Digital Logic - Need help for Basic AND, OR, NAND, NOR gate.

[timmytandian]

I am new to digital logic. i have some question:

1. why are NOR and NAND gate preferable to OR and AND gate?
2. my lecturer told that we can make Q = A + B just by several NAND or NOR gate. Is that true??
2. Is there any disadvantage if i set an output from a logic gate to input from another logic gate?

 

[Mikebits]

Well, you can make all the gates with a NOR and NAND gate. Look at following image.

 

[MikeMl]

Read and understand this.

 

[Gary B]

It is one of those practical issues that have nothing to do with theory. All early logic families used a common emitter output to get a decent fanout. As a result, inversion was built into the gate.

 

[Ratchit]

timmytandian,

1. why are NOR and NAND gate preferable to OR and AND gate?

It depends on what you want to do. You can make any logic term with NAND or NOR gates. You cannot make everything with AND or OR gates because no matter what a genius you are at connecting ANDs and ORs together, you cannot get a inverted output. The fewer gates you use, the less the parts count and propagation delay. If you need to just AND or OR something, why use more than one chip if you don't have to?

2. my lecturer told that we can make Q = A + B just by several NAND or NOR gate. Is that true??

Yes, but the parts count and possibly the propagaion delay will increase.

2. Is there any disadvantage if i set an output from a logic gate to input from another logic gate?

Propagation delay and fanout have to be considered.

Ratch

 

[timmytandian]

wow, thx for your attachment. it helped me, i got new inspiration...

 

[timmytandian]

@ Mike and Mike:wow, thx for your attachment. it helped me, i got new inspiration...
@ Ratchit: thx for your answer.. hmm, now i know that DeMorgan is a very powerful method to solve boolean algebra like Q = A+B...

@all: Now i understand about logic gate thankyou.. btw, what is fan-out??

 

[crutschow]

1. why are NOR and NAND gate preferable to OR and AND gate?

Besides being able to provide inversion, another advantage with NOR/NAND gates is that, with clever design and using logic inversion, you can use one gate type to perform either function. Thus for negative input logic a NAND gate can perform the OR function, i.e. if either input is low, the output is high. Similarly for negative input logic, the NOR gate can produce the AND function, i.e. if both inputs are low, the output is high.

An example of this would be to generate the AND-OR function, AB + CD. This can be performed with three 2-input NAND gates by simply connecting the outputs of the first two gates to the two inputs of the third. The first two generate the /AB and the /CD function. The third gate then will go high when either /AB or /CD is true (output low), generating the desired function.