# Digital Logic design Help PLZ

Discussion in 'Homework Help' started by Ro3a, Nov 6, 2008.

1. ### Ro3a Thread Starter New Member

Nov 6, 2008
5
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Hi everybody ..
I need UR Help ASAP plz .
Im taking digital logic design course & I have some problems to be solved
SO If Anybody could help me I would be thankful
Here R the problems

1) A combinational Logic Circuit has 4 inputs ( A,B,C & D ) and One output Z . The output is iff the input has three consecutive Zeros or three consecutive ones . For example , If A=1, B=0 , c=0 and D=0 then Z=1 , but if A=0 , B=1 , C=0 and D=0 then Z=0
Design the circuit using one 4-input OR gate and 4 3-input AND gates .

2) Design a combinational logic circuit which has one output Z and a 4-bit input ABCD representing a binary number Z . Z should be 1 iff the input is at least 5 , but is no greater than 11 . Use one OR gate ( three inputs ) and three AND gates ( with no more than three inputs each ) .

3) A bank vault has three locks with a different key for each lock . Each Key is owned by a differefnt person . In order to open the door , at least two people must insert their keys into the assigned locks . The signal lines A,B,C are 1 if ther is a key inserted into lock 1,,2 or 3 respectively . write an equation for the variable Z which is 1 iff the door should open .

PLZ PLZ PLZ .. TRY UR Best to help me

2. ### beenthere Retired Moderator

Apr 20, 2004
15,808
294
Just make up truth tables for the problems.

If you are familiar with the various logic gate functions, this is easily resolved by assigning the various inputs to gates that will produce the desired output.

3. ### Ro3a Thread Starter New Member

Nov 6, 2008
5
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Dear hobbyist:

These Qs R copied exactly as they are .
Still we didnt take the Nor , Nand

PLLLLz Try yr best to help me

4. ### hobbyist AAC Fanatic!

Aug 10, 2008
885
87
Sorry I deleted my post, yesterday
because I thought I was giving bad advise..

I came up with a logic diagram to satisfy all the input and outputs required, HOWEVER, I could only do it using 3 "exclusive or" gates
2 "OR" gates and 6 different "AND" gates.

Definately not what you were asking. I tried to use what you gave us, but I personally found it difficult to do this without resorting to my own extra logic gates.

Sorry I can't be much help with this..

5. ### beenthere Retired Moderator

Apr 20, 2004
15,808
294
It is probably a trick question (at least #1). There is no way to know the four states without being able to test by inversion. You can't do much with positive logic alone, but you can do anything with inverting logic.

If your instructor insists that it is possible with the 4 AND gates and the one OR, please post it up. I'd like to see it.

6. ### joed3dge New Member

May 15, 2009
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for part 1:

Z = ABC + BCD + A'B'C' + B'C'D'

7. ### Enos New Member

Dec 7, 2009
2
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for part 2:
Z = AB' + A'BD + A'BC

Just draw the truth table and use a k-map to simplify it.

8. ### Enos New Member

Dec 7, 2009
2
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for part 3:
z = AB + AC + BC