Beginning with the July 1, 2007 General Class Question Pool¹, several new technical areas have been introduced.  If you look at last month's newsletter you'll find a sample test consisting of 25 questions from the  New Question Pool   Two of the new questions deal with Logic Gates; one asks "What is the output of a two-input NAND gate if both inputs are ONE?"  and the other asks "What is the output of a NOR gate given that both inputs are ZERO?".  Fortunately, there are only 5 questions dealing with digital logic circuits and the two above are the only ones asking you to tell the difference between two types of gates.  Here's some information that should make it easy to remember the difference:

First a little background. Basic digital circuits are binary because they deal with only two (bi) states: ON/OFF or HIGH/LOW or TRUE/FALSE.  The terms all mean the same thing and are used interchangeably.  Early computers actually used mechanical switches that were switched on or off, one at a time, to "program" the computer.  It was found that a combination of 8 single pole switches could provide 256 different combinations which was sufficient to describe all the letters in the alphabet both upper and lower case, all numbers from 0 to 9, and several "special characters".  We can answer all the Logic Circuit questions in the new pool with only 3 switches.  Here's why:  One switch, by itself yields only two states, TRUE or FALSE.  It's either on or off.  When you combine that with a second switch you now have 4 possible conditions: All FALSE or OFF, All TRUE or ON, Switch A TRUE, Switch B FALSE, and Switch A FALSE, Switch B TRUE.  Combine these two switches with a third and you double that number of possibilities. Each new switch added doubles the number of possible states.

I've already clouded the issue with too many words.  The use of "Truth Tables" describes the above in much simpler terms:

These are the 5 most common gates

AND Gate	The AND gate is a logic gate that gives an output of 1 only when both of it's inputs are 1. Input-A is True AND input-B is True, therefore Output-Q is True.	Input A Input B Output Q 0 0 0 0 1 0 1 0 0 1 1 1
OR Gate	The OR gate is a logic gate that gives an output of 1 when either of it's inputs are 1. Input-A is True, OR input-B is True, therefore Output-Q is True.	Input A Input B Output Q 0 0 0 0 1 1 1 0 1 1 1 1
NAND Gate	The NAND gate is simply an AND gate that has a built-in NOT gate that changes the output to the opposite state. The NOT gate has changed the output from a 1 to a 0	Input A Input B Output Q 0 0 1 0 1 1 1 0 1 1 1 0
NOR Gate	The NOR gate is an OR gate that has a built-in NOT gate that changes the output to the opposite state. The NOT gate has changed the output from a 1 to a 0	Input A Input B Output Q 0 0 1 0 1 0 1 0 0 1 1 0
NOT Gate	A two terminal device. If the input is 1 then the output is 0 and vice versa. It likes to argue!	Input Output 0 1 1 0

So there you have it.

Question G7B01 asks "Which of the following describes a "Flip/Flop" circuit?"  The answer is "A digital circuit with two stable states".  (The other 3 choices don't even make sense)

Question G7B02 asks "Why do digital circuits use a binary number system/"  The answer is "Binary ones and zeros are easy to represent with an ON/OFF state."

Question G7B03 asks "What is the output of a two-input NAND gate, given both inputs are "one"?"  The answer is "Zero".  If it had been an AND gate the answer would have been "one".

Question G7B04 asks "What is the output of a NOR gate given than both inputs are "zero"?  The answer is "One".  If it had been an OR gate the answer would have been "zero."

Question G7B05 asks "How many states are there in a 3-bit binary counter?"  The answer is "8" (twice as many as a 2-bit device).

Need more?

A really neat animated display of digital logic circuits can be found at this URL: http://www.williamson-labs.com/480_logic.htm

Note 1: The current General Class question pool will be replaced on June 30, 2011