Logic Gate Symbols
A reference to the standard distinctive-shape symbols for the basic gates and the bubble convention that ties them together.
Distinctive shapes
Each basic gate has a fixed silhouette that identifies it at a glance. Learning these shapes is the first step to reading any schematic without a legend.
- AND: a flat back with a rounded, D-shaped nose.
- OR: a curved concave back rising to a pointed nose.
- Buffer and NOT: a triangle pointing toward the output.
- XOR: an OR shape with an extra curved line across its back.
- XNOR: the XOR shape with an output bubble.
The bubble convention
A small circle, the bubble, always means inversion. It converts a gate into its complemented form: an AND with an output bubble is a NAND, an OR with one is a NOR, a triangle with one is a NOT, and an XOR with one is an XNOR. Bubbles on inputs mean the gate responds to a 0 on that line.
Symbol-to-function table
| Symbol | Base shape | Bubble | Function |
|---|---|---|---|
| AND | D-shape | no | conjunction |
| NAND | D-shape | output | not-and |
| OR | curved | no | disjunction |
| NOR | curved | output | not-or |
| NOT | triangle | output | complement |
Two symbol standards
The distinctive shapes above are the traditional style. A rectangular standard also exists, drawing every gate as a box with a qualifying symbol inside, such as an ampersand for AND. Both convey the same functions; distinctive shapes remain the most common in practice.
De Morgan equivalents
Because of De Morgan's laws, one gate has two valid symbols. A NAND may be drawn as an AND with an output bubble or as an OR with two input bubbles. Choosing the form that matches the active levels of the signals makes a schematic far easier to read.