Digital Electronics

1
Digital Electronics

• Understanding truth tables

2
AND gate

• How many lines did a 2-input AND gate truth table
  have?

A B Z (output)
0 0 0
0 1 0
1 0 0
1 1 1

• Answer 4

3
AND gate

• What about if you have a 3 input AND gate?

A B C Z (output)
0 0 0 0
0 0 1 0
0 1 0 0
0 1 1 0
1 0 0 0
1 0 1 0
1 1 0 0
1 1 1 1

• How many lines does our truth table have now?

4
And Gate

• You will have noticed that 0s and 1s are used in
  a truth table.

• This is known as a binary number system. There
  are only two types of digit.

5
AND gate
Number of 1s
Number of 2s
Number of 4s

• Each row represents a decimal number (i.e. the
  numbers we normally use 1,2,3,4,5 9

A B C Z (output)
0 0 0 0
0 0 1 0
0 1 0 0
0 1 1 0
1 0 0 0
1 0 1 0
1 1 0 0
1 1 1 1
Decimal equivalent of this row is 5 i.e. a
4 plus a 1
6
AND gate

• So from the table we see that 001 means decimal
  1 and 101 means decimal 5.

• The maximum value was 111 on the table, but what
  would 1111 mean in decimal?

• Answer 8 4 2 1
• 15