5590Re: How can I send a 4 bit binary code to a 4 to 16 decoder?
- Aug 5, 2008--- In firstname.lastname@example.org, James Tadlock <jamesa4404@...> wrote:
>Output port 4 is used for the hex display, but there is no
> 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.
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?. JimSince 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
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".
> Maybe the trick would be to take D0, D1, D2 and D4 into theI'll assume you meant D0 thru D3, right? I see the 4514
> > decoder.
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.
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