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Excellent, good to hear that. I'll be taking an interest in what you write.

And the collaboration is a good thing, keep that up

12. ### Anon_LG

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Jun 24, 2014
I will *steve*. I hope some more people make resources on different types of components, as I need some links to put in the resource. I have already linked to your resource "Using a bipolar transistor to turn a load on and off" *steve*.

Does anyone have any ideas for the resource icon?

13. ### Supercap2F

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Mar 22, 2014
Look on wikimedia, they should have what you want.

Edit: Lavaguava, have you ever heard of the book "CMOS Cookbook" by Don Lancaster? It would be a good thing to put in a "further reading" section, if you have one. Also the "TTL Cookbook" might be good to (although they are similar in content).

Dan

Last edited: Jul 19, 2014
14. ### Anon_LG

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Jun 24, 2014
This is what I have: (I could not get it off wikimedia, however the image still shows all the logic gates described in the resource)

15. ### Supercap2F

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Mar 22, 2014
Well, this is how your going to see it on the resource:

<-- That's it right there

You really can't see it very well. Maybe you should just try to get a photo of your favorite logic gate?

Regards

Dan

Last edited: Jul 19, 2014
16. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
I agree, use a single gate. The inverter is as good as any.

17. ### Anon_LG

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Jun 24, 2014
The resource icon has been sorted (4001 CMOS NOR gate)

18. ### chopnhack

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Apr 28, 2014
Great article so far - I think the explanations of the gates is clearly presented - at least clearer than I have read about them in the past, especially how you treat the NAND gate, AND followed by NOT! I did lose you at "meaning that all another logic gates can be constructed form NAND gates" Did you mean "all other gates can be constructed from"?

19. ### Anon_LG

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Jun 24, 2014
Thanks!
Yes, sorry it was getting late when I typed that part, I have sorted it now and thank you for pointing it out.

Last edited: Jul 20, 2014
20. ### Anon_LG

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Jun 24, 2014
I think that, unless anyone has any more ideas, this resource needs no further development. If anyone spots something then please could I be notified.