What is difference between positive logic system negative logic system?

Answer 1

If you would like to perform an action by energising to act ( Logic 1 = desired output) then it's positive logic.

If you would like to perform an action by de-energising to act ( Logic 0 = desired output) then it's negative logic.

Here is an example for the positive and negative logic

POSITIVE LOGIC NEGATIVE LOGIC

o/p Y = 1 (energise to act) o/p Y = 0 (de-energise to act)

A B Y A B Y

1 1 1 1 1 0

1 0 0 1 0 1

0 1 0 0 1 1

0 0 0 0 0 1

any how the required action can be performed by using either a positive logic or a negative logic.

Answer 2

First of all, logic level needs to be defined. Lets start with zero. Zero is considered the ground state in most logic systems. Zero has value. In TTL based logic systems, a zero is a voltage beteen 0.0 and 0.8, and most drivers spec out to provide at most 0.4. That is zero. We call it logic low.

Now lets define logic high. In TTL, high is anything higher that 2.0 volts, and it is typical to see 2.4 volts. That "no man's land" between 0.8 and 2.0 is off limits, and reserved for transistion voltages only.

You might ask, why not +5 and ground? See CMOS for that one. This is a discussion of TTL.

At some point in time, you decide what is true and what is false in your circuit. Most engineers would say high is true in low is false. That is fine As long as they are consistent, they are OK. Even if thay change between sections of a circuit, they are find, so long as they pay attention.

So much for the long winded spin up. Here is the answer...

If you have a gate that take two inputs and generates a high output when and only when both inputs are high, you have a positive logic AND gate. Your AND gate takes two inputs, and if they are both true, the output is true. Wonderful.

Take that same gate and pretend the inputs are negative logic and what do you get, but an OR gate. The gate generates a low output when and only when either input is low. You have a negative logic OR gate. Your OR gate takes two inputs, and if one or both of them is true, the output is true. Wonderful.

Beauty is in the eye of the beholder, eh?

Why would you choose one over the other? It turns out that you might have an extra NAND gate or two and you want a NOR gate. Well, with a bit of careful design, one becomes the other.

Another, specific case...

In the Intel 8085 microprocessor, the ALE (Address Latch Enable) pin is positive logic; high true, low false. ALE initiates a sequence of operations known as a bus transaction where an address is emitted and then data is read or written. Along the way, two other pins RD- (Read inverted) and WR- (Write inverted) actually conduct the read or write transaction. RD- and WR- are negative logic, they are true when low and false when high.