Combined Logic Gates
You saw the three simple Logic Gates - AND, OR and NOT on this page. It is possible, and very common, for these to be combined to make more complex logic gates.
When you are working with these more complex gates you need to first identify the individual gates involved and then work your way through the logic for each possible input to draw up the Truth Table.
For example:
This combined gate contains an AND gate and an OR Gate. To help me analyse it I have added the red "d" to the diagram so I can list out the output from the AND gate and the 2nd Input on the OR gate.
AND gate | OR Gate | |||||
Inputs | Output | Inputs | Output | |||
A | B | d | d | C | Z | |
| 1 | 1 | 1 | 1 | 1 | 1 | |
| 1 | 0 | 0 | 0 | 1 | 1 | |
| 0 | 1 | 0 | 0 | 0 | 0 | |
| 0 | 0 | 1 | 1 | 0 | 1 | |
Note - you don't have to list the "d" column twice, I've just done so it to make it easier to see what is going on. Also be aware that some Combined Gates will need more than 4 rows of data in the Truth Table