# Logic Gates

Discussion in 'Homework Help' started by nathalie, Mar 2, 2011.

1. ### nathalie Thread Starter New Member

Jan 9, 2011
14
1
Iam trying to carry out a laboratory experiment in which I have to draw a logic symbol diagram of a 4-input OR gate using two 2-input Or gates. Iam only managing to use a minimum of 3 Or gates how can i simplify this to 2 gates?

Together with this I have to produce a 7input Nand function using a 4 input Nand gate and an OR gate how would this be possible? Would I need to produce the 7 input truth table ?

Thanks

Last edited: Mar 2, 2011
2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
293
Can you show us your current diagrams?

3. ### nathalie Thread Starter New Member

Jan 9, 2011
14
1
for the first one ive connected 2 or gates whose outputs are shorted.

While for the 7 input, Ive used the nand gate together with Or and Ored the final result

4. ### beenthere Retired Moderator

Apr 20, 2004
15,815
293
1. If you work on paper, you will find that your two OR gate solution will not work. You have to be able to look at either output to produce a sensible final output. With the two gates outputs tied together the only way the combined output can go high is with all 4 inputs high. You will need a third gate.

2. If you have 7 signals, you need at least that many inputs to deal with them. The solution will look similar to the OR gate problem.

5. ### Georacer Moderator

Nov 25, 2009
5,177
1,285
Not only you cannot use only 2 ORs to make the 4-input OR, but you must not do it. If the first OR gives a LOW and the second one gives a HIGH, it will try to pull the output of the second gate down, and will most likely succeed. As a resutl will be a false LOW. You could use only two OR gates only if they are Tristate circuits that completely block their output. It will not be much simpler though in few input circuits.