Positive logic defines the high voltage level as 1 and the low voltage level as 0. Negative logic defines the high voltage level as 0 and the low voltage level as 1. Logic gates like NAND and NOR will have different truth tables under positive and negative logic systems. The characteristics of a logic family include speed, fan-in, fan-out, noise immunity, and power dissipation. Speed considers transition times, fan-in is the number of inputs, fan-out is the maximum number of output circuits, noise immunity is the tolerance for noise, and power dissipation should be minimized.

1. •POSITIVE LOGIC
•
In this the state (also called true or on )is considered to be the more positive
voltage level.
The normal values of HIGH and LOW
positive logic
low value =0
high value =1
• NEGATIVE LOGIC
The state1(True or on)corresponds to the less positive voltage
Nagative logic
low value =1
high value=0
NAND gate in positive logic(H=1,L=0)
A B Y
0 0 1
0 1 1
1 1 1
1 1 0
NOR gate in negative logic (H=1,L=0)
A B Y
1 1 0
1 0 0
0 1 0
0 0 1
LOGIC CHARACTERISTICS
The differernt characteristics of a logic family.
1.speed
2.fan-in
3.fan-out
4.noise immunity
5.power dissipation
The speed
It takes into account the trasition times.
FAN-IN
The logic gateb is a number of input's.
FAN-OUT
The fan-out of a logic gate is the maximum number of similar circuits.
NOISE IMMUNITY
It is also called a noise margin.
POWER
The power dissipation of a gate should also be small possbile.