Help me understand this weird logic symbol. - Page 1
Forum section: electronics / beginners

author: MarkS
author_rank: Unknown
timestamp: March 07, 2023, 02:02:42 pm
content: On page three of this4:16 decoderdatasheet, figure 4, you can see that the four address inputs go through inverters. The inverted signal is then split, with half going directly to the NAND gates and the other going to... an inverted inverter? Inverted buffer? What is that?

author: MLXXXp
author_rank: Frequent Contributor
timestamp: March 07, 2023, 02:09:11 pm
content: Just an inverter. It doesn't matter whether the circle is on the input or output.

author: xrunner
author_rank: Super Contributor
timestamp: March 07, 2023, 02:10:24 pm
content: Demorgan inverter?

author: MarkS
author_rank: Unknown
timestamp: March 07, 2023, 02:11:25 pm
content: Quote from: MLXXXp on March 07, 2023, 02:09:11 pmI did not know that! Thank you!Quote from: xrunner on March 07, 2023, 02:10:24 pm Totally read that as "Demogorgon inverter!Coffee hasn't kicked in yet...

author: tooki
author_rank: Super Contributor
timestamp: March 07, 2023, 06:57:42 pm
content: Quote from: MarkS on March 07, 2023, 02:02:42 pmYou can think of it like this: the inverting element is actually the little circle — the triangle is a buffer. Logically, it makes no difference where the inversion takes place.

author: pcprogrammer
author_rank: Super Contributor
timestamp: March 07, 2023, 07:30:16 pm
content: Quote from: tooki on March 07, 2023, 06:57:42 pmThat is a good one. The circle can also be used on inputs of gates to change the logic. Put them on both inputs of a AND gate and you have a NOR gate.Did not know that this is called demorgan, but take a lookherefor more on it.

author: tggzzz
author_rank: Super Contributor
timestamp: March 07, 2023, 07:48:43 pm
content: Think of the circle as meaning an "active low signal", i.e. a signal that is true when the voltage is low.Have a look at the two enable signals. The bar means they are active low. Then look at the and gate, where the input inversion "removes" the effect of the enable signals being active low. Hence the whole circuit is enabled when both E1 and E2 are active (i.e. both inputs are low).

author: Benta
author_rank: Super Contributor
timestamp: March 07, 2023, 08:25:52 pm
content: That data sheet is not very nice.Fig2 and Fig3 are OK, but Fig1 and Fig4 are highly doubtful due to "double inversion" creating confusion.I prefer the term "asserted low" over "active low", BTW, but that's a philosophical thing.The wonderful thing about using DeMorgan symbols is the clarity you can bring to your schematic by matching direct connections and connections with "asserted low" bubbles on each end.If you have a mismatch, it's immediately clear that you're looking at a "disable" function of some sort.William I. Fletcher has a whole chapter on this concept.

author: tggzzz
author_rank: Super Contributor
timestamp: March 07, 2023, 08:42:53 pm
content: Quote from: Benta on March 07, 2023, 08:25:52 pmThat is indeed an excellent book.I'll accept either "asserted" or "active" low; the latter seems more common.

author: MarkS
author_rank: Unknown
timestamp: March 07, 2023, 10:57:09 pm
content: Quote from: tggzzz on March 07, 2023, 08:42:53 pmWhat book? Always looking for a good book!

author: Benta
author_rank: Super Contributor
timestamp: March 07, 2023, 11:53:47 pm
content: William I. Fletcher, "An Engineering Approach to Digital Design". One of the best, (IMO the best).

author: EPAIII
author_rank: Super Contributor
timestamp: March 08, 2023, 09:49:25 am
content: I think the implementation of the two inverters in silicon is exactly the same. Only the drawing style is changed. And the key words in your reference may be "... depending upon the logic being represented in the schematic, the right side symbol will be more intuitive."This seems to say that the person who was drawing the logic diagram thought the reader could grasp the concept of what was going on if things were drawn in certain ways. This may or may not be true, but the circuit artist THOUGHT it to be so.I find this goes along with the general concept of schematic, as opposed to pictorial, ladder, or other types of circuit diagrams, being intended to make the workings of the circuit as easy to understand as possible. If you don't believe me, just try to figure our a Fritzing drawing of a circuit. And imagine something like a 2" video tape recorder of the 70s being documented that way. (The schematics alone needed a 2 or 3 inch, three ring binder.) Give me schematics any day and well drawn ones, not birds' nests.Quote from: pcprogrammer on March 07, 2023, 07:30:16 pm

author: EPAIII
author_rank: Super Contributor
timestamp: March 08, 2023, 10:21:24 am
content: DeMorgan's Theorem(s) is/are not about how schematics are drawn. At least, not directly. Stated in simple terms, they say that for a single logic circuit/gate if you:1. Change the type of logic gate to it's "opposite"AND2. Change the polarity/sense of ALL the inputs and outputs.Then the resulting circuit will operationally be the exact same as the original one. So,An AND gate with all non inverting ins and out could be changed to an OR gate with all inverting ins and out and they would have the same operation/truth table.A Nor gate with two non inverting ins and an inverting out could be changed into a Nand gate with inverting ins and a non inverting out and the result will be the same.Similar arguments apply to And and Or gates.In other words, if you reverse everything, the result is the same.A simple, inverting or non inverting buffer would be exceptional cases and I don't believe DeMorgan included them. I believe he also wrote about gates with three inputs but they are the subject for another discussion.His theorems can provide powerful methods for circuit analysis and also aids in designing logic networks either on the chip or board levels. They are only of peripheral importance in drawing schematic diagrams and too much importance should not be inferred there. The installation of toilet paper rolls, facing inward or outward, is of far greater significance.

author: MarkS
author_rank: Unknown
timestamp: March 08, 2023, 02:21:38 pm
content: I appreciate all of the info! I really love this site. I learn so much here! Now that I'm taking this hobby seriously, and considering that I've been dabbling in electronics for 38~ years, my lack of knowledge is astounding.

author: tggzzz
author_rank: Super Contributor
timestamp: March 08, 2023, 03:14:12 pm
content: Quote from: MarkS on March 08, 2023, 02:21:38 pmDitto, except that it is 58 in my case.There's always more to learn, and there are always areas in which you can be a beginner