# De Morgan alternate symbols

I wonder how many people are using the De Morgan feature in non-logic gate contexts, just as a way of getting alternate symbol graphics. And when it will be generalised to any number of alternatives.

I am a bit guilty here ofā¦not sure what. I am vaguely familiar with alternate symbols for logic gates as you say. I am thinking of squares with inverting wedges at the inputs. Or maybe replacing a NAND with an OR that inverts the inputs? Butā¦I had not thought of it as just (as you say) generally two different graphical representations for the same function. The way I have been doing it presents a minor pain. If I use the same dual diode in multiple places but use different symbols, it shows up as different components on the BOM. Not ideal.

It could also be used to store multiple sizes for the same symbol. This would suit people who want tiny signal transistors and huge power transistors.

I just didnāt know what DeMorgan is originally intended. My brother was one who was using Protel first. When I was taking PCB design I got info: āDeMorgan is simply other graphic. Why it is so strangely named - who knows.ā I got these diodes double representation made as DeMorgan and also all gates but not as you mean. We used DeMorgan to have gate alone or gate with power pins. So using 7400 I was placing 3 standard symbols and one DeMorgan.

It comes from mathematician De Morganās theorem, a search will find it. Itās usually stated in one of the following equivalent forms:

``````not (A or B) = (not A) and (not B)
not (A and B) = (not A) or (not B)
``````

For example, if you are working with inverse logic, say you have a ~RESET line, and you want it to be a conjunction of two negated signals ~A and ~B, then from the second form what you want is an OR gate. Try the little exercise to obtain this result.

From this you can see that the De Morgan alternative for a NAND gate is a NOR gate, and for a NOR gate a NAND gate.

Equations are well known (at the level of being so obvious as to be trivial). I just didnāt remembered that they had a special name. I suppose that we were not told that ever (as both of us (me and my brother) didnāt remembered it). Not all equations have to be named.
I remember only one name associated with digital circuits - Karnaugh table.
In 90s we used GAL to make software protecting key. It took 2h of my program working to find (randomly generating fuse maps) 20 keys that meet my criteria. Then I used (in my program) Karnaugh table to generate the shortest possible source code simulating GAL functioning when programmed with each key.

Actually you definitely know another name associated with digital circuits: George Boole.

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Sorry, I just canāt let this go

The below statement is much easier to comprehend for all those budding young and enthusiastic minds slavishly absorbing the wealth of information found in this forum.

From this you can see that the De Morgan equivalent circuit for a positive logic NAND gate is a negative logic NOR gate, and for a positive logic NOR gate is a negative logic NAND gate.

Thanks. I forgot Iām neither these days. And thatās an inclusive NOR.

Maybe even kittens embarking on an electronics education could comprehend the De Morgan equivalent now?
Maybe even horses?

We had logic at math in secondary school without any reference to electronics or gates and without using ānegative logicā name as negative state representation is an invention of electronics (math alone need not negative logic, I think).
These equations were simply āand of logic states A and B == not or of their negationsā used as the method of transforming logical expressions just like the shorthand multiplication rules are used to transform mathematical expressions. Neither of them were specifically named.
This is why we didnāt hear āDe Morganā before so seeing it in Protel we assumed it is simply the name of alternate graphic representation of element and because of this we had no hesitation in using it that way. Our resistor had total length of 4 grids and had DeMorgan with 3 grids length. Our capacitor had total length of 4 grids and DeMorgan with 2 grids length.

It was the same for me in secondary school in my country. I remember asking why we had to learn this stuff (this was the late 1960s). The teacher replied something like ābecause it is in the syllabusā.

It was about the same time that I started to be seriously interested in electronics. I remember being shocked to realize I could integrate and differentiate with a resistor and a capacitor. This made calculus an enjoyable subject for me. Most of the rest of the class thought I was crazy and couldnāt see any point to calculus.

Yes, I remember my brother teaching me how the exponential charge curve could be derived from applying calculus to first principles from physics.

I was designing an alarm for the green night (the last night during camp when jokes are made). I found that for one R in my circuit if it is going to 0 or to infinity the maximum length of 0.05mm wire to be broken shortens. So somewhere in the middle there should be the optimum (giving the maximum length). I was able to find it only by method of successive approximations. I didnāt needed to have this wire be 30 km length but I was wondering where is the optimum. Being in this design stage I went to school and heard about derivatives for the first time.

First, we deal-with/use Integration/differentiation every day (how far did we go? what speed were we drivingā¦)

My ears perk-up when I hear anything related to Math (I love math!). Thus, you may find this either a 10 second challenge or an all-day head puzzlerā¦

1. divide by 0.

and some extra chars needed by forum.

You win the challenge!

Apparently not anytime soon (!425). While the old file format had supported it, it got knowingly broken with the new file format implementation in KiCad 6. Use Altium instead.

To return to the original question: using DeMorgan equivalents outside logic symbols.
'I do this on a āneed toā or ānice to haveā basis.
The mosr comprehensive example is probably single opamps without a separate power unit.
For those I generate a DeMorgan equivalent with swapped inputs. This leaves the rest of the symbol including power pins intact. Very useful and time saving.

Fpr logic design, I use DeMorgan equivalents all the time (just as suppliers like TI and Nexperia). It results in very clean schematics, ahowing signal function and signal assertion level separately.

If someone wants to pursue this, I can recommend:
William I. Fletcher, āAn Engineering Approach to Digital Designā. Reckon with \$100, fully worth the money.

Not sure what you mean?

My key question/takeaway: If DeMorgan alternates are based on logic equations, isnāt it something of a misnomer to call my two BAW56 symbols DeMorgan variants? Yes I am being nitpicky, but I think we are really getting at any alternate symbols representing the same device? So if we are just drawing op amp power pins differently or repositioning the two diodes in a common anode symbol, that is an equivalent symbol but is NOT a DeMorgan equivalent, right?

Note: I do power and analog but try to avoid logic in my designs. Does that make me illogical?

Actually I recently did use a 74HC4020, a 74HC138, and an NL7SZ97 so this business of avoiding logic is only an un-fulfilled aspiration.