7485 Magnitude Comparator

[Elecman2000]

Why the Cascading inputs has always to be (A>b ,0) (A<b, 0) (A=b , 1 ),, can sm 1 explain pls. thx

 

[Dean Huster]

When you assemble several of these as a multiple-digit comparator, you cascade the < out to the < input of the next stage, = to = and > to >. This way, if the MSD (most significant digit) of A is greater than B, it will force the final output to A>B regardless of what the comparisons of the other digits are. All else being equal (a pun, yes, but if all other digits above the LSD are equal to each other), then the final output will be determined by the LSD.

Rereading the original question, let me then add: A>B, A<B and A=B are all exclusive conditions. One and only one can be true, correct? So at the initial input, we make A=B as the true (1) input while the other two inputs are false (0). This puts the beginning of the comparision on an "equal" footing so to speak. The final output is based upon the digits being compared and not the original input.

These comparators are a lot of fun to work with. I made a digital frequency control for a function generator with a set of them. A keyboard enters the desired frequency into one set of comparator inputs, the output of a counter from the generator is fed to the other set of comparator inputs. The outputs of the comparator go to indicator lamps (to show the status of the control) and to + and - voltage sources that feed a charging capacitor and buffer amplifier which then feeds the VCO input of the functions generator. It's really neat to watch it operate!

 

[Elecman2000]

Thx a lot for HelP Man