# Truth Table Help

Discussion in 'Homework Help' started by Andytos, Jul 16, 2010.

1. ### Andytos Thread Starter New Member

Jul 16, 2010
3
0
Hello everyone,

I need some help with some very simple truth tables. The only problem is that i'm a 3d designer but I'm doing a computer architecture class, so, I'm not really good at it.

I cannot find the attach button, so i'll post a direct link to the document.
I'm having problems at section 1, point 5. http://swift.coventry.ac.uk/Open/As...gust2010ug/120CT Resit Coursework 2009_10.doc

Any help will be really appreciated. If you can do more than point 5, I don't know how i'll repay you, but it would be great.

Andy

2. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
Do you know how the inverter and the OR gate behave when fed with an input? If so, try to cover all possible input variations and find the resulting output for each one. Then compare it with the given truth tables to find the answer.

3. ### Andytos Thread Starter New Member

Jul 16, 2010
3
0
Thanks for replying so fast, but I don't know. If I'd knew, I would've made it by myslef

4. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
Why don't you check Wikipedia for "logic gate"? About at the middle of the page there is a list of the most common gates along with their symbols. At the rightmost column there are tables with inputs, and the resulting outputs. Remember, an OR gate does the logical operation $A + B$. With an inverter in front of one of the OR's input, you calculate the logical operation $A + \overline{B}$.
If you still have trouble, say where you are stuck at, and I 'll be more explanative. It is the forum's policy to not give answers without a little bit of the Original Poster's sweat.

5. ### Andytos Thread Starter New Member

Jul 16, 2010
3
0
I managed to solve almost everything, except for points 5 6 7 and section 2. If anyone could help, I would really appreciate.

6. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
Since the OP went as far as mailiing me to obtain a solution, I 'll try to be a little more desrciptive.
Let's analyze the logic circuit of Section 1, Question 5.

By inspecting the truth tables of the NOT gate, we see that it behaves like this:
$\left[ \begin{array}{c|c} \text{Input} & \text{Output} \\
0 & 1 \\
1 & 0 \end{array} \right]
$

And the OR gate behaves like this:
$\left[ \begin{array}{c|c} \text{Input} & \text{Output} \\
\begin{array}{cc} 0 & 0 \end{array} & 0 \\
\begin{array}{cc} 0 & 1 \end{array} & 1 \\
\begin{array}{cc} 1 & 0 \end{array} & 1 \\
\begin{array}{cc} 1 & 1 \end{array} & 1 \end{array} \right]
$

Let's examine now our own circuit. We will go through all $2^2=4$ combinations of possible inputs and try to make the truth table.
A=0, B=0: As B goes through the inverter, it will become a 1. As a result, the OR gate will receive 0,1 at its inputs and as a result will yield a 1.
A=0, B=1: Again, B is inverted and the OR sees two zeros and gives a 0.
A=1, B=0: 1 OR 1 = 1.
A=1, B=1: 1 OR 0 = 1.
The resulting truth table is the following:
$\left[ \begin{array}{cc|c} 0 & 0 & 1\\
0 & 1 & 0 \\
1 & 0 & 1\\
1 & 1 & 1 \end{array} \right]$

Concerning Question 6: Construct the logic circuit corresponding to the given expression. Now, keep in mind that, in a continuous signal line, eg. for on gate's output to another gate's input, you can insert two inverters and the logic function will remain the same. Naturally, as two NO's equal a YES. Try to convert your circuit with this rule to end to one of the circuits the exercise proposes.

Concerning Question 7
This is a type of latch. Go through your textbook. There is no way you weren't tought of that circuit but are asked to recognise it.

As for Section 2, first of all try to extract the logic functions wich will determine when a segment of the display will light up, in relation to X and Y. If that's too hard for you, construct at least the truth tables asked, and post them, so we can discuss further on them.