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5590Re: How can I send a 4 bit binary code to a 4 to 16 decoder?

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  • randy129
    Aug 5, 2008
      --- In cosmacelf@yahoogroups.com, James Tadlock <jamesa4404@...> wrote:
      >
      > Randy: Thanks for the info. I am running the lovely little
      > ELF2K V85. I re-thought my thinking and figure by using
      > an out 4, (some number between 0 and 15) I should be able
      > to pull the bits I need for input off of D0 to D4 there
      > by having all 16 outputs of the decoder usable.

      Output port 4 is used for the hex display, but there is no
      reason you can't "piggyback" your decoder on the same port.
      You might even consider it an advantage, because you would
      be able to see the current value on the hex display.

      If you want to be able to use the hex display without also
      affecting your decoder, then you need to pick an unused port.
      I skimmed thru the elf2k manual, and found which ports are used
      by the various option boards:

      001 - 1 - vt1802 video board
      010 - 2 - disk board
      011 - 3 - disk board
      100 - 4 - hex display
      101 - 5 - vt1802 video board
      110 - 6 - gp i/o board
      111 - 7 - gp i/o board

      (left column is the 3 n-line values)

      > Wonder if I need to isolate the decoder from the Elf?. Jim

      Since the CD4514 has a built-in latch, you can hang it right on
      the elf data bus. The data inputs, at least. You still need to
      drive the strobe on your latch so it will latch the data bus only
      when a particular output port instruction is executed. This is
      where the 3 n-lines come in.

      If you want to fully decode, say, port 4, that would be
      "n2 and /n1 and /n0". The original PE Elf did not fully
      decode, it just uses "n2" to determine that an output
      inst is executing. That means that on the original Elf
      the hex display will respond to any of "out 4" thru "out 7"
      instructions. Depending on which option boards you have
      (or plan on having) you may or may not need to fully decode.
      If you do not have any of the option boards, then you
      could get away with using just one n-line (either n0 or n1).

      According to the 1802 datasheet, you should also include
      /MRD and TPB in you output port selection logic, for
      instance: "STROBE = N2 and /N1 and /N0 and MRD and TPB"
      (where MRD means //MRD).

      Actually, the datasheet says /MRD, but I am pretty sure
      it is a typo (The bottom of "fig 11. OUTPUT CYCLE TIMING
      WAVEFORMS".) I find double negative logic a bit confusing
      sometimes.

      If you do end up using port 4 for both the hex display and
      the new decoder, you can use the same signal that latches the
      hex display to latch your decoder, but unfortunately, I think
      you will need to invert it. The CD4514 appears to have an
      "active hi" strobe, and if I am reading the elf2k schematic
      right, the hex display strobe is "active low".

      <snip>
      > Maybe the trick would be to take D0, D1, D2 and D4 into the
      > > decoder.

      I'll assume you meant D0 thru D3, right? I see the 4514
      datasheet numbers the data inputs 1 thru 4, but don't let
      that throw ya! :-) You can hook them to to any of the elf
      data lines you like, but the most normal would be 0 thru 3.
      In fact, if you want to output numbers in range 0 to 15, you
      have to use d0 thru d3.

      Randy
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