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I need to construct a decoder 5/32 using any number of 2/4 and 3/8. How do I start? With Enable it's not hard to figure out, but without them it gets complicated. Advice?

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  • \$\begingroup\$ What have you got so far? \$\endgroup\$ – Majenko Nov 16 '14 at 17:12
  • \$\begingroup\$ Looking at the numbers and trying to control the inputs. Not very far. I've got four 3/8 connected to 32 outputs, but I can't figure out how to set up anything before that \$\endgroup\$ – Desperado Nov 16 '14 at 18:39
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I'd suggest working backwards from the output. Since you want only one output active at a time, and because you don't have an Enable on your devices, one simple way to approach it would be to use only a subset of outputs.

For instance, if you did that with a 3-8 decoder, you might use only the low 4 output bits and simply not use the upper 4. The effect is that for any three-bit input, the device would only provide a (connected) output if the high bit is zero. If you work backwards to the input from there, you can come up with a mechanism that will work.

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enter image description hereStart with eight 3/8 decoders and connect only the four outputs of each that have the first bit 1 to loads. This gives 32 outputs and that first bit is essentially an enable. Feed the first three of your five input bits to a 3/8 and wire each output to the first bit of one of the output 3/8s. That is your chip enable. Common the remaining two input bits across the lower two input bits of the eight output 3/8s.

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