1-to-2 demux (was: Using large DRAM modules inside the C64)

From: Magnus Eriksson (mage2254_at_thedreamer.nospammail.net)
Date: 2005-10-14 19:11:32

On Fri, 14 Oct 2005, Baltissen, GJPAA (Ruud) wrote:

> I need a 1-to-2 demultiplexer and then every input must be set
> seperately. I think it simply doesn't exist and found out that
> the next easiest way is to take two 74LS139 who can provide the
> four 'elements' I need.

   "Every input"?  You have one input for the signal to be routed, and
   one input for selecting where it goes, right?

   It seems to me that a 1-to-2 demux is such a simple building block
   that most people would prefer building their own from gates.  A
   quad-NAND would do nicely in your case (if my assumption is right,
   and your mentioning a '139 is just the drugs talking):

      ___      |          +-|-'  ROM CS
  ROM/RAM sel  |          |
  -------------+-=| )-------|-.________
  ___________             +-|-'  RAM CS
  chip select             |
  ---------------=| )-----+
                  signal connected to both inputs, so = inverter

   Or I could just have misunderstood what you meant, YMMV, IANAEE
   (yet), etc.  Surely you would only need half (or "a quarter") of a
   139 for this, and not two?

>     ___
>    / __|__
>   / /  |_/     Groetjes, Ruud
>   \ \__|_\
>    \___|       URL: Ruud.C64.org

   Your ASCII skillz are obviously better than mine.  :-)


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