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Let's say for two inputs, the output values will be the same. So instead of using an AND gate, I was wondering if i can just short them. Please check the attached logisim screenshot to get a better idea of what I'm trying to do.

The 7-seg display will show "1" when the input is 0101 or 0111. Can I just short the two pins in the real decoder, like I've done in logisim? Or would it mess up the thing?enter image description here

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  • \$\begingroup\$ Chances are you will blow one or both of the outputs. \$\endgroup\$
    – Majenko
    Nov 23 '16 at 17:47
  • \$\begingroup\$ Connecting outputs to outputs is asking for trouble unless you exactly know what you're doing like: CMOS inverters in parallel or combining open collector outputs (to make a crude OR function). \$\endgroup\$ Nov 23 '16 at 18:52
  • \$\begingroup\$ Notice the red wire in Logisim means "error" (also interpretable as "undefined value"). \$\endgroup\$
    – user253751
    Nov 23 '16 at 21:35
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It would "mess things up".
Data sheet says that the outputs are " High fan-out, totem-pole, low impedance outputs". This means that outputs actively pull to logic high, or actively pull to logic low. In your attempt to short one output to the other, one will pull high while the other will pull low. They will fight each other, draw excessive current, possibly overheat, and yield logical output that is undefined. Not what you want - don't take this shortcut - decode each state with logic.

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Do not connect push-pull outputs together.

If they are always the same, just use one of them.

There is no advantage to shorting them together if you stay within the recommended fanout (the number of inputs an output can drive) and there could be bad side effects such as a brief shoot-through that causes a glitch if the paths through the decoder are not identical in delay time.

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If you connect two outputs together and they are not in the same state (which they won't be for a 1-of-N decoder) you will get a short circuit between Vcc and GND through those two pins. That will most likely damage one or both of those pins.

Instead you can AND (active LOW) or OR (active HIGH) the outputs together using a 2-input gate to achieve the same reduction of inputs to your other gates.

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