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I was wondering the difference between two symbols (we can found the diagram page 2 at http://www.onsemi.com/pub_link/Collateral/MC14040B-D.PDF )

Between the logic gate NOT (a triangle then a circle) and the other symbol (a circle then a triangle) which is located in the right of the CLOCK10

I searched on the internet but I only find information about the NOT gate (and nothing about the other one)

Thank you very much,

void
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2 Answers2

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The circle notation can be either at the input or output, and this is pretty widely used particularly in integrated circuit circuitry documentation.

This can be viewed as an example of assertion level logic which can help with a proper understanding of the circuit because in general this makes the circuit easier to read.

In this case, when CLOCK is low, the output of the NOR is low, and the input at C is high.

Peter Smith
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Functionally and logically, the two symbols are exactly the same. Both invert and buffer the incoming signal to produce an output.

Notationally, each symbol consists of two components: a triangle that indicates a buffer and a small circle that indicates an electrical logic inversion. These are part of the building blocks for many other electronic logic symbols.

The real difference is that the circle-on-the-output inverter is a US standard symbol, from a 1960s military standard if I remember rightly. It is far more commonly used in schematics and I recommend you favour it so your schematics and sketches are unambiguous to others.

TonyM
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  • But why are two different symbols used in a diagram? Is it just messed up or do they use a different buffer design at ONsemi? Perhaps resulting in different propagation delay? – Ariser Feb 02 '17 at 14:28
  • @Ariser, They can safely use both because it causes no ambiguity in the circuit. Equally, they can show a NAND as an AND with output inverted or as an OR with inputs inverted. The choice often reflects the actual sequence of transistors in the implementation. It might confuse some people so you could call it bad practice if you like but it's a clear logic circuit. – TonyM Feb 02 '17 at 14:34
  • You are skipping over the inverting stage feeding into the buffer or feeding out from the buffer making a difference in propogational analysis if the technology used is known. This is in no way "bad form" it is accurate documentation. – Asmyldof Feb 02 '17 at 14:52
  • @Asmyldof, (a) yes, and (b) quite. – TonyM Feb 02 '17 at 14:55