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I have a requirement for an inverting 3-to-8 decoder circuit (i.e. all 8 output high except the selected one being low), but with an option to have all the outputs go low.

The 74HC137 or 74HC138 is close to what I need, but it's three reset conditions all have all outputs high.

Does anyone know of a real-estate-conserving solution (i.e. not a lot of components)?

EDIT: Desired truth table:

A B C R   7 6 5 4 3 2 1 0
0 0 0 0   H H H H H H H L
0 0 1 0   H H H H H H L H
0 1 0 0   H H H H H L H H
0 1 1 0   H H H H L H H H
...
1 1 1 0   L H H H H H H H
X X X 1   L L L L L L L L

R represents some sort of reset input that enables all the outputs (lines 0-7). A, B, and C are the address inputs.

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  • \$\begingroup\$ What is the problem of hardwiring the "E" inputs to whatever it needed? \$\endgroup\$
    – Eugene Sh.
    Aug 27, 2015 at 16:52
  • \$\begingroup\$ @EugeneSh. The HC138 has no configuration of the E inputs that results in all-low outputs. You only get all-high (i.e. inactive). \$\endgroup\$
    – EBlake
    Aug 27, 2015 at 16:54
  • \$\begingroup\$ You could use 8 AND gates (2x 7408) on the '138 outputs to set them all to 0. \$\endgroup\$
    – Alexxx
    Aug 27, 2015 at 17:21
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    \$\begingroup\$ Each output of the 138 is connected to one input of the AND gate and the other AND gate inputs are all connected together. When this line is low all outputs are 0 when it's high the AND gates pass the decoder outputs' state. \$\endgroup\$
    – Alexxx
    Aug 27, 2015 at 17:36
  • \$\begingroup\$ Depending on your output connection, and if current draw/battery is a concern, I might have an idea. Whats your output connected to? \$\endgroup\$
    – Passerby
    Aug 27, 2015 at 19:41

3 Answers 3

Reset to default
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Maybe you could use a combination of 74HC138 decoder and 74HC245 bus buffer. Switch the 74HC245 off (into z state) for the all zeros state and put a pulldown resistor array to outputs.

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Are inexpensive PLDs or CPLDs still available? This sort of thing is exactly what they are great at doing.

Otherwise, I see at least 3 packages: a hc138 & 2- quad AND gates or a hc137 & 2- quad NOR gates, depending on whether you want active HI or LO for the "All Active" state.

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    \$\begingroup\$ .. or maybe 2 HC86 quad gates (and an HC138) then you can have your druthers. \$\endgroup\$
    – Spehro Pefhany
    Aug 27, 2015 at 18:24
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Googling around I found the 74F538 which seems like it could come pretty close to doing what you want (except your "all outputs" input would be active low rather than active high) by tying together the polarity control input and one of the enable inputs. Unfortunately it no longer seems to be available.

https://www.engineering.uiowa.edu/sites/default/files/ees/files/NI/pdfs/00/95/DS009551.pdf

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