Could somebody please explain the two logic gates with only the one input, A and B, respectively??

NAND gate with single input

It is a NAND gate, with a box just before it.

What is this box?

Why only one input?

  • \$\begingroup\$ @Bimpelrekkie Thank you! If you make that an answer I will mark it as correct. \$\endgroup\$
    – Benj
    Mar 25, 2019 at 13:16

1 Answer 1


The box isn't a box, it shows that the signal goes to both inputs of the 2-input NAND gate. The fact that the line is slightly thicker (as thick as the lines used for drawing the NAND symbols) shows that whoever made the drawing didn't have a clue.

A better drawing would be:


simulate this circuit – Schematic created using CircuitLab

  • 1
    \$\begingroup\$ The book I took the photo from is infamous for being incorrect... Thanks for the great answer! \$\endgroup\$
    – Benj
    Mar 25, 2019 at 13:21
  • 2
    \$\begingroup\$ Perhaps worth mentioning--though it may be obvious--is that the reason this is done is to be able to use NAND gates as NOT gates, because when using discrete gates you tend to have a lot of spares of a single type due to how the 74 and 4000 series (and others) logic chips are designed. Better to use three of the gates on a quad NAND than to use two gates of a hex inverter and one of a quad nand, since then you only need one chip instead of two. \$\endgroup\$
    – Hearth
    Mar 25, 2019 at 15:25

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