What does the bar above a pin name in a datasheet mean?

Question

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?

score 12 · Accepted Answer · answered Apr 26 '18 at 17:45

12

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.

answered Apr 26 '18 at 17:45

Spehro Pefhany

3

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. – Octopus Feb 12 '19 at 21:10

score 1 · Answer 2 · answered Apr 26 '18 at 17:45

1

It usually means an active low pin.

answered Apr 26 '18 at 17:45

Lior Bilia

"usually"? When is it otherwise? :) – Apr 26 '18 at 18:03
5

When technical writers don't follow established standards. – Lior Bilia Apr 26 '18 at 18:25
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. – TimB Apr 26 '18 at 19:48
1

Sometimes we stick a lower-case n at the front or back, unfortunately netlists don't handle over bars very well. – Spehro Pefhany Apr 26 '18 at 20:14
Well, you can't please everyone. – Lior Bilia Apr 26 '18 at 20:39
Unfortunately it is normally only schematic programs that can place an over-bar, and know what they means. The 'font' standards allow for such an object, but no one else has adopted it, not even EE.SE. – Apr 26 '18 at 20:59
Sometimes a tilde is used like this ~EN – mike65535 Apr 27 '18 at 11:11
some use tilde as inverted input, but Arduino prominently marks PWM inputs with tilde to add on confusion. Probably based on fact that ~ looks like PWM signal. – papo Oct 06 '19 at 13:25

2 Answers2