For many years the 7400 quad 2 input NAND gate chip was the backbone of digital electronics. Because using a mass of NAND gates it is possible to construct almost any digital system. Although by today’s standards it would not be the pinnacle of miniaturisation.
This post shows the truth table for a 2 input NAND gate and the pinouts of the common 7400 quad 2 input NAND gate chip.
With thanks to Anthony McCloskey, from the raspberry-vi mailing list for the suggestion of this post and the pinouts.
What is a NAND Gate?
NAND is a short way of saying ‘not and’.
A 2 input NAND gate has, as the name suggests, 2 inputs and one output.
To explain what this means, and how the 2 input NAND gate functions, we can draw what is called a ‘truth table’. This is ‘boolean algebra’, but it is much easier than your worst fears, high school algebra, because we are only interested in 2 states, 0 or 1, off or on, low or high. They are the same thing.
In the table below, 0 is a low, or off value, and 1 is a high, or on value.
Columns A and B show the state of each of the two inputs of a single gate, and the third column shows in what state the output will be for that combination of inputs. The third column is labelled ‘Q’ by convention.
Boolean expression Q = A.B
Read as A AND B gives NOT Q
Or when A and B are high, Q will be low.
The version of the 7400 chip most likely to be encountered and used by an electronic hobbyist or student, is packaged ina typical 14 pin DIL chip. Two rows of 7 pins on each side of the body of the chip.
Holding the chip with the very obvious (easy to find with a fingernail) notch in one end of the body at the top, away from you, the pins are labelled 1 to 14 in an anti-clockwise direction, from the pin immediately to the left of the notch, and ending with pin 14 to the right of the notch.
The chip will bridge two holes ina typical 0.1 inch pitch matrix board, and will also bridge the central divide in a typical breadboard.
I can remember seeing some DIL chips with a small recessed dot next to pin 1 instead of the usual notch. If you cannot find a notch, feel around with the tip of a non-conductive but sharp object and see if you can find this tiny recessed dot.
The following table shows the pin connections.
|1||Gate 1 Input 1|
|2||Gate 1 Input 2|
|3||Gate 1 Output|
|4||Gate 2 Input 1|
|5||Gate 2 Input 2|
|6||Gate 2 Output|
|8||Gate 3 Output|
|9||Gate 3 Input 2|
|10||Gate 3 Input 1|
|11||Gate 4 Output|
|12||Gate 4 Input 2|
|13||Gate 4 Input 1|
It can be seen from the truth table above, that if the two inputs of a NAND gate are connected together, the output of that gate will always be the opposite of the state of the two commonly-connected inputs. Making a NAND gate act as a simple state inverter.