# Help needed for using a decoder

Discussion in 'Electronic Basics' started by [email protected], Dec 29, 2005.

1. ### Guest

Hi all, I was wondering if anyone could help with the following
question.

For the function, F(A, B, C, D) = (0, 2, 4, 5, 8, 10, 12, 13) give the
minimised sum-of-products solution.
Devise a realisation that uses a decoder with active-low outputs and
one other gate.

I have a solution of (NOT B AND NOT D) OR (B AND NOT C) but I dont know
how to use a decoder in this situation. Can I use a 3-line-to-8-line
one for this? Any suggestions?

Also very quickly, why are general purpose encoders not very useful?

Thank you

strictly_mk

2. ### PeteSGuest

As you end up with three terms in your solution, (and I am not sure of
the terminology above to help with your minimisation - is D the least
significant bit?) then a three to eight decoder (such as the '138)
would work, but then you'd have to OR the various outputs together.

I answered a post yesterday that used a '156 decoder, which would be
perfect in this particular case, as you have two two product terms and
the '156 has two 2-4 decoders. You'll still have to OR some outputs
together (but don't be foled by the term OR - a NAND is just as
usefully an OR and a NOR an AND if you know how to do it - see
DeMorgan's theorem).

As decoders decode minterms by definition, the tough stuff is done in
hardware. You'll have to figure out the rest (I could, but someone else
beat you to my 'help with the homework on the season of goodwill'
quota).

Datasheet at:
http://www.onsemi.com/pub/Collateral/SN74LS155-D.PDF
Who says? They have their uses, although they are not generally as
useful as decoders, in terms of the issues we come up against. Great
for encoding banks of switches (although there are other methods)
amongst many other things.
You're welcome

Cheers

PeteS

3. ### Rich GriseGuest

I don't even know what this means.

Can you be more specific?

Thanks,
Rich

4. ### PeteSGuest

A,B,C,D, the function evaluates true when the values of A.B,C,D == 0,
2,4,5,8,10,12,13, or alternatively (for D = msb)

D B C A Y
0 0 0 0 1
0 0 0 1 0
0 0 1 0 1
0 0 1 1 0
0 1 0 0 1
0 1 0 1 1
0 1 1 0 0
0 1 1 1 0
1 0 0 0 1
1 0 0 1 0
1 0 1 0 1
1 0 1 1 0
1 1 0 0 1
1 1 0 1 1
1 1 1 0 0
1 1 1 1 0

Cheers

PeteS

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