This Serial Communications Controller from Zilog lists a few of its pins with a bar above the name. What does this bar mean?

From a schematic I have been looking it seems a few of these are inactive when high and active when low?

Zillog SCC


As you surmise, it means that the function the pin name implies is active when low.

For example, /INT is low to trigger an interrupt and /WR is low to indicate a write.

  • 2
    \$\begingroup\$ I'm surprised you didn't use the word "inverted". Basically, it means "not". While WR would mean the pin denotes that writing has been enabled when it is held high, /WR means that writing has been enabled when it is held low. Essentially an inversion. Or you could reinvert it by including a NOT gate in front of it. Not that you'd want to do that, just that it helps to explain it. \$\endgroup\$ – Octopus Feb 12 '19 at 21:10

It usually means an active low pin.

  • \$\begingroup\$ "usually"? When is it otherwise? :) \$\endgroup\$ – user103380 Apr 26 '18 at 18:03
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    \$\begingroup\$ When technical writers don't follow established standards. \$\endgroup\$ – Lior Bilia Apr 26 '18 at 18:25
  • \$\begingroup\$ When technical writers use Microsoft Word and can't place an overheat without great gnashing of teeth, they will use a trailing \, *, or append _b. It is horrible to read. \$\endgroup\$ – TimB Apr 26 '18 at 19:48
  • \$\begingroup\$ Sometimes we stick a lower-case n at the front or back, unfortunately netlists don't handle over bars very well. \$\endgroup\$ – Spehro Pefhany Apr 26 '18 at 20:14
  • \$\begingroup\$ Well, you can't please everyone. \$\endgroup\$ – Lior Bilia Apr 26 '18 at 20:39

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