How A Computer Works
Table of Contents
1 Simplifying NAND gates
Learn It
- NAND gates are really useful, but it would be annoying to have to draw the transistors every time.
- Instead we use a symbol like the one shown below. You can click the buttons and compare it to the transistor version below it, to see that they are identical in function.
2 Combining NAND gates
- NAND gates are said to be universal logical gates because you can make all other types of logic gate out of them.
- We can take a single NAND gate like this one.
- And use it to make this
- This is called an AND gate
3 Assessment
Badge It - Silver
- Have a look at the below. This is the arrangement for a NOT gate, sometimes called an inverter.
- Construct a Truth table for the arrangement, showing input A output Q.
| A | Q |
|---|---|
| 0 | |
| 1 |
4 Making other gates from NAND gates.
- We've already seen one other gate that can be made using NAND gates - it was called an AND gate.
- Below is the diagram for the NAND gate arrangements for an AND gate again, and a simplified symbol for an AND gate that is commonly used.
- Hopefully you can see that the two are identical. When inputs A and B are on, output Q is on. When either or both A and B are off, Q is off.
- The AND gate has a very similar symbol to the NAND gate (it's just missing the little circle near the output) so make sure you don't get the two confused.
- The truth table for an AND gate looks like this
| A | B | Q |
|---|---|---|
| 0 | 0 | 0 |
| 1 | 0 | 0 |
| 0 | 1 | 0 |
| 1 | 1 | 1 |
5 Assessment
Badge It - Gold
- Look at the diagrams below, showing other NAND gate arrangements. These are called OR, NOR and XOR gates.
- For each one, draw a Truth Table showing inputs and outputs.
- OR gate
- NOR gate
- XOR gate
6 Assessment
Badge It - Platinum
- There is a special arrangement of NAND gates called a D-Latch.
- Use the Web to find out what the arrangement of NAND gates in a D-Latch is.
- Write a description of a D-Latch and explain why it is useful in computing.