In the main portion of the bootcamp, I mentioned that taking the OR of two active low signals was the same as performing an AND on them. In other words, if you have two door sensors that go low when the corresponding door is open, and you want to know when door 1 AND door 2 is open, you can do one of two things:

1. Invert the signals and take the AND. This is potentially inefficient.
2. Use a NOR gate.

Of course, if you wanted the output to be also negative (that is low means true), you could use a NAND or OR gate, as well.

Just like you often see little circles on flip flop inputs to indicate negation, you sometimes see the same thing on logic symbols, as well.

For example, as in the image above, a NAND gate is sometimes drawn as an OR gate with two circles on the inputs. The gate is still a NAND gate. It is simply communicating that the inputs coming in are active low. You might even see a OR gate with two circles on the input and one on the output (which is really an AND gate).

With the advent of computer drawing tools, you don't see this as much as you used to, but you will see it on occasion. It really does help if you are trying to understand what's going on while reading a logic schematic.