4 ms·

> The NAND gate is significant because we can implement any Boolean function using a combination of NAND gate. I would add that the NOR gate is also functional

by y04nn 5y ago

> The NAND gate is significant because we can implement any Boolean function using a combination of NAND gate. I would add that the NOR gate is also functionally complete.

WJW 5y ago
Doesn't the NOR gate take more transistors to build though? IIRC, the NAND gate can be built with only two transistors while you need more for NOR.
- mnw21cam 5y ago
  In standard NMOS or PMOS circuitry, both a NAND gate and a NOR gate require two transistors each (and a resistor, which is usually actually a transistor as well). CMOS requires four transistors for both a NAND gate or a NOR gate. It's only in TTL logic, where you can have a single transistor with multiple emitters that a NAND gate uses fewer transistors.
  - analog31 5y ago
    It's fascinating that silicon has not completely eliminated the asymmetries that were a way of life in vacuum tube circuits.
- kafufle 5y ago
  If you are building a CMOS 2-input gate then both NAND and NOR use 4 transistors. (2 NMOS and 2 PMOS each) The reason NAND is preferred is that NMOS transistors have roughly half the resistance of a PMOS transistor of the same size. This means that to get a gate with pull-up and pull-down networks of equal strength (which is usually desirable), the NOR gate needs very large PMOS transistors and tiny NMOS while the NAND gate has all of its' transistors equally sized and relatively small. The result is that NAND gates are smaller in size and slightly quicker than an equivalent NOR gate.
- xyzzy21 5y ago
  Depends on the transistor technology you are using. It's possible to create bipolar NORs as compactly as CMOS NANDs.
KineticLensman 5y ago
> I would add that the NOR gate is also functionally complete. Yes, one good example being the Apollo Guidance Computer [0], which was constructed wholly using NOR gates. [0] https://en.wikipedia.org/wiki/Apollo_Guidance_Computer#Design https://en.wikipedia.org/wiki/Apollo_Guidance_Computer#Desig...
Woberto 5y ago
And thus why the excellent course "NAND to Tetris"[1] is named as such. [1] https://www.nand2tetris.org/ https://www.nand2tetris.org/

by y04nn 5y ago

> The NAND gate is significant because we can implement any Boolean function using a combination of NAND gate. I would add that the NOR gate is also functionally complete.

WJW 5y ago

Doesn't the NOR gate take more transistors to build though? IIRC, the NAND gate can be built with only two transistors while you need more for NOR.

mnw21cam 5y ago
In standard NMOS or PMOS circuitry, both a NAND gate and a NOR gate require two transistors each (and a resistor, which is usually actually a transistor as well). CMOS requires four transistors for both a NAND gate or a NOR gate. It's only in TTL logic, where you can have a single transistor with multiple emitters that a NAND gate uses fewer transistors.
- analog31 5y ago
  It's fascinating that silicon has not completely eliminated the asymmetries that were a way of life in vacuum tube circuits.
kafufle 5y ago
If you are building a CMOS 2-input gate then both NAND and NOR use 4 transistors. (2 NMOS and 2 PMOS each) The reason NAND is preferred is that NMOS transistors have roughly half the resistance of a PMOS transistor of the same size. This means that to get a gate with pull-up and pull-down networks of equal strength (which is usually desirable), the NOR gate needs very large PMOS transistors and tiny NMOS while the NAND gate has all of its' transistors equally sized and relatively small. The result is that NAND gates are smaller in size and slightly quicker than an equivalent NOR gate.
xyzzy21 5y ago
Depends on the transistor technology you are using. It's possible to create bipolar NORs as compactly as CMOS NANDs.

KineticLensman 5y ago

> I would add that the NOR gate is also functionally complete. Yes, one good example being the Apollo Guidance Computer [0], which was constructed wholly using NOR gates. [0] https://en.wikipedia.org/wiki/Apollo_Guidance_Computer#Design https://en.wikipedia.org/wiki/Apollo_Guidance_Computer#Desig...

Woberto 5y ago

And thus why the excellent course "NAND to Tetris"[1] is named as such. [1] https://www.nand2tetris.org/ https://www.nand2tetris.org/