Hi,
Could anyone help plz. I want a bit more explanation on double complement identity as I'm beginner.Double complement is describe at the bottom of link no:1. Also I don't know where CR1 and CR2 come from in the diagram link no:2. Following are the links.
link no: 1 http://www.allaboutcircuits.com/vol_4/chpt_7/3.html
link no: 2 http://sub.allaboutcircuits.com/images/14033.png
Many Thanks+

 

CR1 & 2 are presumably control relays. The so-called relay "ladder" logic diagram is a representation of how the double complement could be physically implemented using relay logic.

See ..

http://en.wikipedia.org/wiki/Ladder_logic

and

http://en.wikipedia.org/wiki/Relay_logic

 

And if what is tripping you is the concept behind the 'double complement' identity, it just means that if you logically invert something twice you get the original value back. To prove this, make a Truth Table with A, A', and (A')' for all possible values of A (i.e., all two of them) and you should see that A is the same as (A')' for all possible values. Thus, it is an identity.

 

In Boolean Algebra, you have only two values, 0 and 1.

Make sure you understand what complement is.
Complement resembles ordinary negation in that it exchanges values.
So...complement of 0=1, complement of 1=0.

If you double complement (invert twice) 0, you get 0.
And if you double complement 1, you get 1. Therefore (A')'=A.

This is a pretty basic thing to understand.

 

Many thanks to everyone for helping me to find the answer.