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I’m working on a 4 bit computer/calculator that’s fully NAND (just for the heck of it) and I needed some tri-state buffers for the registers. I think that using an AND works the same, but nowhere online shows a layout like this for a tri state buffer.

Is there a reason we don’t use these, or are they everywhere?

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2 Answers 2

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No you can't:

This is a tri-state truth table (this one also inverts, also the EN is B in the diagram):

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Source:http://karmic23.blogspot.com/2011/01/cmos-inverting-tri-state-buffer.html

An 'AND' gate only output's high and low's

A tri-state will float when the output is in High-Z, this means you can connect two or more to the same wire and if they are in High-Z mode, it won't interfere with the other outputs of other tri-state buffers, which makes them useful for constructing busses. If you connect the outputs of two and gates together, your likely to get a over current condition when one goes high and the other low.

In short: Tri-state buffers are for 'buffering' the outputs can be connected (though two should not be enabled at the same time). 'AND' gates outputs should not be connected.

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That truth table for the tri state isn’t Quite correct, when enable is low the output of the tristate is actually “high impedance” - it looks as if it’s not connected at all.

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  • \$\begingroup\$ So if I'm correct, when enable is either low or the gate is turned off (depending on the buffer) the truth table should have a Z for high impedance? \$\endgroup\$
    – zvolk4
    Commented Jul 9, 2020 at 21:00
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    \$\begingroup\$ When enable is low only the tri state buffer will be “z” or “hi-z”, whereas the AND gate will output a real zero. Tri states are usually used with their outputs in parallel with other outputs, and the enable lines are sequenced in such a way that the outputs never are in conflict. The AND gate has no such feature - if you were to parallel it with another logic output there could be conflicts, leading to undefined voltage levels. \$\endgroup\$
    – Bryan
    Commented Jul 9, 2020 at 21:06
  • \$\begingroup\$ Your answer is correct, but in the future try to add more substance to your answers. One-line answers are frowned upon, and possibly lacking in details. Just a heads-up. \$\endgroup\$
    – user105652
    Commented Jul 11, 2020 at 4:18

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