3
\$\begingroup\$

In logic diagrams found in datasheets (e.g. Texas Instruments 74HC316) I've often seen both inverters and buffers with active-low inputs ("negated buffers" or "inverting buffers" or whatever they are called).

enter image description here          enter image description here

I wonder why they use different symbols, since from a logic level point of view they act the same (output is the logical NOT of the input after all).

Are there some differences in the implementation of the two? It seems to me that it is also manufacturer-dependent, since the datasheets of the same parts from other manufacturers may also use plain inverter symbols instead of "negated buffers" (e.g. NXP 74HC3416).

\$\endgroup\$

1 Answer 1

5
\$\begingroup\$

There is no difference. Many times the choice of where to draw the bubble is made as to where the signal level is active low. As such the symbols that you show can be implemented with the same circuit or chip.

In similar manner experienced engineers will also choose to draw other logic gates on the schematic according to the actual usage as to where the low true and high true signals are located. Thus it is possible to use gates on the left below as the logic equivalent on the right. The part data sheets most often shows the left symbol but, depending upon the logic being represented in the schematic, the right side symbol will be more intuitive.

enter image description here

\$\endgroup\$
2
  • \$\begingroup\$ Ok. Thanks for the detailed answer (I knew about DeMorgan's equivalent and their use, but I guess the details are useful for completeness). The fact that the "inverted buffer" can be used on inputs to be coherent with the usage of such pins makes sense (e.g. the /E enable pin in the TI DS). What still puzzles me is that that symbol is also used deep inside the logic diagram, where no external connections are available. Does this give any useful information to the reader? It doesn't seem to help understanding the inner working of the circuit any better. Am I missing something? \$\endgroup\$ Nov 23, 2014 at 15:05
  • \$\begingroup\$ BTW, sorry if I keep pointing out that DS example, the question was triggered by it, but it's not specific about that chip. I've had this doubt for ages and I've seen this different usage of inverter vs. inverted buffers many times. I just don't have more examples at hand now. \$\endgroup\$ Nov 23, 2014 at 15:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.