Logic Gates

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First, if your book doesn't cover this material, you need a different book. You'll see these circles on inputs and outputs in various schematic diagrams.

Try a Google search for 'logic gate symbols'.

The first hit I got was

Richard Seriani

A small circle on either input or output means an inversion takes place, there. So an OR gate with a ball on the output means that the output is low when any input is high, instead of high when any input is high. It just takes the output from an OR gate and inverts it. The result is called a NOR (not OR). It would have been more logical if it had been called a ORN (OR not) but that is harder to pronounce, I guess.

Bart, An excellent resource when working with a digital IC is the device's datasheet. There are many places to find the datasheets, but one in particular is datasheetarchive.com. You will need to do a search for whichever part you are looking. The sheets are often many pages, containing quite a bit more information than you might need. But the first page (sometimes the second) shows the truth table for the device. The truth table is simply a table indicating all the possible inputs to the chip, and the resulting output. The datasheet also has the pinout diagram, which is always nice to have.

Flank

On Mon, 12 Dec 2005 20:54:04 -0500, via , "Bart" spake thusly:

No.

On Mon, 12 Dec 2005 23:43:33 -0500, via , "Bart" spake thusly:

"Rats" spelled backwards is "Star."

Thanks a bunch, makes sense now! Oh, if the output is high I assume I can source an led to turn on, and if an output is low, I can sink an led to come on?

Bart

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Usually, yes. A typical logic gate is capable of sourcing or sinking enough current to drive a typical LED. The output voltage may be out of spec when drawing this much current, but that doesn't matter if you are just driving a LED and not another logic gate as well. Some gates have open-collector outputs which can only sink current.

Dave :)

Source, yes. Usually.

Sink, no. Unless it's an open collector.

yes, depending in the device some devices can't source much or can't sink much.

Bart, Not all chips supply/sink the same amount of current. For the most part, ICs cannot supply or sink much current at all. An led needs around 5-15 mA. This is stretching it for some ICs. A 7404 (hex inverter gate) can sink a pretty fair amount a current. I typically wire my LEDs such that the high pin is at 5V, and the low pin is tied to the output of an inververter gate through a 330 ohm resistor. Then, when I send a high signal to the input of the hex gate, the output goes low and turns of the LED. Also, it is a good idea to isolate the signal used to drive your LEDs away from the logic signals. An LED can draw enough current from a gate to cause the gate to malfunction.

Flank

No, it's because it's not an "Or Not". An "Or Not" has only one input inverted. (X = A + /B) ;-)

Cheers! Rich