Help needed for using a decoder

Thread view:

Enable EMail Alerts

Start New Thread

Thread rating:

strictly_mk@hotmail.com - 29 Dec 2005 14:09 GMT

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

Reply to this Message

PeteS - 29 Dec 2005 15:50 GMT

> Hi all, I was wondering if anyone could help with the following
> question.
[quoted text clipped - 7 lines]
> how to use a decoder in this situation. Can I use a 3-line-to-8-line
> one for this? Any suggestions?

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

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

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.

> Thank you
>
> strictly_mk

You're welcome

Cheers

PeteS

Reply to this Message

Rich Grise - 30 Dec 2005 01:57 GMT

> 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.

I don't even know what this means.

Can you be more specific?

Thanks,
Rich

> Devise a realisation that uses a decoder with active-low outputs and
> one other gate.
[quoted text clipped - 8 lines]
>
> strictly_mk

Reply to this Message

PeteS - 30 Dec 2005 09:02 GMT

> > Hi all, I was wondering if anyone could help with the following
> > question.
[quoted text clipped - 10 lines]
>
>From my teaching days, that would mean that given four digital inputs,

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

> > Devise a realisation that uses a decoder with active-low outputs and
> > one other gate.
[quoted text clipped - 8 lines]
> >
> > strictly_mk

Reply to this Message

Electronics Forum / Basics / December 2005

Help needed for using a decoder