A question of style or consistency: considering below schematic symbol which notation to indicate a pin with inverted logic is preferred:

  1. Dot and bar above pin label (possible double-negation?!)
  2. Dot only, pin label without bar
  3. No dot but bar above pin label

enter image description here

  • 4
    \$\begingroup\$ My personal opinion: I prefer the dot if possible, but I am fine with the overbar. But please don't use BOTH to indicate negation, that hurts my sense of logic. \$\endgroup\$ Oct 2 '14 at 18:53
  • 8
    \$\begingroup\$ I'd prefer to have the bar, or the bar plus the dot. Because when you start writing about the signal in text (say in a description of your schematic), you still want to have the reminder that it's low-active. And you want the names in your text to match the names used in your schematic. \$\endgroup\$
    – The Photon
    Oct 2 '14 at 18:57
  • \$\begingroup\$ I consider using the bar for complex parts where the pin name matters and the dot for basic logic gates. Still, I'm curious what accepted industry practice is \$\endgroup\$
    – Arne
    Oct 2 '14 at 18:58
  • 2
    \$\begingroup\$ In my experience looking at datasheets, 3 is more common, followed by 2. 1 seems like a double negative and is very confusing. \$\endgroup\$
    – Dave
    Oct 2 '14 at 19:44
  • \$\begingroup\$ @Dave I don't see how a double negation is confusing in any way. It's expressed in very clear terms, not much different from a double bar. It's just very wrong here. \$\endgroup\$ Oct 2 '14 at 21:51

The ultimate answer here is "Do whatever your coworkers do". They're the ones who will need to understand it after all.

That being said, 3 is the clearest and most difficult to misunderstand, and probably the best general practice for that reason when you don't have a coworker to ask.

  • \$\begingroup\$ Excellent point. Floris's corollary to Murphy's Law: "If anything can be misunderstood, it will be." Making it "the most difficult to misunderstand", while itself a somewhat convoluted phrase (and thus open to misunderstanding) is a sound approach. \$\endgroup\$
    – Floris
    Oct 3 '14 at 0:17

You can use as a reference, any regulation or recommendation, such as ANSI / IEEE Std 91-1984, but more important is to maintain consistency across all documents.

Use a standard, has the advantage that when you refer to it in your documents, there is no possibility of ambiguity.

You can also write an internal document to your organization (such as a long time ago) where you specify the documentation system chosen. If the documentation gets to outsiders, you can always attach a copy of the internal standard. This has the advantage that it can write the internal document agreeing between all stakeholders.

For this particular question, the standard referred to above, implements only dot without adding the bar on the label.


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